CARD #1 The Shape of the Land: Effects of Crustal Tilting When we look at a birds-eye view of the Great Lakes, it is easy to assume the lakes are all at a similar elevation, but viewed in this way, we see a very different perspective. Depth, distance and drainage are more apparent.
CARD #2 The Force of Ice Shaping the Landscape Toronto on Ice During the last ice age 100000 to 10000 years ago most of Canada was buried under an ice sheet more than a kilometre thick! This pushed the land down 400 m below its present level. As the climate warmed up, the ice started to melt and Glacial Lake Iroquois formed. It covered Lake Ontario and extended north of Lawrence Ave. Gradually, as the tremendous weight of the glacier decreased, the ground bounced back, (like memory foam does) Lake Iroquois drained and Lake Ontario shrank to its current size. Picture Ice as a Mammoth Bulldozer Try to imagine what a piece of ice this size might weigh. Remember it s a solid mass. Think of the largest piece of ice/snow you have held up, what size was it? Now, imagine an ice sheet the size of the glacier depicted in this photo (over 1 km above ground level) moving over the land. What kind of impact will it have? What do you think will happen to the land below that ice?
CARD #2 The Force of Ice Shaping the Landscape Here s how the skylines of Toronto, Chicago, Boston and Montreal compare against the ice sheets covering those locations during the last ice age. What is a Glacier? Glaciers are large, slow-moving rivers of ice, rock and sediment that move under the influence of gravity. They are formed by accumulation and compaction of falling snow. Glaciers grow and shrink in response to climate. How do Glaciers Form? Glaciers begin to form when snow remains in the same area yearround, where enough snow accumulates to transform into ice. Each year, new layers of snow bury and compress the previous layers. This compression forces the snow to re-crystallize, forming grains similar in size and shape to grains of sugar. Gradually the grains grow larger and the air pockets between the grains get smaller, causing the snow to slowly compact and increase in density. The topographic map to the right, comes from The Laurentide and Innuitian ice sheets during the Last Glacial Maximum by Dyke et al. and was used as a reference for the infographic.
CARD #3 Land left behind Impacts of Glaciation Glaciers are made up of more than just ice and snow. They contain water, rocks and sediments. When a glacier melts, all of the rock, sand and mud that it was carrying gets left behind. Geologists call this mixture of sediment, till. An esker is a long, winding ridge of sand and gravel caused when meltwater streams under the ice carried and left a trail of rock and gravel along its path. A drumlin, from the Irish word droimnín ("littlest ridge"), is a long oval-shaped hill created by glacial ice movement. Terminal (end) moraines are irregular ridges of glacial sediments that form at the margin or edge of the ice sheet. A moraine is an accumulation of sediment deposited at the edge and front of glaciers. Examples include rocks, sand, gravel, and clay. A kame is an irregularly shaped hill or mound composed of sand, gravel and till that accumulates in a depression on a retreating glacier, and is then deposited on the land surface with further melting of the glacier. A kettle (kettle hole, pothole) is a shallow, sediment-filled body of water formed by retreating glaciers or draining floodwaters.
CARD #1 Ice Age history How the Great Lakes were formed 14000 years ago Until now, Canada and northern USA are covered in ice up to 2 km thick. Then about 14000 years ago the climate warmed and the Great Lakes Basin began to take shape. Meltwaters filled the deep depressions left by retreating glaciers, creating the first ancestors of the Great Lakes 9000 years ago The glaciers withdrew from the Great Lakes Basin by about 9000 years ago. As the ice disappeared, the waters of Lake Iroquois dropped drastically. Drainage patterns changed too. Lake Algonquin which would form Lakes Michigan, Huron and Superior started flowing east through what is now Lake Nippising and the Ottawa Valley to the St. Lawrence River. 7000 years ago to Present The evolution of the Great Lakes is not only the result of retreating glaciers but also a phenomenon called crustal tilting. Freed from the weight of the ice, the land northeast of the Great Lakes Basin began to rise. This uplift eventually caused the upper lakes to empty down the St. Clair and Detroit Rivers, ensuring drainage of all the Great Lakes through the St. Lawrence Valley. Crustal tilting is still shaping the Great Lakes today.