By the time the lake had covered an area of twenty thousand square miles the lowest point in the rim of the basin was reached and the overflow began. No map will tell you where the outlet was, for no river exists there now. If you could explore the shore lines of this ancient lake, which has been called Bonneville after the noted trapper, you would find two low spots in the mountains which hem the waters in, one upon the south, facing the Colorado River, the other on the north toward the Snake River. The one on the north happened to be a little lower, so that the break occurred there. First as a little, trickling stream, then as a mighty, surging river, the water poured northward down the valley of a small stream, widening and deepening it until, passing the spot where now the town of Pocatello stands, it joined the Snake River. This old outlet is now known as Red Rock Pass, and it forms an easy route for the Oregon Short Line from Salt Lake City to the plains of southern Idaho. The old river-bed is marked by marshes and fertile farms. With an outlet established, Lake Bonneville could rise no higher, and its waves began the formation of a well-defined terrace or beach, just as waves are sure to do along every shore. The level of the water could not remain permanently at the same height, for the rocks at the outlet were being worn away by the large volume of water which flowed over them. In the course of years the level of the lake was lowered four hundred feet. The sinking was not uniform, but took place by stages, while at each period of rest the waves made a new beach line. The lake during all this time must have been a beautiful sheet of fresh water filled with fish. Its shores, also, must have been much richer in vegetation than they are now. [Illustration: FIG. 54.--RED ROCK PASS, SOUTHERN IDAHO Outlet of Lake Bonneville] The water remained for a long time at the level of four hundred feet below its highest stage. This fact is evident from the width of the wave-cut terrace, which is the most prominent of all those that mark the old levels along the sides of the mountains. Finally, for some reason the climate began to change, the streams supplied less water to the lake, and the evaporation from its surface became greater because the air was drier. As a result the lake was lowered to such an extent that it lost its outlet. The mighty river flowing down through Red Rock Canon grew smaller and at last dried up altogether. In this manner the lake was again cut off from the ocean, as it had been during its earlier history. The waters still continued to recede, but not at a uniform rate. During periods of greater rain its level remained stationary, so that the waves added new terraces to those already formed.