The mountains of Glacier National Park are made up of many layers of limestone and other rocks formed from sediments deposited under water. The rocks show ripple marks which were made by waves when the rock material was soft sand and mud. Raindrop impressions and sun cracks show that the mud from time to time was exposed to rains and the drying action of the air. These facts indicate that the area now known as "Glacier National Park" was once covered by a shallow sea. At intervals muds were laid down which later became consolidated into rocks known as "shales" and "argillites." Limy or calcareous muds were changed into limestone. The geologist estimates that these depositions were made several hundred million years ago. In the plains area east of the mountains are other lime and mud formations. These are younger and softer than the rocks which make up the mountains but were undoubtedly formed under much the same conditions. These contain much higher forms of life, such as fish and shells. When originally laid down all these layers must have been nearly horizontal, just as they are deposited today in bodies of standing water all over the world. Then came a time when the sea slowly but permanently withdrew from the area by an uplift of the land, which since that time has been continuously above sea level. This uplift, one of the greatest in the history of the region, marks the beginning of a long period of erosion which has carved the mountains of Glacier National Park. The geologist observes that the rock layers are no longer in the horizontal position in which they were laid down. There are folds in the rocks and many breaks or faults cutting across the layers. Furthermore, the oldest rocks in the region are found to be resting on the younger rocks of the adjacent plains. One of the best examples of this is to be seen at Chief Mountain where the ancient limestone rests directly on the young shale below (fig. 1). The same relationship is visible in Cutbank, St. Mary, and Swiftcurrent Valleys. In these areas, however, the exact contact is not always so easy to locate principally because of the debris of weathered rocks that have buried them. What has happened? How did this peculiar relationship come about? The answers to these questions unravel one of the grandest stories in earth history. Forces deep in the earth slowly gathered energy until finally the stress became so great that the rocky crust began to move.