pamp@bcsaic.UUCP (pam pincha) (08/19/85)
-------------------------------------------------------- Being a geologist (who is taking an indefinite sabbatical to pursue an insatiable AI infatuation) I thought I'd add my two cents worth here. -------------------------------------------------------- ran@ho95e.UUCP asks: >How did glaciers manage to dig these lakes so deep?????? >Other glacial lakes (Manitoba, Great Slave) are relatively >shallow. The Great Lakes were formed (carved out) by a series of continental ice sheets (at times 2 km thick) that have formed periodically over the North American continent in the last 1.5 million years (the last sheet formed was approximately 15,000 years ago). The Great Lakes are gouge features resulting from movement of these ice sheets, whereas the other lakes listed are more likely the result of outwash plain lakes (lakes formed from ice meltwaters, often very extensive (upwards to 100,000sq.mi. Lake Agassiz) when ice or moraine deposits dammed drainage routes). (That is, I think that Manitoba, and Great Slave are such, but all my references are still packed up. If they aren't, then they may be more recent features than the Great Lakes, which seem to have been gouged, then re-gouged, with each new ice sheet advance.) >I'm wondering if some plate tectonics might be involved here. the only >other inland areas below sea level I can think of had tectonic causes >(Death Valley,Dead Sea, Caspian Sea, Loch Ness). Might the Great Lakes >be the remnant of a failed rifting (which was much later covered by >ice)? The only way I can think that Plate tectonics could be tied into the Lake's formation is as follows: - One reason the lakes formed in that area is that there was previous topographical depression formed by previous drainage. -The previous drainge in the area may have been controlled by remanents of the mid-continent rift faulting that currently controls the Mississippi river drainage. Hence, features formed by the very early rifting (Late Paleozoic, I believe( I'll retract if my sources indicate otherwise)) may have controlled the placement of the Great Lakes. Mind you, this is mainly conjecture from what I remember about the area. I'm not really sure that the Mid-continent rift extends as far as the location of the Great Lakes. I do know that continental ice sheets have a great carving power. The carving out of the Great Lakes would not be at all difficult for them. (Oh, by the way, Loch Ness is formed along a fault line not a rifting zone like the other inland seas you mentioned.) Pam Pincha-Wagener (bcsaic!pamp) (Note: My comments are the gems of my geological expertise, Boeing has nothing to say on the matter -- or at least they haven't told me anything about it....)