Giant glacial grooves both cut across and wrap around the north end of the Mission Range. Some of these grooves are straight, others crescentic, but all appear to be independent of stratigraphic units and of the gross structure of the range. They were seemingly localized by preexisting stream valleys whose original trends were determined by joints. The grooves are cut in bedrock, U-shaped in cross section, thickly veneered with till, and roughly parallel. Most range in length from 0.5 km to 3.5 km. Each groove maintains a uniform width, but widths range from about 50 in to 275 in. Depths of individual grooves vary widely, ranging from about 10 m to about 60 in. The grooves are probably contemporaneous in age, but their time of formation is uncertain; it seems to have been prior to late Pinedale time but after the pre-Bull Lake glaciations. Of the various interpretations offered to explain their origin, the one that appears to fit most of the facts suggests that those grooves at the northern tip of the mountains were carved by a lobe of the south-flowing Cordilleran ice sheet, whereas those farther south were likely formed by the westward deflection of a north-flowing Swan valley glacier.
