The valley of Deer Creek is a deep, heavily timbered gulch heading at about 7,000 feet of altitude in the Bitter Root Range and extending about 7 miles to the Saint Regis River near Deborgia, Missoula County, Montana. Streams of ice flowing from the Chief, Diamond, Crystal, and other cirques, two of which contain typical moraine-dammed glacial lakelets, coalesced in the main valley into a glacier that during an earlier stage extended almost to the mouth. It ground the Belt quartzites and preglacial gravels into a ground moraine of light-colored clay, with boulders, cobbles, pebbles, and sand grains heterogeneously distributed through it; this is locally known as “white clay.”
After the ice had melted back to near the head of the valley, Deer Creek eroded a broad shallow channel in the surface of the white clay and floored it with stream gravel. Then in . . .