Although the Lewis overthrust is exposed in a great number of places very few of these are easily accessible, and at only one does a trail provide a close approach to the actual contact between Belt and Cretaceous rocks. The latter site lies along Roes Creek only a few hundred yards from East Glacier Campground. Before reaching the fault at the base of a high cliff of Altyn limestone, the trail crosses several outcrops of Cretaceous sandstone replete with fossil pelecypods (clams) and gastropods (snails). The fault surface is covered by loose rock where the trail crosses it, but on the opposite side of the stream a zone of crushed Altyn limestone and Cretaceous shale is visible. From U. S. Highway No. 2 just east of Marias Pass an excellent distant view of the thrust may be obtained. About three miles to the north it appears as a nearly horizontal line high on the side of Summit Mountain. Above it is a vertical cliff in which white Altyn and red Grinnell are prominent, and below is a gentler slope composed of gray-brown Cretaceous shale. Cretaceous rocks with relatively low resistance to Earth stresses, were strongly crumpled and folded during the period of overthrusting. The folded zone extends several miles eastward from the mountains (Figure 3 D), and may be seen to good advantage along Blackfeet Highway on the north side of Two Medicine Ridge, where a series of thin shales and sandstones has been squeezed into anticlines and synclines. It is because of the Lewis overthrust that there are no significant foothills on the east side of the Lewis Range. The fault has brought into direct contact the massive and resistant Belt rocks which stand up as mountains, and the relatively weak shales of the plains which are carved into subdued landscape features by erosion. [Illustration: LEWIS OVERTHRUST AT BASE OF MT. WYNN SEEN FROM HIGHWAY EAST OF MANY GLACIER HOTEL. OVERTHRUST LIES AT BASE OF CLIFF. CRETACEOUS ROCKS OUTCROP ON GENTLE SLOPE BELOW THRUST. (DYSON PHOTO)] After the Lewis overthrust had taken place, and probably following a period of erosion, the western part of the block broke along a vertical fault and sank several thousand feet. For a short period of time a lake, in which clay was deposited, covered the floor of this depressed area. The present valley of the North Fork of the Flathead River lies on this downfaulted block (Figure 3D), and the western boundary of the Livingstone Range marks the trace of the fault. Because the fault is of the high-angle variety the front of this range is much straighter than that of the Lewis Range which is formed by the notched eastern edge of the relatively thin overthrust block. The Belton Hills and Apgar Mountains near the park’s west entrance are isolated blocks separated from the Livingstone Range by normal faults probably dating from the time the North Fork Valley subsided. The Effect of the Ice Age