Friday, December 19, 2008

Jefferson Ravine Headwall with Goldthwait's schrund line and the Ravine downslope showing absence of terminal moraine

These two photos taken in 2007 show Jefferson Ravine, one of the nine cirques included by Goldthwait in his theory of local and continental glaciation in the White Mountains. The schrund (black) line is where Goldthwait's placed the upper limit of the alpine glacier that existed in Jefferson Ravine prior to the inundation by the Wisconsinan ice sheet. Goldthwait described the glacier in Jefferson Ravine as a tributary of the local glacier in Great Gulf and that also incorporated another small glacier that formed in a small cirque below Spinx Col. This compound glacier curved down and out to the lower valley in the back ground of this photo before curving again towards the North (left in the photo). The Goldthwaits estimated this glacier to be eight miles long at times; the longest of the Presidential Range glaciers.

The notable controversy in the discussion is whether the cirques were formed before the Wisconsinan epoch or afterwards. Billings and Goldthwait concluded that there was a major period of local glaciation before the continental ice inundated the White Mountains. Their conclusion is based on several observations including (1) the absence of terminal moraines. A more technical and much more controversial note (2) is the question of “drift’. Drift is a general term for the rock debris left by the glaciers and Goldthwait (and other geologists) carefully cataloged drift found in the cirques on the northern slope, in king’s Ravine for instance, and determined that it came from a few miles north of the mountains and was deposited there by the Wisconsinan ice sheet after local glaciation ceased. Goldthwait (1970) makes several more points in his argument for the theory that the local glaciers created the alpine cirques before the arrival of last continental ice sheet that you can read in his paper. I won’t include them here.