Glaciation of north dakota, U.S.A.

Abstract

Publisher Summary This chapter discusses that the first synthesis of statewide glaciation in North Dakota was also by the U.S. Geological Survey. Their Preliminary Glacial Map of North Dakota showed the south-western comer of North Dakota as being non-glaciated, with increasingly younger glacial boundaries to the north and east. End moraines were delineated, as were ground moraine, outwash deposits, stagnation, or collapse moraine and former glacial lakes. This map showed that most till deposits formed as the result of both subglacial and marginal thrusting, or of collapse of debris-veneered stagnant and dead ice. Attempts have been made to interpret the mode and stages of glaciation in North Dakota. Each end moraine was initially interpreted as resulting from a significant readvance of glacier ice. Subsequently, most end moraines were interpreted as being the result of a slight readvance or period of stillstand of the ice margin during retreat of the ice sheet. The marginal shear masses have impeded further advance of the ice, thereby marking the position of the ice margin for some undefined period of time. The significance is that such ridges were not the result of climatic changes, as most true moraines are traditionally interpreted. The definition and interpretation of these transverse constructional ridges in North Dakota is still unresolved. The tendency is to separate distinct thrust ridges from the more continuous end moraines.