EVIDENCE FOR THE MILNOR BEACH, AN EARLY LAKE AGASSIZ SHORELINE IN SOUTHEAST NORTH DAKOTA

The earliest history of glacial Lake Agassiz in SE North Dakota is poorly understood. We report on weakly developed, discontinuous strandline features in an effort to reconstruct the lake's paleogeography. On the eastern side of the Cottonwood Slough are a series of discontinuous ridges that have been referred to as the Milnor Bars. These ridges are parallel to, and found 8m above the Herman beach and have been described as being 'wave cut and washed with silt', but little more data than that exists.

Topographic profiles across the ridges indicate an asymmetrical geometry, with a steeper northeast face. The base of the steeper northeast face (333 m) dips toward the lower Herman shoreline, and the profile closely resembles that of the Herman shoreline (325 m).

Using a vibracore and bucket auger methodology, it was determined that the ridges are composed of planar-laminar, medium grained sand of suspected lacustrine origin. The top of the ridges are capped with well-sorted sand and sampled by bucket auger only. The bedded sand from the vibracores is truncated on the northeast slope, indicating that the ridges are an erosional feature. Moreover, because the elevation at the base of each ridge is at the same elevation, they are interpreted to be strandline features. The well-sorted sand caps on the ridges are interpreted to be eolian in origin, similar to the sand caps with deflation hollows observed on the Herman beach ridges.

The restricted geographic distribution and discontinuous nature of the strandline ridges, indicates that the ice-margin was probably nearby and that this stage of Lake Agassiz was short-lived. We conclude that the Milnor bars represent the Milnor beach, and are erosional shoreline features from one of several possible ephemeral lake levels predating the Herman stage.