020

• Format: Text
• Language: unknown
• Published: State Historical Society of North Dakota
• Subjects: Ice Sheet
• Online Access: http://cdm16921.contentdm.oclc.org/cdm/ref/collection/ndsl-books/id/55470

GEOLOGY OF THE HILL COUNTRY 19 was.swept over or not by each succeeding ice-sheet may be uncertain, but it seems evident that each ice-sheet removed and carried farther south the drift deposited by its predecessor. There is recognizable only the Wisconsin driftsheet spread over the hill country and even that exists as a thin veneering to the far older Cretaceous clays and shales that bodily form the hill townships and which extends westward. Such bowlder clay as exists in the hill country can be distinguished from the older marine clay by the fact that the former has a yellowish tint and contains gravel, pebbles and bowlders of different sizes, largely small ones. Where the driftsheet is no more than two feet thick, the underlying Cretaceous clay is apt to have a whitish tint, resembling potter's clay. At a depth of a dozen or more feet the marine clay is 3pt to have a bluish tint and hence is commonly spoken of as the "blue clay." Usually, perhaps, the bowlder clay does not extend downward over a dozen feet from the surface and often much less. Shale rock sometimes occurs next below the drift- sheet. On the tops of some of the higher eminences there is no driftsheet at all, the whitish marine clay occurring beneath eight or ten inches of gravelly loam with an occasional bowlder on the surface. There appear to have been old drainage courses in the hill country that were filled by water-assorted drift—clay and sand -—during the advance movement of the Wisconsin ice-sheet. A part of a coulee being blocked up by the front of the glacier, but open for a while above, the depression would become filled with water and sediments and finally overridded and buried by the common driftsheet. A well was dug at the Arnold place 24 feet deep which did not reach to the bottom of the sedimentary beds of the kind mentioned. If formed 33.1 the way indicated, bowlders brought from the north by soinei. previous ice-sheet might lie at the bottom of such deposits. During the recession of the ice-sheet of the Wisconsin stage, which occupied a long period of time, the part of it covering Scanned with a Zeutschel Zeta book scanner at 300 dpi. Edited in Multi-page TIFF Editor.