During the Proterozoic Era of Earth history a long, narrow section of North America extending from the Arctic Ocean southward, probably as far as Arizona and southern California, slowly sank to form a large, shallow, sea-filled trough known as a geosyncline (Figure 1). Streams from adjacent lands carried muds and sands into the sea, at times almost completely filling it. Inasmuch as thousands of feet of sediments were deposited, the geosyncline must have continued to sink throughout the period of sedimentation. Eventually the muds were compacted into shales, or limestones if they contained a lot of lime, and the sands into sandstones. These are the rocks we now know as the Belt series. The surfaces of many of the sandstone layers are covered with ripple marks which could have been made only by wave and current action in shallow water. Mudcracks on many of the shale beds prove that at times the sediments, probably near the mouths of rivers were exposed to the air long enough to dry out. Great thicknesses of limestone and numerous fossils of calcareous algae, primitive marine plants, are evidences that the body of water was a sea. FIGURE 1. BELT GEOSYNCLINE Throughout the geologic past the appearance and disappearance of seas on the continents have been frequent events. In fact such changes are slowly taking place even today. Hudson Bay and the Baltic and North Seas are examples of shallow seas situated on the continents. The area around Hudson Bay is rising; as attested by the fact that some of the fish weirs constructed in water along the shore during the past several hundred years are now a considerable distance inland. We know also that our Atlantic coast has been subsiding for a number of years at an annual rate of about 0.02 feet. To be sure, these movements are slow, but if continued over a long period they might conceivably make some rather profound changes, even as the birth and death of the Belt sea. Within Glacier National Park the Belt series is divided on the basis of lithologic differences into six distinct formations. Because each has a characteristic color, these formations can easily be identified, often from distances of several miles. Usually two, sometimes three or four, of them comprise a single mountain, the oldest always at the mountain base and the youngest on the summit, this being the relative position in which they were deposited in the form of sediment. [Illustration: MUD CRACKS ON A LAYER OF THE APPEKUNNY FORMATION (PHOTO BY C. L. FENTON. OUR AMAZING EARTH. DOUBLEDAY AND CO.)] [Illustration: RIPPLE MARKS ON A LAYER OF THE SHEPARD FORMATION NEAR LOGAN PASS. THEIR ASSYMMETRICAL FORM INDICATES FORMATION BY CURRENTS IN SHALLOW WATER. (DYSON PHOTO)] The Belt Formations ALTYN FORMATION.