PEI has experienced many ice ages over its long history, but the landscape we know and love today was shaped by the most recent – the Wisconsin Glaciation – which was at its peak about 24,000 years ago. Just like the other chapters in our natural history, you can read this story in our landscape.
Earth has gone through periods where winter snow accumulation exceeded summer melting. When this continues over many centuries, the snow gets compressed into huge sheets of ice. During the Wisconsin Glaciation, the ice sheets covering PEI were an incredible three to four kilometres (1.9 to 2.5 miles) thick!
Glaciers not only advance as cooler temperatures extend further south, but they can also move. Once the ice gets to be about 60 metres (roughly 200 feet) thick, its weight causes the base to liquify. This water serves as a lubricant, allowing the glacier to move across the landscape at rates ranging from a few metres to as much as 100 metres (more than 300 feet) per year. It’s a brutal scrape rather than a smooth slide, and evidence of glacial advance and retreat can be found across PEI.
One example is glacial striations (Photo 1). These are scratches in our native sandstone, made when debris in the base of the glacier scraped across it. It can sometimes be hard to tell glacial striations from marks made by annual sea ice, but characteristics to look for are scratches that are straight, parallel, more than a metre long, regular in width, length and depth, and that run in only one or two directions. The site shown is on Malpeque Bay, where the bank is eroding and revealing striated rock with regular, parallel scratches just over three metres in length.
Ice retreated from PEI from east to west, and our Island was entirely ice-free by about 12,500 years ago. The immense weight of the glaciers had pushed the land down, much like what happens when you sit on a mattress. Once that weight was removed, it took some time for the land to rebound (think of it a bit like memory foam). At this time, western PEI was around 45 metres (about 150 feet) lower than it is today and under water.
We can read the story of melting glaciers and rebounding land at sites like this one in West Prince (Photo 2). The lowest level is prehistoric sandstone, but above this is a much more recent layer of rounded cobbles tightly packed into sediment. As glaciers melted, the resulting rivers of meltwater carried with them rocks and sediment that had been picked up by the ice. This layer was created by that meltwater.
The top layer is fine sand, evidence of a raised beach. This layer was formed by the same coastal processes that build our beaches today. As the land rebounded, it carried the beach with it; what is above my head in this photo would have been below my feet thousands of years ago.
The Island rebounded at a pace exceeding the rising sea levels caused by melting glaciers and, for a time, PEI was connected to the rest of the Maritimes by a land bridge. As glaciers continued to melt, sea level rose by some 70 metres (230 feet), eventually flooding the land bridge, creating the Northumberland Strait, and making PEI an island around 6,000 years ago.
There is evidence of all this sea level change as well. Photo 3 shows one of many drowned forests around the coast of PEI. That’s not driftwood on the beach, as a closer look reveals (Photo 4). It’s the rooted remains of trees that once grew there when sea level was much lower, and this was an inland forest rather than a coastal beach.
Of course, some of the most obvious remnants of the Wisconsin glaciation are glacial erratics (Photo 5). As I’ve described in recent posts, the Island’s native rocks are sedimentary, the only exception being a sill of igneous rock in Malpeque Bay. But other igneous rocks – such as the granite boulders shown here – are scattered around PEI. They were carried from New Brunswick and Quebec by glaciers and deposited here when the ice melted.
Glacial erratics and other evidence of the last ice age are all parts of PEI untamed!
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