PROTEROZOIC Sea withdrew and region was eroded at end of era. Area covered by sea in which Belt sediments were deposited. Algae lived in sea. Intrusions (diorite sill and dikes) from flows (Purcell) of igneous material. ARCHEOZOIC 2,110,000,000 B.C. ? ERAS, PERIODS, AND DATES IN THIS CHART ARE IN ACCORDANCE WITH THOSE WHICH HAVE BEEN ADOPTED AS OFFICIAL BY THE NATIONAL PARK SERVICE. The Story Begins The most striking feature of the mountains—certainly the one which comes first to a visitor’s attention—is the color banding. No matter where one looks this feature greets his view. If he enters the park at the St. Mary Entrance, there ahead on the sides of Singleshot and East Flattop Mountains are white and purple bands. Should he enter first the Swiftcurrent Valley, he would soon note the banding in the mountains lying to his right and left, and finally culminating in the precipitous Garden Wall at the head of the valley. The visitor soon realizes that every mountain within the park is composed of rock layers of various colors. With very few exceptions these strata are of sedimentary origin; that is, they accumulated by depositions of muds and sands in a body of water and are now mainly limestones, shales, and sandstones. These sedimentary rocks all belong to a single large unit known as the Belt series, so named because of exposures in the Little Belt and Big Belt Mountains farther south in Montana. In Glacier National Park these rocks, which have a maximum thickness of more than 20,000 feet, are in the form of a large syncline (downfold), the east and west edges of which form the crests of the Lewis and Livingstone Ranges (Figure 3D). Throughout the large area of western Montana, northern Idaho, and southern British Columbia where Belt rocks occur, they are important mountain-makers. In addition to the ranges already mentioned they are the principal rocks in many others, including the Mission, Swan, and Flathead in the region south of Glacier Park; the Bitterroot and Coeur d’Alene between Idaho and Montana; and the Purcell in British Columbia. Further, rocks of similar age form the core of the Uinta Range in Utah and the lower section of the Grand Canyon in Arizona.