During investigations for a damsite on the Sand River, Alberta, an 80 m thick silt deposit in a valley some 500 m across was encountered. The most distinctive feature of the deposit is the uniformity of the silt, with thicknesses of up to 50 m occurring as a single layer uninterrupted by seams of significantly different material. The silt unit is believed to be the result of very uniform, prolonged depositional conditions in a subglacial river. The silt consists largely of silica grains between 10 and 60 μm, and although it contains clay, in some aspects its behaviour is close to that of sand.Evidence suggests that the present valley is a tunnel valley formed by meltwaters flowing through the underside of a glacier. The implications for the engineering geology of the damsite investigated are discussed and some geotechnical properties of this unusual silt deposit are presented, particularly as they affect settlement prediction.Key words: tunnel valley, glacial silt, earth dam, settlement, airphoto interpretation.