Movement along this fault was slow—so slow that had people been present at the time they probably would not have been aware that anything of an unusual nature was occurring. Occasionally along many large faults, however, there is sudden movement of small magnitude, usually not more than a few inches, but strong enough to vibrate the crust. These vibrations are earthquakes, and their frequent occurrence in California and elsewhere along the Pacific coast indicates the presence of numerous active faults. Their occurrence also in the northern Rockies, as at Helena, Montana in 1935 and 1936, attests to the fact that some of the faults here are still active. The Lewis overthrust comes to the surface at the base of the Altyn formation along the entire precipitous east front of the Lewis Range and can be traced nearly 100 miles northward into Canada and for almost an equal distance south of the park. The section lying within the park is tilted very gently toward the southwest, the angle of dip seldom exceeding ten degrees. In some places it is practically horizontal. For this reason the lower courses of all the largest, and some of the small, valleys on the east side of the Lewis Range have been cut entirely through the upper block (overthrust) of Belt rocks down into the weak Cretaceous shales underneath. This causes the trace of the overthrust to be very sinuous and also accounts for the deep indentations in the mountain front formed by Swiftcurrent, St. Mary, Two Medicine, and other valleys. The floors in the lower courses of these valleys, because they lie below the level of the thrust surface, are composed of Cretaceous shales. In most places these rocks are covered by glacial moraine, but they are exposed along the highway from Babb into the Swiftcurrent Valley, especially along the shore of Sherburne Reservoir and near the entrance station. Because these shales readily disintegrate when exposed to the atmosphere they give rise to slumps and landslides which, although of small proportions, cause a great deal of damage to the highway, sections of which must be rebuilt annually. At most damaged spots along the route the shales appear as a dark mud or clay in the roadcuts. The bumpy topography of the whole slope lying north of the road has been formed by innumerable such small landslides. A deep well located near Cameron Falls in Waterton Townsite (Waterton Lakes National Park) about one mile west of the edge of the mountains passes through 1,500 feet of Belt rocks and then penetrates the Lewis overthrust and the Cretaceous shales beneath.