I’m going from notes here and I can’t seem to find this particular show on National Geographic’s website as part of its series Naked Science. So here it goes. It’s long but very interesting especially for Michiganders. On Sunday, April 5th, I viewed a series on Nat Geo about the origin and age of the Great Lakes. The show’s starting point was around 20,000 years ago when the Laurentide Ice Sheet began to melt and retreat. The show culminated with the current assessment that Michigan is tilting.
Since Michigan has the greatest telltale signs of this massive ice sheet—the Great Lakes, it was a good place to study its after affects. However, this was not the initial intention of the study. The study was simply trying to date the Great Lakes, but as research continued, the study shifted with new findings and the combination of 2 theories as to how the lakes were formed.
The study began with a focus on Niagra Falls where all central U.S. water floods over the edge at 150,000 gpm. This study revealed that the falls are retreating or moving back 1 ft. per year toward Lake Erie. It used to retreat 3-4 ft. just 100 years ago, but the introduction of hydroelectric power slowed that progress. The perpetual destruction of the falls is a giant timepiece. One would think that the weight of the water would harden the surface rock it flows over and it does. It’s called cap rock and is extremely hard, but there is weaker shale behind the cap rock and at the base where it is perpetually pummeled with 70 mph water flow. The base is honed back leaving a cantilever of rock at the top of the falls, which eventually breaks away and lines the river edge below. So Niagra Falls at some point in time will end up in the basin of Lake Erie.
The researchers couldn’t carbon date the rock left behind but they could carbon date the clam shells they found from Lake Erie that remained stuck in crevices as the falls regressed. So the further away from the falls, the older the shells with the oldest being 7 miles downriver. Carbon dating these shells puts Niagra Falls at 12,600 years old. Now onto dating the lakes.
The Laurentide Ice Sheet covered most of N. America and Canada and was a 1000 times bigger than our largest glacier. When it began to melt and retreat, it carved out Lakes Michigan and Erie first. The bedrock in Mohawk Bay in Lake Erie seems to support this theory. Core samples of sediment there showed that it ground away 1 inch of bedrock every 100 years. Ice streams in glaciers don’t freeze but flow and move 10 times faster than the glacier and grind rock 10 times faster. This fast stream could have carved out the lakes in as short a time as 10,000 years. Core samples coincide with this grinding theory because samples near the top are 1700 years old, the middle 7500 years old, and near the base of the lake are 9,000 years old. The rocks deposited along the shores of Mohawk Bay are both worn smooth by water running over them and jagged as newer hard cut pieces.
The early lakes were lifeless, cold, and harsh with a milky appearance that didn’t allow light to penetrate until the bedrock sediment settled. Once clear, the biggest difference in the lakes then and now was that the lakes were disjointed and the floodwaters of the retreating glacier ran south down the Mississippi to the Gulf. Researchers found 14,000 year old freshwater seashells in the Gulf of Mexico. What else could have dumped that much freshwater into the Gulf back then? One more catastrophic event must have happened that joined the lakes and created the 2000-mile long water system—the St. Lawrence Seaway.
At the bottom of Lake Ontario is a crater ½ mile wide. None of the other lakes have craters. The theory is that a comet may have caused this crater on first impact. Since comets are actually ice there would be little evidence left behind. But besides this crater, researchers did find grains of iridium along Lake Michigan and black dots of pure carbon compressed so tightly they formed millions of tiny diamonds called impact diamonds.
It appears that 12,900 years ago, a comet did strike the Great Lakes area. Wildfires from it quickly broke up the last of the retreating Laurentide glacier. Debris from trees and rocks, giant ice forms, and flood waters from the melting glacier stopped up the flow down the Mississippi backing up the lakes to connect them all and forge the St. Lawrence Seaway as an outlet.
Researchers recently found tree trunks still rooted and wood in Lake Huron in 40 ft. of water. The area was never surveyed before. It was a forest of cedar and pine carbon dated as 6,400 to 7,900 years old. Lake Huron was a great degree smaller than Lake Erie, which was believed to be much bigger than now and more turbulent because wild rice was found 5 miles inland from Erie. Rice needs well-oxygenated water to grow. This rice was 4,200 years old. This huge backwash effect caused Lake Huron to swell and swamp that forest and carve out the St. Claire River to Lake Erie. When the seaway was finally carved out, Erie’s shoreline retreated also. But why to the east?
Michigan is tilting. Researchers used GPS monitoring to measure whether the shed housing the GPS equipment was rising or falling. They found Michigan tilting higher in the north and lower in the south with the west part of the state rebounding more quickly than the east. Earth’s surface can be compressed and the Laurentide Glacier weighed upwards of 10 million billion tons. It depressed the land around the Great Lake ½ a mile over time. The land is recovering and the rebound is responsible for not only Michigan but also much of N. America to tilt in odd ways. Global warming is accelerating this rebound. The future of the Great Lakes may well be as turbulent as its past.
To make it easier to understand how global warming affects the rebound, think of cake batter in a pan in an uneven oven. The dry heat of the oven causes the shallow, less dense batter to rise while the other side—well, it’s just a lopsided cake, and we all know that water will seek the lowest point.