Date of Award

1987

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Geology

Abstract

During the Pleistocene, the Wisconsinan ice sheet retreated from North Dakota leaving a veneer of glacial drift over the landscape. In the north-central area of the state, as the Souris Lobe retreated, Glacial Lake Souris formed and deposited mostly sand and silt over a large area of till. The purpose of this study was to characterize the till beneath Lake Souris sediments and to determine which descriptive parameters are most useful for till data analysis.

The area of study is in McHenry and western Pierce Counties and is contained entirely within the Glacial Lake Souris basin. The North Dakota Geological Survey drilling rig was used to drill holes, 36 of which penetrated till; a total of 109 samples were collected. Till was characterized on the basis of textural and lithologic analyses, in addition to visual hand-sample descriptions. The textural analyses provided sand, silt, and clay percentages, whereas the lithologic analysis provided percentages of fragments of certain rock types (crystalline, carbonate, sandstone and siltstone, shale, and lignite) in the very-coarse sand fraction of the till. The best parameters for characterizing the till were percent normalized silt (silt % / silt % + clay %) plotted against percent normalized crystalline ( xtal % / xtal % + carbonate %) and percent sand.

The lithologic data show an increase in percent crystalline rock fragments and a corresponding decrease in percent sandstone and percent carbonate fragments in a southeasterly direction. Percentages of sandstone, siltstone, and lignite fragments peak in the center of the lake basin and decrease toward the edges. The textural data show that percent clay changes little throughout the basin while silt values are high in the basin center and sand is high near the basin margins.

The lithologic data indicate that differential resistance of rocks resulted in relative increases in the resistant rock types in the down-glacier direction. With the data obtained from the study, it was possible to identify two tills within the study area. The first till, an older unit, was found in several holes that formed a northwest-to-southeast-trending linear band near the southwest margin of the basin. The second, a younger till, was found throughout the basin.

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