Not until the Cretaceous period of Earth history, about 100 million years ago, did the geologic record again become clear. At that time a great thickness of mud and sand was deposited in the geosyncline burying deeply the ancient Belt and other rocks which had accumulated as sediment during the preceding several hundred million years. Life had made tremendous advances in this interval, and the abundance of fossils in Cretaceous rocks indicates that the sea swarmed with shelled creatures during that period. THE LEWIS OVERTHRUST. Toward the end of Cretaceous time tremendous crustal forces, principally from the west, were directed against the geosyncline with the result that its rocks were compressed and uplifted, converting the site of the former sea into a mountainous region. Similar activity took place throughout the length and breadth of the entire geosyncline, which resulted in the formation of the Rocky Mountain system stretching between Mexico and Alaska. A number of mountains were formed on other continents during this period. So widespread and tremendous was the deformation, especially in the present day Rocky Mountain region, that it is known as the Rocky Mountain, or Laramide (after the Laramie Range in Wyoming), revolution. Mountain-building forces continued for several million years in the Glacier Park area, finally squeezing the rocks into a great fold (anticline). Continued pressure from the west overturned the fold and put additional strain on the rock layers, eventually causing them to break along a great low-angle fault. The western limb of the fold, now a great slice of the crust, was driven upward and eastward over the eastern limb ultimately reversing the order of rock layers by placing older on top of younger ones (Figure 3). These younger layers are Cretaceous shales and sandstones underlying the plains immediately east of the mountains. The mountains themselves have been carved by streams and glaciers from the Belt formations comprising the upper block of older rock, that slice of the crust which has been moved more than 15 miles toward the east. The surface over which it was pushed is the Lewis overthrust. At the time this great break occurred the part of it now exposed in Glacier National Park was deeply buried. It was long after that when removal by erosion of overlying Belt rocks, possibly several thousand feet of them, finally exposed the fault. FIGURE 2. MAP OF WATERTON-GLACIER INTERNATIONAL PEACE PARK LEGEND 1. TRACE OF LEWIS OVERTHRUST 2. ALLUVIAL FANS 3. RANGE CRESTS 4. HIGHWAYS