Permian to Present 5: Post-glacial evidence
In the last edition of ‘PEI: Permian to Present’ we looked at the Island during and just after the last ice age. Today, let’s examine a few examples of evidence of that period found in today’s PEI landscape!
Some of the most obvious remnants of the Wisconsin glaciation are glacial erratics (Photo 1). As we’ve seen in previous editions of this series, 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 – can commonly be found on PEI. (You can find metamorphic rocks, too. I’ll have more on those in the next edition). As massive glaciers moved across the landscape, they picked up rocks and debris from northern areas and carried them further south. As the ice sheets melted, these hitch-hikers were deposited in new areas, far from where they started. Many of the glacial erratics you can find around PEI today originated in the Miramichi Highlands of New Brunswick and Quebec’s Gaspé Peninsula.
Glacial movement is a brutal scrape rather than a smooth slide, and in some places on PEI you can find evidence of this in glacial striations (Photo 2). 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 sea ice, especially in locations like this one on the shore of Malpeque Bay. 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. At the site shown, the bank is eroding and revealing the striated rock with regular, parallel scratches just over three metres in length.
As we saw in the last edition of this series, Maritime coastlines changed as glaciers melted, land rebounded, and sea levels rose. We can see evidence of all this today. Photo 3 shows a drowned forest along the coast of Murray Harbour in eastern 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.
On the shores of Egmont Bay in western PEI, you can find ancient roots of Cat-tails (Typha latifolia, Photo 5). Cat-tails grow only in fresh water, so what are they doing on this salty coastline? They grew when this site was far inland, with a forest and freshwater wetland. The roots are surrounded by peat, which formed over thousands of years as rising sea levels and marine sediments drowned the forest, wetland, and associated plants.
In the next edition of Permian to Present, we’ll take a quick look at metamorphic rocks before we move on to what the pollen record tells us about changes in PEI’s plants over the last 10,000 years or so.