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A Journey to a New Land

Multimedia Library

Peter Locher

Department of Archaeology, Simon Fraser University

Sea Level Change During Glaciation

There was quite a variety of sea level changes that happened during the last glacial maximum and at the end of the ice age. In some areas, for example on the west coast of Vancouver Island, and in Haida Gwaii (or the Queen Charlotte Islands) sea levels were actually much lower than they are today. In other areas, like the Lower Mainland, sea levels were much higher than they are today. So, for example, the sea levels out at Haida Gwaii were up to 150 meters lower than they are today, while they are at the same time, up to 200 meters higher here in the inner coast on the Lower Mainland.

There are several reasons for this. One reason is that global sea levels were very different at that time because of the water that was locked in the ice sheets that covered all of Canada and northern Europe. The sea levels during the height of the glaciation were up to 130 meters lower across the whole globe and, when the ice started to melt, of course those sea levels would slowly rise again.

Another thing that happened during the ice age is isostatic depression or uplift, as we call it. This is caused by ice masses, which are very big in some areas, (in Vancouver they were two to two and a half kilometers thick). These ice masses basically press the crust of the earth and the mantle below the crust. The mantle is kind of a plastic viscous material that can be depressed so, in the area around here, the mantle was probably depressed two to three hundred meters. This means that, even though the global sea levels were lower, in this area the relative sea levels were higher because the whole Lower Mainland was depressed by two hundred to three hundred meters and it was just in front of the big ice field. So that explains why we have such a difference in sea levels on the coast here.

There is a third effect that also comes in here, that's called the forebulge effect. So again, because the mantle and crust underneath the continental ice is depressed, we have the material in the mantle kind of pushed outwards, towards the edge of the ice margin and the ice margin was of course just right here, in the Lower Mainland and the Vancouver Island area. So that means that while the crust and the mantle was depressed in the inner coast, all the material was pushed out towards the outer coast, so the outer coast was actually uplifted at the same time as the inner coast was depressed. That even increased the difference in sea levels between the outer coast and the inner coast. And, of course, after the ice retreated, those forces would balance themselves out so we would have the crust rising again, and we would have isostatic uplift on the inner coast or Lower Mainland here, and we would have isostatic falling of the area in the outer coast or a collapse of the forebulge, you could call it as well.