pread over thousands of square miles, overrunning the
desert valleys and making islands of the scattered mountain ranges.
It extended from north to south across Utah, into southern Idaho
and almost to the Arizona line, until this body of water, which
arose from so small beginnings, had become a veritable inland sea,
three hundred miles long, one hundred miles wide, and one thousand
feet deep.

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 tho