1 00:00:02,447 --> 00:00:09,967 Earth, a 4.5- Billion-year-old planet, still evolving. 2 00:00:09,967 --> 00:00:18,167 As continents shift and clash, volcanoes erupt, glaciers grow and recede, 3 00:00:18,167 --> 00:00:22,767 the Earth's crust is carved in numerous and fascinating ways, 4 00:00:22,767 --> 00:00:26,396 leaving a trail of geological mysteries behind. 5 00:00:29,127 --> 00:00:31,447 In this episode, the Great Lakes of North America, 6 00:00:31,447 --> 00:00:37,527 the largest expanse of freshwater on the planet, are investigated. 7 00:00:37,527 --> 00:00:41,407 They hold 20% of the world's freshwater 8 00:00:41,407 --> 00:00:46,407 and provide drinking water for nearly 10% of Americans. 9 00:00:46,407 --> 00:00:51,207 These five lakes are among the world's greatest natural wonders. 10 00:00:51,207 --> 00:00:54,207 But their origins are a mystery. 11 00:00:54,207 --> 00:00:59,887 Now geologists are investigating, piecing together the clues that lie hidden 12 00:00:59,887 --> 00:01:07,247 in this extraordinary landscape, delving deep into a vast underground salt mine 13 00:01:07,247 --> 00:01:14,607 behind the torrential flow of Niagara Falls, climbing a mile-high glacier, 14 00:01:14,607 --> 00:01:22,607 where clues to understanding the Great Lakes' formation also provides a window 15 00:01:22,607 --> 00:01:26,441 into the formation of the Earth itself. 16 00:01:36,487 --> 00:01:42,527 The five Great Lakes, Superior, Michigan, Huron and Erie, 17 00:01:42,527 --> 00:01:48,238 pour over one of the world's great waterfalls, Niagara, into Lake Ontario. 18 00:01:53,447 --> 00:01:56,567 The mighty torrent of the falls empties excess water 19 00:01:56,567 --> 00:02:00,719 from four of the five Great Lakes out to the sea. 20 00:02:02,047 --> 00:02:06,607 For geologists, the lakes are a natural wonder and a puzzle, 21 00:02:06,607 --> 00:02:09,967 and scientists are on the trail of how they were formed, 22 00:02:09,967 --> 00:02:15,724 with rocks as their clues, and ice, lava and water as their suspects. 23 00:02:17,487 --> 00:02:22,087 Their investigation begins at these seemingly ordinary industrial buildings 24 00:02:22,087 --> 00:02:24,043 beside Lake Huron. 25 00:02:25,127 --> 00:02:30,042 Hundreds of feet below ground here, there's a remarkable secret. 26 00:02:34,687 --> 00:02:39,847 Deep below Lake Huron, and also Lake Michigan, are vast salt mines 27 00:02:39,847 --> 00:02:43,806 carved out directly beneath freshwater lakes. 28 00:02:48,287 --> 00:02:54,047 Right now we're at 1,750 feet below the surface of the Earth. 29 00:02:54,047 --> 00:02:57,447 We're in the largest underground salt mine in the world. 30 00:02:57,447 --> 00:03:01,156 And we're below Lake Huron, a large freshwater lake. 31 00:03:02,247 --> 00:03:06,718 Amazingly, this salt deposit was uncovered by accident. 32 00:03:07,887 --> 00:03:09,567 They were drilling for oil. 33 00:03:09,567 --> 00:03:11,327 And they hit salt. 34 00:03:11,327 --> 00:03:13,807 And that was the end of looking for... looking for oil. 35 00:03:13,807 --> 00:03:15,684 They just kept on digging for the salt. 36 00:03:16,687 --> 00:03:22,207 This salt deposit is the investigator's first clue, 37 00:03:22,207 --> 00:03:26,644 evidence that there was once an ancient sea here. 38 00:03:31,807 --> 00:03:36,447 Many years ago, the... the salt was formed in a great salt lake 39 00:03:36,447 --> 00:03:41,607 and the evaporation, dry seasons, the salt dropped out, evaporated out, 40 00:03:41,607 --> 00:03:44,075 and formed this salt that we're actually mining in. 41 00:03:46,327 --> 00:03:50,567 There are hundreds of layers of salt, leading investigators to conclude 42 00:03:50,567 --> 00:03:54,967 the sea must have dried up and refilled hundreds of times. 43 00:03:54,967 --> 00:04:01,047 Scientists would later prove this sea finally evaporated millions of years ago. 44 00:04:01,047 --> 00:04:05,767 35% of North America's salt comes from these mines - 45 00:04:05,767 --> 00:04:10,167 salt used to melt ice on frozen roads and sidewalks, 46 00:04:10,167 --> 00:04:11,887 salt used to season food - 47 00:04:11,887 --> 00:04:15,687 the remains of million-year-old seas. 48 00:04:15,687 --> 00:04:19,839 All coming from beneath Lake Huron and Lake Michigan. 49 00:04:21,007 --> 00:04:22,567 McCUE: The salt deposit is massive. 50 00:04:22,567 --> 00:04:26,127 There's probably trillions of tons of salt in the deposit. 51 00:04:26,127 --> 00:04:29,087 It extends all the way down to Detroit. 52 00:04:29,087 --> 00:04:33,717 All of Lake Huron, the salt is under it. And all of Michigan. 53 00:04:35,807 --> 00:04:37,367 The salt is soft, 54 00:04:37,367 --> 00:04:41,807 and over millions of years the salt layers should have worn away. 55 00:04:41,807 --> 00:04:43,847 Why haven't they? 56 00:04:43,847 --> 00:04:49,127 It's because the salt is protected by a vast impenetrable layer of rock 57 00:04:49,127 --> 00:04:53,567 that lies like a giant basin beneath Lakes Michigan and Huron 58 00:04:53,567 --> 00:04:55,967 and stretches under Lake Erie. 59 00:04:55,967 --> 00:05:01,519 Like the porcelain lining a bath tub, the rocky basin holds the lakes' freshwater. 60 00:05:04,927 --> 00:05:08,767 Geologist John Zawiskie and a team of divers are hunting for clues 61 00:05:08,767 --> 00:05:11,847 to the rocky basin's origins. 62 00:05:11,847 --> 00:05:16,637 They're heading for Thunder Bay, a small island at the edge of Lake Huron. 63 00:05:19,087 --> 00:05:23,447 As he walked along the beach, Zawiskie discovered some crucial evidence, 64 00:05:23,447 --> 00:05:27,607 seemingly insignificant rocks that were overlooked for decades. 65 00:05:27,607 --> 00:05:32,927 But Zawiskie suddenly realised what he was looking at - 66 00:05:32,927 --> 00:05:36,317 fossilised remains of ancient sea creatures. 67 00:05:38,327 --> 00:05:41,407 ZAWISKIE: I was seeing something that many geologists had never seen 68 00:05:41,407 --> 00:05:43,007 when they visited this island. 69 00:05:43,007 --> 00:05:46,007 There were the heads of giant lime-secreting sponges 70 00:05:46,007 --> 00:05:47,998 that were some of the main reef builders. 71 00:05:49,287 --> 00:05:52,447 Zawiskie uncovered a perfectly preserved fossil 72 00:05:52,447 --> 00:05:56,963 of a giant sea sponge that must have come from an ancient coral reef. 73 00:06:01,527 --> 00:06:05,127 For the past five years, Zawiskie's divers have been surveying the lake 74 00:06:05,127 --> 00:06:09,087 to discover the size of the ancient coral reef. 75 00:06:09,087 --> 00:06:15,327 They believe it's hundreds of feet thick and extends deep below Lake Huron. 76 00:06:15,327 --> 00:06:20,047 And Zawiskie has proof these rocks are extremely old. 77 00:06:20,047 --> 00:06:22,647 The time period can be pretty confidently bracketed 78 00:06:22,647 --> 00:06:25,445 at right around 385 million years ago. 79 00:06:27,167 --> 00:06:30,207 America was then a very different place. 80 00:06:30,207 --> 00:06:32,287 385 million years ago, 81 00:06:32,287 --> 00:06:35,687 its land mass lay in the southern hemisphere, 82 00:06:35,687 --> 00:06:39,965 a land covered by ancient warm coral seas. 83 00:06:41,327 --> 00:06:44,687 This region was just south of the equator, in tropical conditions, 84 00:06:44,687 --> 00:06:49,920 and shallow seas had swamped many of the land areas of the Earth at that time. 85 00:06:51,327 --> 00:06:52,847 Year in, year out, 86 00:06:52,847 --> 00:06:59,607 coral reefs decay naturally and turn into a soft rock, limestone. 87 00:06:59,607 --> 00:07:03,047 And much of the rock Zawiskie's divers find under Thunder Bay Island 88 00:07:03,047 --> 00:07:08,838 consists of layer upon layer of this limestone from successive coral reefs. 89 00:07:10,687 --> 00:07:12,647 But millions of years ago, 90 00:07:12,647 --> 00:07:17,007 some of this soft limestone near the surface was changed. 91 00:07:17,007 --> 00:07:20,967 When the salty briny sea evaporated, it turned the limestone 92 00:07:20,967 --> 00:07:23,407 into a second, much harder rock, 93 00:07:23,407 --> 00:07:28,162 something which would decide the very shape of the Great Lakes. 94 00:07:30,447 --> 00:07:32,847 This rock is limestone. 95 00:07:32,847 --> 00:07:37,127 This other piece was once the exact same material. 96 00:07:37,127 --> 00:07:41,407 However, it's been converted by a process of brines 97 00:07:41,407 --> 00:07:44,247 creating the conditions for recrystallisation into a rock 98 00:07:44,247 --> 00:07:45,967 that we call dolostone. 99 00:07:45,967 --> 00:07:50,207 It's much harder than limestone, more weathering resistant, 100 00:07:50,207 --> 00:07:53,967 and I can easily demonstrate the difference between these two. 101 00:07:53,967 --> 00:07:57,167 Calcium carbonate, calcium magnesium carbonate. 102 00:07:57,167 --> 00:08:00,807 To show the relative hardness of the two rocks, 103 00:08:00,807 --> 00:08:04,516 Zawiskie uses an essential tool in the geologist's arsenal. 104 00:08:06,847 --> 00:08:09,122 Let me put a little acid on here. 105 00:08:10,127 --> 00:08:14,607 Acid easily attacks and dissolves soft rocks. 106 00:08:14,607 --> 00:08:17,201 First, how will the limestone react? 107 00:08:19,287 --> 00:08:21,847 You can see a very violent reaction there. 108 00:08:21,847 --> 00:08:26,079 Carbon dioxide gas is being released from the limestone. 109 00:08:28,167 --> 00:08:31,567 Next, the hard dolostone. 110 00:08:31,567 --> 00:08:34,407 Let's go ahead and do the acid test on it. 111 00:08:34,407 --> 00:08:37,247 And you can see we don't get this violent reaction. 112 00:08:37,247 --> 00:08:39,647 Almost no reaction at all. 113 00:08:39,647 --> 00:08:43,527 Zawiskie has proved the dolostone layer is harder and more resistant 114 00:08:43,527 --> 00:08:45,367 than the limestone. 115 00:08:45,367 --> 00:08:50,287 The ancient ocean's salty water converted the top layer 116 00:08:50,287 --> 00:08:55,042 of the limestone deposit into a cap of hard, resistant dolostone rock. 117 00:08:56,647 --> 00:09:02,527 It's this that forms the super tough rock basin under three of the five lakes - 118 00:09:02,527 --> 00:09:04,995 Michigan, Huron and Erie. 119 00:09:06,687 --> 00:09:10,687 Scientists were beginning to piece together the chain of events that led 120 00:09:10,687 --> 00:09:13,927 to the formation of the Great Lakes. 121 00:09:13,927 --> 00:09:17,247 The clues uncovered so far - 122 00:09:17,247 --> 00:09:23,127 vast salt deposits provide evidence of an ancient ocean. 123 00:09:23,127 --> 00:09:30,247 The briny ocean changed soft, fossilised limestone into hard dolostone. 124 00:09:30,247 --> 00:09:35,275 Dolostone makes up the rocky basin under Lakes Michigan, Huron and Erie. 125 00:09:38,287 --> 00:09:40,927 The tip of the rock basin, the rim, 126 00:09:40,927 --> 00:09:44,442 forms steep cliffs that tower above these three lakes. 127 00:09:46,487 --> 00:09:50,767 This immense wall of rock, called the Niagara Escarpment, 128 00:09:50,767 --> 00:09:53,167 forms the boundaries of these lakes, 129 00:09:53,167 --> 00:09:57,647 and makes possible one of the world's greatest natural spectacles, 130 00:09:57,647 --> 00:09:59,558 Niagara Falls. 131 00:10:02,687 --> 00:10:05,167 Over this hard dolostone cliff, 132 00:10:05,167 --> 00:10:10,007 3,000 tons of water a second tumble from four of the five lakes. 133 00:10:10,007 --> 00:10:12,927 But it's more than just a miracle of nature. 134 00:10:12,927 --> 00:10:17,247 Niagara Falls is a vital clue that helps scientists date 135 00:10:17,247 --> 00:10:23,163 when freshwater first began flowing into what we now call the Great Lakes. 136 00:10:32,127 --> 00:10:34,607 The Great Lakes of North America. 137 00:10:34,607 --> 00:10:38,327 Geologists have discovered three of the lakes were formed 138 00:10:38,327 --> 00:10:42,807 in a vast rock-lined basin, laid down by an ancient lagoon. 139 00:10:42,807 --> 00:10:47,487 The question is, when? And they think the answer lies here. 140 00:10:47,487 --> 00:10:49,247 Niagara Falls. 141 00:10:49,247 --> 00:10:55,083 Behind this curtain of water lies the evidence to when the lakes were made. 142 00:10:57,007 --> 00:11:01,407 Like the overflow from a bath tub, excess water from four of the five lakes, 143 00:11:01,407 --> 00:11:05,087 Superior, Michigan, Huron and Erie, 144 00:11:05,087 --> 00:11:08,807 spills over the falls into Lake Ontario. 145 00:11:08,807 --> 00:11:14,287 And all that water is changing the falls, change that can be measured 146 00:11:14,287 --> 00:11:18,405 and used to calculate the age of the lakes themselves. 147 00:11:20,407 --> 00:11:22,207 The falls were first studied 148 00:11:22,207 --> 00:11:27,287 by one of modern geology's founding fathers, Charles Lyell. 149 00:11:27,287 --> 00:11:31,607 Lyell, who pioneered the early understanding of Earth's secrets, 150 00:11:31,607 --> 00:11:35,967 was intrigued by the concept of geological time. 151 00:11:35,967 --> 00:11:40,367 Charles Lyell came to Niagara region in the 1840s, 152 00:11:40,367 --> 00:11:44,087 and he made very important observations at Niagara Falls. 153 00:11:44,087 --> 00:11:48,967 Lyell was using the principle that things that we see are going on today 154 00:11:48,967 --> 00:11:52,243 can be used as examples for what went on in the past. 155 00:11:54,727 --> 00:11:59,087 Lyell believed the world wasn't shaped in a few days or even years 156 00:11:59,087 --> 00:12:03,407 but by slow change over millions and billions of years. 157 00:12:03,407 --> 00:12:06,567 This directly contradicted the much shorter time 158 00:12:06,567 --> 00:12:10,767 biblical scholars said the world had been in existence. 159 00:12:10,767 --> 00:12:16,327 Lyell realised that dramatic geological change was going on in front of his eyes 160 00:12:16,327 --> 00:12:18,287 at Niagara Falls. 161 00:12:18,287 --> 00:12:22,599 If he could measure it he might be able to calculate the falls' age. 162 00:12:24,287 --> 00:12:27,407 Lyell's technique was brilliantly simple. 163 00:12:27,407 --> 00:12:30,687 He noticed below the falls was a great gorge 164 00:12:30,687 --> 00:12:33,807 which locals said was steadily increasing in length 165 00:12:33,807 --> 00:12:37,087 as the water wore away the ledge of the falls. 166 00:12:37,087 --> 00:12:41,558 The falls, they said, were moving slowly upstream. 167 00:12:45,607 --> 00:12:48,927 Head to the base of the falls and you can see why. 168 00:12:48,927 --> 00:12:52,203 The cliff face is being worn away. 169 00:12:54,767 --> 00:12:56,767 The falls are formed by a cliff 170 00:12:56,767 --> 00:13:00,527 capped with a ledge of the same hard dolostone rock 171 00:13:00,527 --> 00:13:03,687 created, as we've seen, by seawater. 172 00:13:03,687 --> 00:13:09,603 Beneath the tough dolostone cap is a layer of much softer rock called shale. 173 00:13:10,607 --> 00:13:14,007 As the water crashes over the dolostone, it erodes out 174 00:13:14,007 --> 00:13:17,087 these soft shales that are underlying the dolostone, 175 00:13:17,087 --> 00:13:20,927 and the blocks can fall down from the face. 176 00:13:20,927 --> 00:13:24,807 On the right, then, you can see these massive blocks of dolostone 177 00:13:24,807 --> 00:13:28,038 that have fallen down at the bottom of the waterfall. 178 00:13:31,407 --> 00:13:36,647 Each time the dolostone ledge collapses, the falls move further upstream. 179 00:13:36,647 --> 00:13:40,807 Lyell believed this process had been going on for thousands of years, 180 00:13:40,807 --> 00:13:42,638 and was still continuing. 181 00:13:47,047 --> 00:13:49,807 It had begun as the lakes were first formed 182 00:13:49,807 --> 00:13:54,722 when water began wearing away the hard dolostone ledge of the falls. 183 00:13:55,767 --> 00:13:58,047 To discover the age of the falls, 184 00:13:58,047 --> 00:14:01,927 all Charles Lyell needed was some simple math. 185 00:14:01,927 --> 00:14:05,647 BURCIK: He realised that the falls had started at the Niagara Escarpment 186 00:14:05,647 --> 00:14:09,247 which is about 35,000 feet from here, 187 00:14:09,247 --> 00:14:14,367 so if the falls receded at one foot per year and receded 35,000 feet, 188 00:14:14,367 --> 00:14:19,122 that would give an age for their present position of 35,000 years. 189 00:14:20,927 --> 00:14:26,047 Lyell's calculation was based on simple measurements but wrong guesswork. 190 00:14:26,047 --> 00:14:29,647 He thought the falls were receding by one foot a year. 191 00:14:29,647 --> 00:14:33,686 But today we have much better records to go on. 192 00:14:35,327 --> 00:14:37,887 This plaque commemorates Table Rock, 193 00:14:37,887 --> 00:14:41,967 which is where the falls were at the beginning of the 19th century. 194 00:14:41,967 --> 00:14:46,006 Since that time, they've receded about 600 feet to my right. 195 00:14:48,207 --> 00:14:49,967 So in the last 200 years, 196 00:14:49,967 --> 00:14:53,767 the falls have steadily retreated at a rate of not one foot, 197 00:14:53,767 --> 00:14:56,807 but an astonishing three feet a year. 198 00:14:56,807 --> 00:15:01,447 So instead of Lyell's calculation of 35,000 years old, 199 00:15:01,447 --> 00:15:07,527 the Niagara Falls were a third of that figure, just 12,000 years old. 200 00:15:07,527 --> 00:15:13,159 A mere blink of an eye in Earth's 4.5 billion year history. 201 00:15:15,207 --> 00:15:17,767 In the search to find what created the Great Lakes, 202 00:15:17,767 --> 00:15:23,364 scientists now had a crucial clue, the age of one of their key features. 203 00:15:24,527 --> 00:15:26,847 Born at the same time, the falls is 204 00:15:26,847 --> 00:15:29,407 the overflow for all the upper lakes 205 00:15:29,407 --> 00:15:32,847 into Lake Ontario and the sea. 206 00:15:32,847 --> 00:15:36,887 So if the falls have only been around for 12,000 years, 207 00:15:36,887 --> 00:15:42,120 then it means the lakes themselves must also be incredibly young. 208 00:15:44,087 --> 00:15:48,007 Now that scientists had worked out when the lakes were created, 209 00:15:48,007 --> 00:15:50,927 the next question was how? 210 00:15:50,927 --> 00:15:56,967 What immense force could have created not one but five huge lakes? 211 00:15:56,967 --> 00:16:01,407 A force so powerful it must have left a trail of incriminating evidence 212 00:16:01,407 --> 00:16:03,602 across the region. 213 00:16:09,727 --> 00:16:14,047 Geologist John Menzies scans the landscape to track the mysterious force 214 00:16:14,047 --> 00:16:16,163 that created the Great Lakes. 215 00:16:17,407 --> 00:16:20,807 And he's spotted something unusual - 216 00:16:20,807 --> 00:16:26,127 strange teardrop-shaped hills, one after another, called drumlins. 217 00:16:26,127 --> 00:16:28,847 MENZIES: Some are small, fat and streamlined, 218 00:16:28,847 --> 00:16:31,361 some are extremely elongated. 219 00:16:32,727 --> 00:16:37,447 This one is about... almost a mile in length, 150 feet high, 220 00:16:37,447 --> 00:16:39,438 and about 200 feet across. 221 00:16:40,887 --> 00:16:44,567 This is the evidence that John Menzies has been looking for. 222 00:16:44,567 --> 00:16:46,847 There are many drumlin fields in North America, 223 00:16:46,847 --> 00:16:48,807 but this one is a particularly large field. 224 00:16:48,807 --> 00:16:51,207 It has anywhere between 60 and 80,000. 225 00:16:51,207 --> 00:16:53,767 So it's truly an enormous drumlin field. 226 00:16:56,407 --> 00:16:59,647 Each drumlin points in the same direction, north, 227 00:16:59,647 --> 00:17:03,407 to where an immense force came from. 228 00:17:03,407 --> 00:17:08,007 This tells Menzies they were all created by the same powerful object, 229 00:17:08,007 --> 00:17:11,087 but what was it? 230 00:17:11,087 --> 00:17:16,207 The answer lies 4,000 miles away, high in the Swiss Alps. 231 00:17:19,927 --> 00:17:25,445 Here the culprit is plain to see - snow and ice. 232 00:17:26,567 --> 00:17:30,327 Switzerland is home to some of Europe's largest glaciers. 233 00:17:30,327 --> 00:17:34,206 They're giant rivers of ice that flow down mountain valleys. 234 00:17:36,847 --> 00:17:39,287 Glaciologist Dr Andreas Bauder studies 235 00:17:39,287 --> 00:17:42,807 how glaciers can transform the landscape. 236 00:17:42,807 --> 00:17:47,881 What he discovers here could also point to how the Great Lakes were made. 237 00:17:49,247 --> 00:17:52,527 We measure the movement of the ice. 238 00:17:52,527 --> 00:17:55,567 This reflector reflects the laser signal 239 00:17:55,567 --> 00:17:57,127 coming from a theodolite 240 00:17:57,127 --> 00:17:59,407 giving us the position of this stake. 241 00:17:59,407 --> 00:18:02,127 And then we can calculate the movement. 242 00:18:02,127 --> 00:18:06,247 My colleagues down here are drilling deep holes 243 00:18:06,247 --> 00:18:10,607 down to the base of the glacier to install instruments 244 00:18:10,607 --> 00:18:15,522 to understand how the glacier is changing here. 245 00:18:18,487 --> 00:18:23,322 Bauder's measurements reveal this glacier moves over ten feet every month. 246 00:18:25,767 --> 00:18:30,287 Here, a seemingly stationary glacier is shown moving down the mountain, 247 00:18:30,287 --> 00:18:33,723 recorded by time-lapse photography over a year. 248 00:18:38,087 --> 00:18:42,365 To find out what's driving it, Bauder climbs high up the glacier. 249 00:18:44,727 --> 00:18:49,881 This glacier is thousands of years old and almost a mile thick in some places. 250 00:18:51,167 --> 00:18:58,207 Ice that's a mile thick weighs a colossal 3.8 billion tons per square mile. 251 00:18:58,207 --> 00:19:02,807 That's the weight of 59,000 fully laden supertankers. 252 00:19:02,807 --> 00:19:04,767 And it's this immense weight 253 00:19:04,767 --> 00:19:08,447 that makes the glacier such a force to be reckoned with. 254 00:19:08,447 --> 00:19:12,607 Its weight is slowly pushing the glacier down the valley, 255 00:19:12,607 --> 00:19:16,885 gathering anything in its path, collecting rocks and debris. 256 00:19:21,127 --> 00:19:26,167 The rocks act like the blades of a giant bulldozer, scouring the ground, 257 00:19:26,167 --> 00:19:29,796 digging up yet more and more rock and soil. 258 00:19:30,847 --> 00:19:37,047 But when the temperatures rise, the glacier melts, retreating up the valley, 259 00:19:37,047 --> 00:19:41,687 and leaving rocks and debris behind in huge piles. 260 00:19:41,687 --> 00:19:44,127 This is how the teardrop-shaped drumlins 261 00:19:44,127 --> 00:19:46,527 back in North America were formed. 262 00:19:46,527 --> 00:19:50,767 They were bulldozed, landscaped by a powerful glacier. 263 00:19:50,767 --> 00:19:55,283 A glacier that may also have gouged out the Great Lakes. 264 00:19:57,647 --> 00:20:00,687 The evidence is coming together. 265 00:20:00,687 --> 00:20:05,687 Niagara Falls, dated just 12,000 years old. 266 00:20:05,687 --> 00:20:09,567 This suggests the lakes themselves are very young. 267 00:20:09,567 --> 00:20:11,687 The presence of thousands of drumlins 268 00:20:11,687 --> 00:20:14,884 pointing to ice that carved out the Great Lakes. 269 00:20:17,087 --> 00:20:19,367 It's a convincing case. 270 00:20:19,367 --> 00:20:20,967 But there's one problem. 271 00:20:20,967 --> 00:20:26,047 The Great Lakes cover an area five times the size of Switzerland. 272 00:20:26,047 --> 00:20:31,047 No glacier that size has ever been known to exist. 273 00:20:31,047 --> 00:20:34,807 Geologists were on the hunt for something even more powerful 274 00:20:34,807 --> 00:20:38,207 that could have created such huge destruction, 275 00:20:38,207 --> 00:20:42,519 a kind of prehistoric monster roaming over North America. 276 00:20:51,527 --> 00:20:53,927 Geologists are scouring the landscape, 277 00:20:53,927 --> 00:20:57,367 searching for evidence of a massive force. 278 00:20:57,367 --> 00:21:01,727 One that was capable of gouging out 12 trillion tons of solid rock, 279 00:21:01,727 --> 00:21:05,967 enough to create the Great Lakes of North America. 280 00:21:05,967 --> 00:21:13,442 It would be a body of ice so large that it would break every record, defy all logic. 281 00:21:14,807 --> 00:21:19,087 Geologist John Menzies hunts for evidence of this prehistoric monster 282 00:21:19,087 --> 00:21:21,476 just south of Niagara Falls. 283 00:21:22,687 --> 00:21:25,047 This whole area was covered by the ice 284 00:21:25,047 --> 00:21:28,167 with a tremendous torrent of sediment and water 285 00:21:28,167 --> 00:21:30,607 between the ice and this bedrock. 286 00:21:30,607 --> 00:21:32,287 And as this sediment moved across, 287 00:21:32,287 --> 00:21:37,998 it produced these superb striations and parallel scratches and marks. 288 00:21:39,807 --> 00:21:41,447 And there's another clue. 289 00:21:41,447 --> 00:21:45,645 Giant boulders of hard crystalline rock called granite. 290 00:21:49,407 --> 00:21:54,127 These hard, massive rocks sit in a flat, sandy landscape. 291 00:21:54,127 --> 00:21:55,719 They shouldn't be here. 292 00:21:57,207 --> 00:21:59,967 This is what we refer to as an erratic boulder. 293 00:21:59,967 --> 00:22:03,647 It's granite. It weighs some 80 to 100 tons. 294 00:22:03,647 --> 00:22:07,167 It would actually be frozen up into the base of the ice and then moved, 295 00:22:07,167 --> 00:22:10,447 kind of like a conveyor belt, along on the base of the ice 296 00:22:10,447 --> 00:22:12,487 down to this part of Southern Ontario, 297 00:22:12,487 --> 00:22:16,127 some 400 or 500 miles to the south from the Canadian Shield, 298 00:22:16,127 --> 00:22:19,607 where, with ice retreat and the eventual melting of the ice, 299 00:22:19,607 --> 00:22:22,838 this boulder has been left to sit, as we see it today. 300 00:22:24,447 --> 00:22:28,607 Erratic boulders moved hundreds of miles from northern Canada. 301 00:22:28,607 --> 00:22:31,807 Scratches on the bedrock and drumlin hills - 302 00:22:31,807 --> 00:22:34,196 the evidence is mounting. 303 00:22:35,207 --> 00:22:39,847 There was ice here once - lots of ice. 304 00:22:39,847 --> 00:22:42,607 Geologists map these glacial features together 305 00:22:42,607 --> 00:22:45,807 and an extraordinary picture emerges. 306 00:22:45,807 --> 00:22:50,727 Not of a glacier, but of a vast ice sheet one mile thick 307 00:22:50,727 --> 00:22:53,366 and over 2,000 miles long. 308 00:22:54,487 --> 00:22:56,487 It stretched all the way from the North Pole 309 00:22:56,487 --> 00:23:03,727 as far south as Chicago and New York, leaving a trail of destruction in its path. 310 00:23:03,727 --> 00:23:08,847 Here was a force powerful enough to create the Great Lakes. 311 00:23:08,847 --> 00:23:12,327 But even this vast sheet of ice couldn't have gouged out 312 00:23:12,327 --> 00:23:16,367 basins that are over 1,300 feet deep. 313 00:23:16,367 --> 00:23:20,076 It seemed the culprit wasn't working alone. 314 00:23:22,887 --> 00:23:26,447 At Scarborough Bluffs, just 100 feet from Lake Ontario, 315 00:23:26,447 --> 00:23:31,167 John Menzies has spotted an unusual deposit at the cliff face. 316 00:23:31,167 --> 00:23:35,407 Layers of rock provide him with a kind of geological time machine. 317 00:23:35,407 --> 00:23:39,719 The deeper he looks, the further back in time he goes. 318 00:23:40,927 --> 00:23:44,847 You could say that this is a journey through the last 60,000 years 319 00:23:44,847 --> 00:23:47,447 of geological history in this part of Canada. 320 00:23:47,447 --> 00:23:52,167 This lower formation is 65,000 to about 40,000 years ago. 321 00:23:52,167 --> 00:23:57,400 The next layer is between 25,000 and 10,000 years ago. 322 00:23:58,687 --> 00:24:03,807 Menzies focuses on the dark layers sandwiched between the light ones. 323 00:24:03,807 --> 00:24:06,647 What we have here is a sequence of sediments 324 00:24:06,647 --> 00:24:09,487 which illustrate the movements of the ice front, back and forward 325 00:24:09,487 --> 00:24:10,927 across this part of Canada. 326 00:24:10,927 --> 00:24:16,367 These dark layers mark the exact end of each Ice Age - 327 00:24:16,367 --> 00:24:21,727 formed of organic material when plants grew again at warmer temperatures. 328 00:24:21,727 --> 00:24:26,007 Here, John Menzies has proof that Ice Ages returned twice 329 00:24:26,007 --> 00:24:29,886 to this spot during their cycles of destruction. 330 00:24:32,807 --> 00:24:35,447 In fact, across the Great Lakes region, 331 00:24:35,447 --> 00:24:42,047 geologists have found evidence of up to ten separate enormous ice sheets. 332 00:24:42,047 --> 00:24:43,887 As each new ice sheet advanced, 333 00:24:43,887 --> 00:24:47,767 it carved the Great Lake basins deeper and wider, 334 00:24:47,767 --> 00:24:50,998 eventually forming the largest lake system in the world. 335 00:24:52,087 --> 00:24:55,887 But the ice left vast areas unscathed. 336 00:24:55,887 --> 00:24:59,007 It suggests there was some other force at play, 337 00:24:59,007 --> 00:25:02,127 something in the lakes' ancient past that set them apart 338 00:25:02,127 --> 00:25:03,567 from the surrounding landscape, 339 00:25:03,567 --> 00:25:08,004 making them particularly vulnerable to the ice sheets' attack. 340 00:25:09,407 --> 00:25:12,087 Menzies decided to dig deeper, 341 00:25:12,087 --> 00:25:16,319 down to the landscape that existed before the Ice Ages. 342 00:25:17,687 --> 00:25:20,007 Going back 2.5 million years, 343 00:25:20,007 --> 00:25:23,047 he found evidence of a chain of ancient rivers flowing 344 00:25:23,047 --> 00:25:26,447 across what's now the Great Lakes region. 345 00:25:26,447 --> 00:25:30,407 MENZIES: The pre-glacial topography of the Great Lakes basin mirrors 346 00:25:30,407 --> 00:25:34,639 the existing Great Lakes system and Great Lakes basin that we see today. 347 00:25:35,887 --> 00:25:38,247 The ancient rivers' pattern and flow 348 00:25:38,247 --> 00:25:42,247 exactly mirrored the shape and position of today's lakes. 349 00:25:42,247 --> 00:25:44,367 It's no coincidence. 350 00:25:44,367 --> 00:25:49,727 These rivers formed valleys that affected the way the ice sheets moved. 351 00:25:49,727 --> 00:25:52,847 MENZIES: As the ice sheet advanced to the south 352 00:25:52,847 --> 00:25:55,967 it would tend to follow the pre-glacial rivers, 353 00:25:55,967 --> 00:25:58,887 and so you get these really fast-moving zones of ice 354 00:25:58,887 --> 00:26:00,807 which create a tremendous amount of erosion 355 00:26:00,807 --> 00:26:03,446 in these pre-existing depressions. 356 00:26:04,607 --> 00:26:06,727 The ancient river valleys funnelled the ice sheets 357 00:26:06,727 --> 00:26:10,727 into fast-moving super ice floes. 358 00:26:10,727 --> 00:26:14,167 Menzies believes the coarse sediments the rivers left behind 359 00:26:14,167 --> 00:26:17,045 dramatically accelerated the ice sheets' flow. 360 00:26:18,287 --> 00:26:20,567 This sediment acts as a kind of lubricant, 361 00:26:20,567 --> 00:26:24,007 a bit like ball bearings underneath the ice. 362 00:26:24,007 --> 00:26:27,807 It would actually speed it up quite... quite appreciably. 363 00:26:27,807 --> 00:26:32,801 These fast streams of super ice were even more destructive to the landscape. 364 00:26:35,247 --> 00:26:37,767 The case is coming together. 365 00:26:37,767 --> 00:26:39,887 Drumlins clustered across the landscape 366 00:26:39,887 --> 00:26:43,038 testify to the vast ice sheets' brutal power. 367 00:26:44,567 --> 00:26:49,367 Dark layers of rock reveal the ice was a serial attacker, 368 00:26:49,367 --> 00:26:51,527 while a network of ancient rivers 369 00:26:51,527 --> 00:26:54,887 left some areas more vulnerable to these attacks, 370 00:26:54,887 --> 00:26:59,915 turning slow, lumbering ice into destructive, fast-moving super ice. 371 00:27:02,727 --> 00:27:05,767 These gouged out all the loose rock and sediment 372 00:27:05,767 --> 00:27:10,887 down to the hard dolostone layer, the rocky lake floor. 373 00:27:10,887 --> 00:27:16,007 The result, the basins of the Great Lakes. 374 00:27:16,007 --> 00:27:21,527 Case closed for three of the five lakes inside the rocky basin. 375 00:27:21,527 --> 00:27:28,407 But not for the other two. Lakes Ontario and Superior are outsiders. 376 00:27:28,407 --> 00:27:30,367 The theory doesn't fit. 377 00:27:30,367 --> 00:27:33,327 They're simply too deep. 378 00:27:33,327 --> 00:27:35,407 In an attempt to find out why, 379 00:27:35,407 --> 00:27:39,767 a daring underwater expedition would investigate Lake Superior, 380 00:27:39,767 --> 00:27:43,887 the largest, deepest, greatest lake of all. 381 00:27:43,887 --> 00:27:46,117 (RADIO CHATTER) MAN ON RADIO: Roger that. 382 00:27:50,247 --> 00:27:55,767 The hunt is on to discover what formed the Great Lakes of North America. 383 00:27:55,767 --> 00:28:00,007 Geologists have found compelling evidence that the central lakes lie 384 00:28:00,007 --> 00:28:05,007 in a vast rock-lined basin laid down by an ancient lagoon, 385 00:28:05,007 --> 00:28:08,204 gouged out by giant ice sheets. 386 00:28:10,327 --> 00:28:14,407 But when it comes to Lake Superior, the theory doesn't fit. 387 00:28:14,407 --> 00:28:19,247 The greatest of all the lakes, at over 1,300 feet deep, 388 00:28:19,247 --> 00:28:22,927 it could almost submerge the Empire State Building. 389 00:28:22,927 --> 00:28:25,725 And it lies outside the rocky basin. 390 00:28:26,727 --> 00:28:29,687 Lake Superior isn't just deeper than the other lakes, 391 00:28:29,687 --> 00:28:34,207 its floor is the lowest place on the North American continent. 392 00:28:34,207 --> 00:28:39,287 Over half of this mighty lake lies below sea level. 393 00:28:39,287 --> 00:28:41,926 The question is why? 394 00:28:42,967 --> 00:28:47,087 Canadian geologist Henry Halls was convinced the explanation 395 00:28:47,087 --> 00:28:50,567 could be found at the very bottom of the lake. 396 00:28:50,567 --> 00:28:54,487 The opportunity came up to study a very remote part of the lake, 397 00:28:54,487 --> 00:28:58,607 it's almost in the geometrical centre, and it's called the Superior Shoal. 398 00:28:58,607 --> 00:29:02,087 And people didn't know what the rocks were there 399 00:29:02,087 --> 00:29:04,078 and they didn't know why it was there. 400 00:29:06,487 --> 00:29:08,767 In the summer of 1987, 401 00:29:08,767 --> 00:29:13,247 Halls led an expedition to the lake's dark unexplored depths. 402 00:29:13,247 --> 00:29:17,527 HALLS: We went down. It took us about 15 minutes to go down. 403 00:29:17,527 --> 00:29:22,647 And it gets completely black, apart from the searchlights of the submersible. 404 00:29:22,647 --> 00:29:24,207 And when we reached the bottom, 405 00:29:24,207 --> 00:29:27,207 the... the pilot, he said, "This is very strange." 406 00:29:27,207 --> 00:29:30,167 He said, "I'm getting echo sounds coming back," 407 00:29:30,167 --> 00:29:32,158 he said, "more or less from all directions." 408 00:29:33,207 --> 00:29:35,927 MAN: It does look almost vertical. 409 00:29:35,927 --> 00:29:38,207 SECOND MAN: It is vertical. 410 00:29:38,207 --> 00:29:42,200 MAN: More than vertical, we've heard. In fact, it's hanging over us. 411 00:29:43,647 --> 00:29:47,847 Deep in the centre of the lake, on the border between Canada and America, 412 00:29:47,847 --> 00:29:51,687 Halls came across a strange rock formation. 413 00:29:51,687 --> 00:29:53,527 HALLS: The pilot, he said, 414 00:29:53,527 --> 00:29:57,047 "It seems that we were in some sort of a chimney," or something like this. 415 00:29:57,047 --> 00:29:59,197 He said, "I'm not sure what it is." 416 00:30:00,567 --> 00:30:02,767 Halls and his submersible 417 00:30:02,767 --> 00:30:07,167 were in a deep canyon 1,200 feet below the surface. 418 00:30:07,167 --> 00:30:11,763 Intrigued, he took an even closer look at the canyon walls. 419 00:30:13,407 --> 00:30:18,047 And as we climbed, I started to see striations like this. 420 00:30:18,047 --> 00:30:25,327 They were actual glacial striae on the sides of what presumably was a canyon. 421 00:30:25,327 --> 00:30:30,527 MAN: We are continuing to move up this vertical face. 422 00:30:30,527 --> 00:30:34,127 Halls had uncovered a vast canyon lined with striations 423 00:30:34,127 --> 00:30:37,967 or scratches from the glacier that had carved out the lake. 424 00:30:37,967 --> 00:30:41,327 But it was the type of rock that was the clue 425 00:30:41,327 --> 00:30:44,205 to Lake Superior's exceptional depth. 426 00:30:45,807 --> 00:30:51,847 He used the sub's robotic arms to take rock samples from the canyon walls. 427 00:30:51,847 --> 00:30:55,967 The canyon was made of dark basalt rocks. 428 00:30:55,967 --> 00:31:01,439 The discovery of this rock took the investigation in a surprising direction. 429 00:31:02,447 --> 00:31:07,123 Basalt could only have been formed by intense volcanic activity. 430 00:31:09,327 --> 00:31:16,167 Basalt is created when hot magma deep within the Earth wells up to the surface. 431 00:31:16,167 --> 00:31:21,047 A billion years ago, immense forces pulled the Earth's crust apart here, 432 00:31:21,047 --> 00:31:23,287 forming a rift valley. 433 00:31:23,287 --> 00:31:27,527 Hot magma seeped up through the cracks in the thin crust. 434 00:31:27,527 --> 00:31:32,367 As it cooled, it lined the valley with a layer of hard basalt. 435 00:31:32,367 --> 00:31:39,087 Then, over millions of years, the rift was filled with soft, sedimentary rocks. 436 00:31:39,087 --> 00:31:41,807 So there's a tremendous thickness of infill in that lake 437 00:31:41,807 --> 00:31:46,961 lying above those volcanic rocks, and all of this is relatively soft. 438 00:31:49,687 --> 00:31:53,687 Many geologists believe the exact same volcanic action 439 00:31:53,687 --> 00:31:58,007 accounts for the formation of the fifth and final lake. 440 00:31:58,007 --> 00:32:01,477 Ontario, on average, is the second deepest lake. 441 00:32:02,487 --> 00:32:04,767 A separate rift valley appeared here 442 00:32:04,767 --> 00:32:07,407 much later than the one under Lake Superior. 443 00:32:07,407 --> 00:32:11,607 The volcanic split in the landscape stretched as far as the ocean, 444 00:32:11,607 --> 00:32:15,043 creating Lake Ontario and the St Lawrence Seaway. 445 00:32:17,527 --> 00:32:20,727 Millions of years later, the mile-high ice sheet 446 00:32:20,727 --> 00:32:24,167 easily carved out the weakened rift valley structures 447 00:32:24,167 --> 00:32:27,762 under Lake Superior and Lake Ontario. 448 00:32:30,527 --> 00:32:36,207 The extraordinary story of how the Great Lakes were made is almost complete. 449 00:32:36,207 --> 00:32:39,567 Ice sheets repeatedly carved out soft rock 450 00:32:39,567 --> 00:32:42,286 down to the hard basins of the central lakes. 451 00:32:44,887 --> 00:32:49,007 And to the north, ice attacked billion-year-old rift valleys 452 00:32:49,007 --> 00:32:51,967 to make the deepest lake, Lake Superior. 453 00:32:51,967 --> 00:32:55,880 The same action was repeated at Lake Ontario. 454 00:32:56,927 --> 00:33:01,247 When the ice melted for the last time 14,000 years ago, 455 00:33:01,247 --> 00:33:04,047 it filled the lakes with freshwater. 456 00:33:04,047 --> 00:33:08,087 It sounds straightforward. But there's a problem. 457 00:33:08,087 --> 00:33:09,967 There's so much ice, 458 00:33:09,967 --> 00:33:14,047 the Great Lakes should be many times bigger than they are today. 459 00:33:14,047 --> 00:33:17,527 Just when geologists thought they'd solved the mystery 460 00:33:17,527 --> 00:33:21,047 of how the lakes were formed, a new puzzle emerges. 461 00:33:21,047 --> 00:33:23,561 Where did all the water go? 462 00:33:25,567 --> 00:33:29,527 Geologist John Menzies is investigating exactly what happened 463 00:33:29,527 --> 00:33:33,047 at the end of the last Ice Age, when a vast ice sheet, one mile thick 464 00:33:33,047 --> 00:33:37,643 and stretching to the North Pole, started to melt. 465 00:33:40,047 --> 00:33:44,327 He believes it was so large it should have created far bigger lakes 466 00:33:44,327 --> 00:33:46,687 than the ones we see today. 467 00:33:46,687 --> 00:33:50,566 He's looking for evidence of one of these prehistoric lakes. 468 00:33:51,807 --> 00:33:55,927 As the ice sheet melted, a vast freshwater lake appeared 469 00:33:55,927 --> 00:33:59,047 that geologists call Iroquois. 470 00:33:59,047 --> 00:34:03,407 Then, later, as Lake Iroquois dried up, it left beaches 471 00:34:03,407 --> 00:34:05,967 which can still be seen today. 472 00:34:07,767 --> 00:34:11,527 Menzies believes he can detect these ancient beaches 473 00:34:11,527 --> 00:34:15,486 in the gently sloping landscape surrounding Lake Ontario. 474 00:34:16,487 --> 00:34:20,807 As Menzies drives uphill, away from the present-day lake, 475 00:34:20,807 --> 00:34:26,487 he's travelling back in time across Lake Iroquois' ancient shores. 476 00:34:26,487 --> 00:34:29,087 We're crossing one shoreline after another. 477 00:34:29,087 --> 00:34:32,247 The reason we know they're shorelines is that they contain 478 00:34:32,247 --> 00:34:34,007 large zones of sand, 479 00:34:34,007 --> 00:34:39,327 beach sands and beach bars and spits, the oldest being about 12,000 years ago, 480 00:34:39,327 --> 00:34:43,639 the bottom shoreline being about 6,000 years ago. 481 00:34:44,927 --> 00:34:50,327 Getting to the top of the hill, 400 feet above the level of today's Lake Ontario, 482 00:34:50,327 --> 00:34:54,798 Menzies is standing on the ancient shore of the original lake. 483 00:34:55,967 --> 00:34:59,447 The present-day Lake Ontario is off there in the mist 484 00:34:59,447 --> 00:35:03,007 and we're sitting about 400 feet plus on this beach 485 00:35:03,007 --> 00:35:07,487 which is... was formed maybe 10,000, 11,000 years ago, 486 00:35:07,487 --> 00:35:11,196 and then the ultimate oldest beach is about 12,000 years ago. 487 00:35:13,567 --> 00:35:17,447 These ancient beaches, now buried under the surrounding landscape, 488 00:35:17,447 --> 00:35:20,519 are evidence of a colossal freshwater lake. 489 00:35:21,727 --> 00:35:24,607 We're looking at a vast amount of water, and when you think of the water, 490 00:35:24,607 --> 00:35:28,407 it stretched from here to beyond the present lake, way into New York State, 491 00:35:28,407 --> 00:35:33,487 beyond into Rochester, so it's a huge, enormous, inland sea. 492 00:35:33,487 --> 00:35:35,127 Despite their size, 493 00:35:35,127 --> 00:35:40,567 the Great Lakes today are just a small fraction of these vast prehistoric lakes. 494 00:35:40,567 --> 00:35:42,762 The water has vanished. 495 00:35:45,287 --> 00:35:48,643 Geologists want to know how they emptied. 496 00:35:51,447 --> 00:35:55,447 50 miles east of Toronto, at Indian River Canyon, 497 00:35:55,447 --> 00:35:58,644 Menzies picks up the trail of the missing water torrents. 498 00:35:59,807 --> 00:36:04,087 OK, what we have here is an enormous subglacial pothole, 499 00:36:04,087 --> 00:36:08,527 formed by subglacial meltwater exiting underneath the ice sheet, 500 00:36:08,527 --> 00:36:11,807 typically formed with a large roller ball which rolls around 501 00:36:11,807 --> 00:36:15,767 in these really torrential vortices. 502 00:36:15,767 --> 00:36:21,207 The meltwater is chock full of... of boulders and sediments, 503 00:36:21,207 --> 00:36:24,916 and in this instance it's drilled itself the whole way through. 504 00:36:26,607 --> 00:36:30,527 These potholes are evidence of a catastrophic flood, 505 00:36:30,527 --> 00:36:34,127 of huge volumes of water moving at high speed. 506 00:36:34,127 --> 00:36:37,447 This flood needed an escape route, 507 00:36:37,447 --> 00:36:40,807 and Menzies believes he's found the place. 508 00:36:40,807 --> 00:36:44,567 This would be an enormous torrent, possibly at least a couple of miles across 509 00:36:44,567 --> 00:36:47,407 and could easily have been two, three, four hundred feet deep, 510 00:36:47,407 --> 00:36:49,637 moving at an incredible velocity. 511 00:36:50,647 --> 00:36:52,887 Nearby, a steep gorge, 512 00:36:52,887 --> 00:36:57,247 yet more evidence of the floodwater's terrifying power. 513 00:36:57,247 --> 00:37:02,567 The stream that remains today couldn't have cut such a huge amount of rock. 514 00:37:02,567 --> 00:37:05,247 MENZIES: And what we've got left is what we call a misfit stream, 515 00:37:05,247 --> 00:37:09,327 which is the fairly small Indian River, and this, if you like, is the remnant 516 00:37:09,327 --> 00:37:11,921 of that enormous torrential flood. 517 00:37:17,567 --> 00:37:20,887 Geologists believe as the ice sheet retreated, 518 00:37:20,887 --> 00:37:24,167 it uncovered this ancient Indian River outlet, 519 00:37:24,167 --> 00:37:28,843 allowing vast amounts of meltwater to tear down towards the sea. 520 00:37:30,847 --> 00:37:33,087 Finally, 12,000 years ago, 521 00:37:33,087 --> 00:37:34,687 the ice retreated, 522 00:37:34,687 --> 00:37:36,767 freeing the St Lawrence Seaway, 523 00:37:36,767 --> 00:37:41,283 and allowing the lakes to settle into their present flow. 524 00:37:42,967 --> 00:37:45,807 The story of the Great Lakes is coming together. 525 00:37:45,807 --> 00:37:50,127 Ice sheets repeatedly ground out deep basins, 526 00:37:50,127 --> 00:37:53,807 digging out ancient weaknesses in the Earth's crust. 527 00:37:53,807 --> 00:37:58,087 Prehistoric beaches show that when the final ice sheet melted, 528 00:37:58,087 --> 00:38:03,407 the water flooded the basin to create vast superlakes like Iroquois. 529 00:38:03,407 --> 00:38:08,327 And as the ice finally retreated 12,000 years ago, 530 00:38:08,327 --> 00:38:13,924 the excess water drained away to leave the Great Lakes we know today. 531 00:38:16,287 --> 00:38:19,847 But even now, as we know how the Great Lakes were formed, 532 00:38:19,847 --> 00:38:22,007 they are still changing. 533 00:38:22,007 --> 00:38:27,161 And scientists predict one day the lakes might disappear forever. 534 00:38:34,887 --> 00:38:38,727 The Great Lakes evolved over a billion years. 535 00:38:38,727 --> 00:38:43,407 Today, they're a vital link between the cities bordering the lakes and the sea. 536 00:38:43,407 --> 00:38:47,167 They provide over 20 million people with drinking water 537 00:38:47,167 --> 00:38:50,125 and irrigate crops throughout the Midwest. 538 00:38:52,047 --> 00:38:57,567 But in the past few years, fears have grown about the Great Lakes' future. 539 00:38:57,567 --> 00:39:00,367 Water levels are falling. 540 00:39:00,367 --> 00:39:02,727 People who have worked the lakes for years 541 00:39:02,727 --> 00:39:05,527 believe they can already see a change. 542 00:39:05,527 --> 00:39:07,567 We noticed a drastic decrease 543 00:39:07,567 --> 00:39:10,567 in water levels right after the September long weekend, 544 00:39:10,567 --> 00:39:12,807 where the water in a week dropped a foot 545 00:39:12,807 --> 00:39:15,847 and, throughout the... the remaining of the fall, 546 00:39:15,847 --> 00:39:17,967 it went down about another two feet. 547 00:39:17,967 --> 00:39:22,927 And you can notice that by the pinker or the brighter coloured rock 548 00:39:22,927 --> 00:39:26,647 versus the rock that is typically exposed to the weather. 549 00:39:26,647 --> 00:39:29,047 And what we saw there was a clear example 550 00:39:29,047 --> 00:39:33,882 of how the water has dropped a good three to four feet. 551 00:39:37,567 --> 00:39:42,687 Many have been quick to blame global warming for the fall in lake levels. 552 00:39:42,687 --> 00:39:46,077 But geologists believe there is another force at work. 553 00:39:48,647 --> 00:39:51,727 The ice sheet that cut out the lakes was so heavy, 554 00:39:51,727 --> 00:39:55,247 it pushed down on the Earth's crust. 555 00:39:55,247 --> 00:40:00,727 Now the ice sheet has gone, the crust is bouncing back. 556 00:40:00,727 --> 00:40:05,087 Incredibly, 9,000 years since the end of the last Ice Age, 557 00:40:05,087 --> 00:40:08,487 the ground is still lifting. 558 00:40:08,487 --> 00:40:11,887 In the north, where the ice was thickest, land has risen 559 00:40:11,887 --> 00:40:16,278 by as much as 1,800 feet since the ice melted away. 560 00:40:18,887 --> 00:40:23,527 Toronto's famous CN Tower appears to be getting higher. 561 00:40:23,527 --> 00:40:28,447 As the crust bounces back, the land it's built on, beside Lake Ontario, 562 00:40:28,447 --> 00:40:31,687 rises nearly an inch each year. 563 00:40:31,687 --> 00:40:33,767 The CN Tower is part of the landmass here, 564 00:40:33,767 --> 00:40:35,847 so in fact, it's rising out of the land, 565 00:40:35,847 --> 00:40:38,725 in fact, the whole land surface is rising slowly. 566 00:40:40,927 --> 00:40:45,557 Lake Nipissing today is a small body of water to the north of Lake Huron. 567 00:40:46,567 --> 00:40:49,527 12,000 years ago, when the ice began to melt 568 00:40:49,527 --> 00:40:54,207 and Lake Nipissing first formed, it lay at sea level. 569 00:40:54,207 --> 00:40:56,767 MENZIES: Lake Nippising, an enormous lake there, again, 570 00:40:56,767 --> 00:41:00,007 as the land rebounds, so the lake eventually drained out, 571 00:41:00,007 --> 00:41:04,797 and the land rose slowly, so the land is now 400, 450 feet above sea level. 572 00:41:06,527 --> 00:41:09,407 Geologists call this crustal rebound 573 00:41:09,407 --> 00:41:11,407 and it dramatically affects the delicate balance 574 00:41:11,407 --> 00:41:16,047 of the network of small rivers that feed the lakes. 575 00:41:16,047 --> 00:41:17,687 This is an interesting example 576 00:41:17,687 --> 00:41:20,967 if we... if we think of trying to explain crustal rebound, 577 00:41:20,967 --> 00:41:24,727 and we look at this river as it flows out into the lake at the moment. 578 00:41:24,727 --> 00:41:27,007 If we have crustal rebound, the land comes back up, 579 00:41:27,007 --> 00:41:30,556 this river, in fact, will cease flowing out into this lake. 580 00:41:32,407 --> 00:41:34,247 It's this crustal rebound 581 00:41:34,247 --> 00:41:38,167 that's partly responsible for the fall in level of the lakes. 582 00:41:38,167 --> 00:41:41,927 And as the lakes empty, their weight decreases, 583 00:41:41,927 --> 00:41:45,327 allowing the crust to bounce up even faster. 584 00:41:45,327 --> 00:41:47,727 Lake levels will fall so the amount of water in the basin 585 00:41:47,727 --> 00:41:49,487 will in fact become less, 586 00:41:49,487 --> 00:41:53,607 and the effect of that will be to increase the rate of crustal rebound. 587 00:41:53,607 --> 00:41:57,680 The land will come up even faster than it's already doing and continues to do. 588 00:41:59,567 --> 00:42:03,487 As the crust rises, the lakes slowly empty. 589 00:42:03,487 --> 00:42:05,447 But in a few thousand years, 590 00:42:05,447 --> 00:42:09,087 the lakes will face another, even more dramatic, change. 591 00:42:09,087 --> 00:42:11,527 One of the exciting things about geology these days 592 00:42:11,527 --> 00:42:13,967 is not only looking at the past, but is looking into the future, 593 00:42:13,967 --> 00:42:16,687 in other words, having the ability to start to predict what might happen 594 00:42:16,687 --> 00:42:18,245 in the next several millennia. 595 00:42:21,207 --> 00:42:26,967 And the future is here at Niagara Falls, at least in geological terms. 596 00:42:26,967 --> 00:42:30,807 Every year, the falls are retreating three feet upriver. 597 00:42:30,807 --> 00:42:38,087 Only 12 miles and 21,000 years to go before they're back into Lake Erie. 598 00:42:38,087 --> 00:42:42,687 When that happens, everything will change, and fast. 599 00:42:42,687 --> 00:42:45,807 If the falls eroded all the way back to Lake Erie, 600 00:42:45,807 --> 00:42:47,887 which would take some thousands of years, 601 00:42:47,887 --> 00:42:50,527 the levels of all the upper Great Lakes, 602 00:42:50,527 --> 00:42:52,287 Huron, Superior and Michigan, 603 00:42:52,287 --> 00:42:56,599 would adjust to the lowered level of Lake Erie by dropping as well. 604 00:42:58,447 --> 00:43:02,527 The land between the falls and the lakes acts as a block. 605 00:43:02,527 --> 00:43:06,805 It's the Niagara Escarpment, topped with hard dolostone rock. 606 00:43:08,607 --> 00:43:11,607 When the falls cuts its way through this rock, 607 00:43:11,607 --> 00:43:13,887 the water levels in all the lakes to the west 608 00:43:13,887 --> 00:43:20,007 would drop by a staggering 180 feet, the height of Niagara Falls. 609 00:43:20,007 --> 00:43:23,204 Almost all of Lake Erie would drain away. 610 00:43:25,367 --> 00:43:28,687 One day the lakes may disappear altogether. 611 00:43:28,687 --> 00:43:33,807 But geologists also predict a new cycle of Ice Ages will begin again. 612 00:43:36,007 --> 00:43:40,167 So an Ice Age will begin, and this Ice Age would then cover, 613 00:43:40,167 --> 00:43:43,247 we would expect, at least 30% of the land's surface, 614 00:43:43,247 --> 00:43:45,522 as it did in the previous Ice Ages. 615 00:43:46,527 --> 00:43:48,287 And when the ice returns, 616 00:43:48,287 --> 00:43:53,367 the lake basins will be cut even deeper before filling again with water. 617 00:43:53,367 --> 00:43:59,087 The largest freshwater lake system in the world has had an extraordinary past. 618 00:43:59,087 --> 00:44:03,767 A basalt-lined canyon discovered at the bottom of Lake Superior 619 00:44:03,767 --> 00:44:10,327 shows that two great rifts opened up below Lakes Superior and Ontario. 620 00:44:10,327 --> 00:44:14,647 Fossilised sea sponges are evidence of an ancient briny sea 621 00:44:14,647 --> 00:44:20,727 that laid down the rocky bowl that holds Lakes Michigan, Erie and Huron. 622 00:44:20,727 --> 00:44:24,647 Thousands of drumlin hills are proof that vast ice sheets 623 00:44:24,647 --> 00:44:27,445 repeatedly scoured out the lake basins. 624 00:44:28,567 --> 00:44:33,047 Born just 12,000 years ago, the Great Lakes as we know them today 625 00:44:33,047 --> 00:44:34,927 are just a transient feature. 626 00:44:34,927 --> 00:44:38,527 They've only existed for the geological blink of an eye. 627 00:44:38,527 --> 00:44:41,807 But their story hasn't ended yet. 628 00:44:41,807 --> 00:44:44,407 The Great Lakes are changing and evolving - 629 00:44:44,407 --> 00:44:48,241 an endless process, like the Earth itself.