Permain to Present 4: Ice Age

For this fourth edition of ‘PEI: Permian to Present’, let’s jump ahead about 246.9 million years to the time of the last Ice Age. Our Island has experienced many ice ages over its long history, but the landscape we know and love today was shaped by the most recent (called the Wisconsin Glaciation) which was at its peak about 24,000 years ago.

Earth has gone through periods where winter snow accumulation exceeded summer melting. When this continues long enough – over many centuries – the snow gets compressed into huge sheets of ice. Those that covered PEI during the Wisconsin Glaciation would have been three to four kilometres (1.9 to 2.5 miles) thick!

These ice sheets 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.

The Wisconsin glaciers extended past Sable Island and the Scotian Shelf, and when they started to recede around 19,000 years ago these were the first areas to become ice-free. Ice retreated from PEI from east to west, and our Island was entirely ice-free by about 13,000 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 (Photo 1).

Atlantic Canada sea levels about 13,000 years ago. Map by John Shaw, Bedford Institute of Oceanography.

Over the next few thousand years, the land continued to rebound at a pace that exceeded rising sea levels caused by the melting glaciers, and PEI was connected to the rest of the Maritimes by a land bridge (Photo 2).

Sea levels about 9,000 years ago. Map by John Shaw, Bedford Institute of Oceanography.

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 by about 6,000 years ago (Photo 3).

Sea levels about 6,000 years ago. Map by John Shaw, Bedford Institute of Oceanography.

In the next edition of Permian to Present, I’ll show you some modern-day evidence of the Wisconsin Glaciation and its associated sea level changes.