Then the great overthrust followed. Side-pressures of inconceivable power forced upward the western edge of this crack, including the entire crust from the Algonkian strata up, and thrust it over the eastern edge. During the overthrusting, which may have taken a million years, and during the millions of years since, the frosts have chiselled open and the rains have washed away all the overthrust strata, the accumulations of the geological ages from Algonkian times down, except only that one bottom layer. This alone remained for the three ice invasions of the Glacial Age to carve into the extraordinary area which is called to-day the Glacier National Park. The Lewis Overthrust, so called because it happened to the Lewis Range, is ten to fifteen miles wide. The eastern boundary of the park roughly defines its limit of progress. Its signs are plain to the eye taught to perceive them. The yellow mountains on the eastern edge near the gateway to Lake McDermott lie on top of the Blackfeet Indian Reservation, whose surface is many millions of years younger and quite different in coloring. Similarly, Chief Mountain, at the entrance of the Belly River Valley, owes much of its remarkable distinction to the incompatibility of its form and color with the prairie upon which it lies but out of which it seems to burst. The bottom of McDermott Falls at Many Glacier Hotel is plainly a younger rock than the colored Algonkian limestones which form its brink. Perhaps thousands of years after the overthrust was accomplished another tremendous faulting still further modified the landscape of to-day. The overthrust edge cracked lengthwise, this time west of the continental divide all the way from the Canadian line southward nearly to Marias Pass. The edge of the strata west of this crack sank perhaps many thousands of feet, leaving great precipices on the west side of the divide similar to those on the east side. There was this great difference, however, in what followed: the elongated gulf or ditch thus formed became filled with the deposits of later geologic periods. This whole process, which also was very slow in movement, is important in explaining the conformation and scenic peculiarities of the west side of the park, which, as the tourist sees it to-day, is remarkably different from those of the east side. Here, the great limestone ranges, glaciered, cirqued, and precipiced as on the east side, suddenly give place to broad, undulating plains which constitute practically the whole of the great west side from the base of the mountains on the east to the valley of the Flathead which forms the park's western boundary. These plains are grown thickly with splendid forests. Cross ranges, largely glacier-built, stretch west from the high mountains, subsiding rapidly; and between these ranges lie long winding lakes, forest-grown to their edges, which carry the western drainage of the continental divide through outlet streams into the Flathead.