In the southern part of Glacier National Park just north of Marias Pass, Debris Creek has cut a hole or “window” (known as a fenster by geologists) through the overthrust block (Figure 2). Thus a small area of Cretaceous rock completely surrounded by the Belt series lies within the mountains. This is the only such Cretaceous outcrop in the park, but like the well at Waterton, it serves as a reminder that the rocks of this period are everywhere present under the mountains, and their surface constitutes the “sliding board” over which the upper, more massive block of Belt rocks was pushed. And so we see that the mountains of Glacier National Park, unlike many of the world’s great ranges, have no roots, for they rest on a base of greatly different and much less resistant material, the Cretaceous shales. Presumably the Lewis overthrust and Cretaceous rocks beneath it would be penetrated by a well drilled anywhere within the mountains, although in the Livingstone Range the depth of such a well would be very great (Figure 3D). FIGURE 3. HISTORY OF LEWIS OVERTHRUST A HYPOTHETICAL SECTION NEAR THE CLOSE OF CRETACEOUS TIME. B PRESSURE FROM WEST CREATES LARGE FOLD AND OVERTURNS IT TOWARDS EAST. THE NEW MOUNTAINS ARE CUT DOWN BY EROSION. C CONTINUED PRESSURE BREAKS THE STRATA AND THE LEWIS OVERTHRUST IS FORMED. DOTTED LINE SHOWS APPROXIMATE PRESENT OUTLINE OF SURFACE D SECTION ACROSS THE PARK SHOWING RELATIONSHIPS OF ROCK FORMATIONS AND THE LEWIS OVERTHRUST (VERTICAL SCALE GREATLY EXAGGERATED) Erosion in the eastern part of the overthrust block, in addition to producing its crenulated edge, has left several isolated remnants (outliers) east of the main mass of the mountains. The best known of these is Chief Mountain situated near the northeast corner of the park several miles west of the Chief Mountain International Highway. It is a mass of Altyn limestone rising vertically on its east, south and north sides for a distance of 1,500 feet. The Lewis overthrust is well exposed all around its base. Two smaller pinnacles immediately to the west are similar outliers, and, like Chief Mountain, were once part of the main mass of the Lewis Range (Figure 3D and cover sketch). Divide Peak, at the west end of Hudson Bay Divide, is another outlier. It, too, is composed entirely of the Altyn formation.