One of the cool things about glaciers (sorry, couldn't resist) is that they were here yesterday and gone today. It takes a lot of sleuthing and imagination to postulate about glaciers. Yet, if you observe and know a little bit about how glaciers work, the evidence tells a fabulous story. That is what this post is about. Today we write about features observed from our travels, features that were created when continental ice sheets were here or were in the process of melting and receding from where we are today.
Welcome to whale country! Well, not exactly. Whales used to swim here around Quebec City when the Atlantic Ocean flooded this area all the way up the Saint Lawrence River Valley up the Champlain Sea in Vermont. How do we know that? Because there are whale fossils in the Champlain Valley. But we are getting ahead of ourselves.
Let's look at the big picture. All of this discussion is based on Four Billion Years And Counting, a fabulous resource on Canada's geology.
The white is where glacial geologists believe there was a continental ice sheet. The dates come from radiocarbon dating of debris deposited at the margins of the ice sheet.
So, at about 21,400 years before present (BP), the last ice sheet (there have been four of these during the so-called Pleistocene Period that started about 2.5 million years ago to about 11,000 years ago) was at its maximum.
Look at some things that may be familiar to you, such as the Great Lakes and mid-West or where Long Island is. These were all under thousands of feet of ice.
Now, jump ahead to 15,000 years ago, almost at the end of the Pleistocene Period. Ice is melting at a rapid rate, but the land is still depressed from the weight of all that ice. So, you can see a little green of land extending out into what will eventually become water. Even off of Nova Scotia and Newfoundland, there are areas of dry land. Evidence of this can be found in boreholes drilled on today's continental shelf.
Now let's jump ahead to 9,000 years ago.
Large glacial lakes exist in central Canada. Glacial Lake Ojibway is an example of this. The last picture, at 7,800 years ago, shows that the land is still depressed, especially around Hudson's Bay and the sea is more extensive than it is today.
Now let's zero in to our region.
Here, you can see that over time from 18,500 years ago forward, the southern margin of the Laurentide Ice Sheet recedes and exposes the Great Lakes and the Saint Lawrence River. At 13,500 years ago, though, look at the Champlain Sea. It existed through 9,500 years ago.
It gave the whales a chance to swim all the way up to Vermont's Champlain Valley because the land was still depressed by the weight of the ice sheet and the ice sheet in Quebec north of the Saint Lawrence would dam the glacial meltwaters and prevent them from draining to the Atlantic.
By 6,500 years ago, the landscape was very similar to today in this region, although the land was still rebounding.
Now, let's zoom in even further to the area right around Quebec City.
Notice that at 12,000 years ago, the immediate area is ice-free. Think of how this compares with the more regional pictures above for the same time period as 12,000 years ago.
At 11,000 years ago, the land continues to rebound. Quebec City is still underwater, but the highest parts of the island just downstream of Quebec, Ile d'Orleans, are beginning to be exposed.
In Vermont, the Champlain Sea is draining and if you are a whale, your chance to escape to the Atlantic has probably passed.
At 9,000 years ago, the higher parts of Quebec City are now exposed. The land continues to rebound faster than the ocean is rising from glacial meltwater.
Today, the area around Quebec City looks familiar to us. But the area is blessed with fertile soils deposited from the sea water that covered the area. The region grows bountiful crops and Quebec and Montreal are fortunate.
Over the next few days, we plan to drive Brio up towards Montreal and then south along the Richilieu to Lake Champlain. Sadly, we won't see any whales. But the climate will certainly be more temperate and that's a good thing.
Cheers,
Brio
You can also see that in front of the glacier there are glacial lakes and bodies of water that exist because the receding glacier prevents the meltwater from draining away and emptying the glacial lakes.
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