Date of Award

1977

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Geology

Abstract

The term ''drumlin" is of Gaelic origin being derived from druim, a word for a mound or rounded hill, and has been used in glaciological literature since 1866. But with the increased study of subglacial ·shear features the terminology has become very inconsistent. The glacially formed longitudinal ridges found in northeastern North Dakota can be referred to as either drumlins or drumlinized ground moraine depending on their size, shape, composition, and interpreted origin.

Most of the drumlins observed in North Dakota are cored with glacial outwash deposits but exceptions do exist. Drumlins cored with Pierre shale occur north of Langdon, North Dakota, and till-cored drumlin ridges are present .south of Devils Lake, North Dakota. Longitudinal ridges that are generally less than 5 feet in height and .made up entirely of till are present in many locations in northeastern North Dakota.

Based on limited field observations, a study of drumlin literature, and an application of several principles of soil mechanics, a model for the formation of drumlins cored with sand and gravel has been developed. Internal pore-water pressure, effective stress (intergranular stress on the sediment), load pressure of the overlying ice, and permeability of the sediment are taken into consideration. Shear strength of the sediment is a function of effective stress. High pore-water pressure reduces the strength of the material.

If outwash from a previous ice advance contained channelized sand and gravel deposits (great permeability--rapid discharge capacity) surrounded by sediment with a large amount of silt and clay (slight permeability--slow discharge capacity) shear strength would be an important factor in erosion occurring during a second advance. A sand and gravel body with a greater effective stress would survive a shear stress that would erode the surrounding material. The sand-and-gravel-cored drumlin ridges could be the remnants of an advance where shearing forces eroded the surrounding sediment.

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