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. APPEKUNNY FORMATION. Lying on top of the Altyn are 3,000 or more feet of prevailing greenish shales and argillites[2] comprising the Appekunny formation. Slabs of these rocks, because of their great hardness, have been used as flagstones in the walks at the Many Glacier Ranger Station and adjacent Park Service residential area. Mud cracks and ripple marks are common. The formation is prominent on the side of Singleshot Mountain near the St. Mary entrance to the park, and everywhere immediately overlying the lighter-hued Altyn along the east edge of the Lewis Range where, especially when seen from a distance, it appears to have a purplish color. It also outcrops along the western base of the Livingstone Range (Figure 3D), but such exposures are as a rule obscured by a cover of dense forest. Accessible outcrops can readily be examined along Going-to-the-Sun Highway for several miles east of Sun Point and near McDonald Falls, and also along the lower part of the Grinnell Glacier trail. GRINNELL FORMATION.