Glacial Growth and Retreat on the Isles of Scilly
I have been looking again at this rather interesting animation showing how the last glacial invasion of the nothern Isles of Scilly supposedly happened. The intentions of the CADARN learning portal were obviously very worthy, but I'm surprised it was not checked for reliability before being put on on YouTube.
I'm intrigued by the idea that it can keep on snowing for centuries or millennia without any of the snow accumulating on the frozen ground surface. It's all very dramatic, the way that the distant approaching glacier front gets closer and closer, and inexorably overwhelms the land, bulldozing a mighty frontal moraine along as it approaches. Wild fantasy, for the most part. In glacial troughs this may indeed be what happens, although this is not the way in which most terminal moraines are formed.
The Bulldozer Fantasy
Here, on the front edge of the Irish Sea Ice Stream, with no constraining topography, the actual course of events will have been wildly different. The onset of glaciation would have been marked not by a bulldozing ice front pushing across the landscape but by a gradual transformation of seasonal snow-cover to a perennial snow-cover, with snowpatches and snowfields gradually coalescing and thickening.
So almost always in situ snow is converted to firn and then to glacier ice, thickening rather than advancing until at last the ice starts to move, pressurised by the thickening and growing glacier from the north. Then material starts to move, with the formation of till, the transport of erratics and the creation of moraine banks at the ice edge.
This is the story that is told by the sequence of deposits on the northern edge of the Scilly Archipelago -- and this is what happened at least twice during the passage of the Quaternary Ice Age.
Here is a reminder of a relevant post:
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