The shorelines of Glacial Lake Missoula in Montana, USA. These are nicely picked out by winter snowbanks. Shorelines or terraces like these are excellent indicators of ephemeral lakes impounded by ice margins. They may or may not have formed in a sequence, as lake levels fell from the highest to the lowest marked position.
Laminated or banded sediments left high and dry when Lake Missoula was finally drained. These are relatively course, made of "rock flour" -- a sign of turbulent water and very heavy sediment loads coming into it from the melting ice margin. Note also the dropstones which have dropped into the sediments on the lake bed as icebergs and ice floes melted on the surface of the lake.
Shorelines of Lake Bonneville -- another vast lake formed during the wastage of the ice sheets in the United States. The history of these lakes, and of the landforms associated with them, is now quite well known.
A detailed map from a paper by Krister Jansson of the glacial lake sequence in a part of Labrador-Ungava. Here the cold-based ice sheet was retreating broadly northwards, down the surface slope. So the situation would have been broadly similar to that of Glacial Lake Brynberian. Ponded water was effectively held up by the retreating ice margin. Lake deposits, washed surfaces, spillways and shorelines all go to build up a picture of how the ice edge retreated.
In seeking to discover whether Glacial Lake Brynberian really did exist, we really need to know whether there are glacial lake sediments within its proposed edges, whether there are shoreline traces (even very faint ones) like those in the illustrations above, and whether there are Spillways worthy of the name, by which the lake might have drained. At the moment I am not entirely convinced on any of those fronts. Work in progress......
Brian,
ReplyDeleteGreat post. Certainly such evidence is sufficient to prove the existence of a glacier lake. But is it necessary? That is the question.
My sense is the evidence left behind also depends on other geographical processes. If, for example, the draining of a lake is very gradual and the lake basin is solid bedrock would we still get the stepped terrace shorelines of Lake Missoula?
Kostas
The survival of shorelines like these depends on many factors -- including the length of time that a water level is maintained, the nature of the bedrock on the slope concerned, the amount of wave action (which in turn depends on the extent of the lake surface) and the number of months per year when the lake is free of surface ice. In some cases sediment supply on the shoreline is also important. I agree that if shoreline evidence is all we have to go on, we might decide that there never was a Lake Brynberian......
ReplyDeleteBrian,
ReplyDeleteFrom what you know of the landscape and its geology, what evidence would be reasonable to expect if a Lake Brynberian once existed? Would that evidence be different if the Lake was solidly frozen?
Kostas