Friday, December 19, 2008

The northwestern slope of the Presidential Range with Mt. Jefferson (left) and Mt. Washington (right)

The last continental glacier, the Wisconsinan, beginning roughly 110,000 years ago moved very slowly over the White Mountains and south across most of New England on a south by south east course as far as Cape Cod, Martha’s Vineyard and Nantucket. The Cape and the islands, built mostly of sand, represent the terminal moraine of the Wisconsinan glacier. In the photo above you see the northwest 'flank' of the northern Presidentials, with Mts. Jefferson, Clay and Washington, as it looks today. You can visualize the impact of the Wisconsinan glacier at this point where the immense ice sheet met the rock. In this case the rock in the Presidential Range is a hard, metamorphic mica schist of the Littleton Formation. As the glacier “felt” the resistance of this rock it was able to push up over it lubricated with water melted out of the glacier by the enormous pressure of the ice crushing against the hard mica schist. I often try to picture the ice pushing slowly up this ridge and am filled with curiousity what it would have been like to watch it all in person. It’s very likely the White Mountains, in general, looked a lot different then today, in their 'youth', 500,000 to 1,000,000 years ago, and it would have been neat to see them then, too. If we only had a time machine.

Because of fluctuations in the climate and a gradual warming the glacier began to retreat through “ablation”, or, in other words, a process of melting, probably 15,000 years ago. This ablation process may have been interrupted periodically by short local and global shifts to colder climates during which the glacier may have re-advanced slightly or remained stationary. In other words the downwasting of the glacier was probably static, but it’s safe to say it had completely melted back from the White Mountains by roughly 11,000 years before the present, (BP)

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