ALTYN FORMATION.
This is the oldest of the several formations and thus occupies a stratigraphic position at the base of the entire series. It is composed mainly of sandy dolomites (magnesian limestones) and limestones which weather to a light buff color. It outcrops all along the base of the eastern front of the Lewis Range and comprises the entire block of Chief Mountain. Because of its comparatively great resistance to weathering and erosion it usually forms a conspicuous ridge or terrace wherever it crosses a valley. In the Swiftcurrent Valley it forms the dam which holds in Swiftcurrent Lake and creates Swiftcurrent Falls. In Two Medicine Valley the highway crosses a similar terrace which gives rise also to Trick Falls. In the St. Mary Valley it creates the Narrows and forms the imposing wall in lee of which East Glacier Campground is located. The rock of this formation can best be examined on the ridge immediately east of Many Glacier Hotel (between hotel and parking lot) and above Swiftcurrent Falls. Its average thickness is about 2,300 feet.