GLACIOTECTONICS OF THE OKANOGAN LOBE, NORTH CENTRAL WASHINGTON STATE: IMPLICATIONS FOR THE ORIGIN OF THE WITHROW MORAINE AND THE LANDSCAPE AROUND UPPER MOSES COULEE

Glaciotectonic (GT) landforms (Aber and Ber, 2007), created by the Okanogan Lobe of the Cordilleran Ice Sheet in north central Washington State, are more abundant and extensive between the Columbia River to the north and the terminal Withrow Moraine to the south than previously reported (Hanson, 1970). Most named buttes and hills in the area, such as Chester Butte and the Wheeler Hills, are GT hills located leeward of the associated holes from which they originated.

Moses Coulee, part of the megaflooded Channeled Scablands of eastern Washington, originates in this landscape. GT landforms parallel the sides of uppermost Moses Coulee and cross its headwaters, complicating the puzzle of how floodwater got into the coulee.

The distinctive jointing and mechanical properties of Columbia River Basalt (CRB), the substrate, account for the GT hills having properties that differ from GT landforms made of soft sedimentary strata. A fine-grained sedimentary interbed between basalt layers was likely a pressurized aquifer/GT failure plane.

Angular basalt scree that mantles steep GT hillsides suggests interiors composed solely of CRB. Basalt-dominated glacial diamicton with granitic and metamorphic erratics covers other GT surfaces. “Haystack” rocks to 10 m diameter made of CRB entablature stand out on GT hill surfaces. GT may be the primary mechanism which excavated the haystack rocks across the area.

The Withrow Moraine, which has been described as atypical, consists of hills and ridges of basalt-dominated rock debris with haystack rocks on top, like the GT hills. Some previous studies have identified the morainal hills, and some of the GT hills, as kames. However, the GT hills and Withrow Moraine knobs do not appear to be fluvial deposits.

West of Moses Coulee, linear glacial-flow features (drumlins, flutes) track up-glacier from the Withrow Moraine to escarpments eroded into CRB. The similarity of morainal knobs to GT hills, and of the flowlines between knobs and basalt escarpments to those between GT hills and source areas, suggest the moraine formed by GT transfer from the escarpments.

Further study will document the full range and character of GT on this landscape, determine how the GT landforms relate to the glaciofluvial and glacial surge landforms that are present on the same landscape, and constrain how Moses Coulee originated.