Lake Bonneville, whose present representative is Great Salt Lake, is the subject of a recent monograph by Mr. G. K. Gilbert, from which it appears that this ancient body of water occupied 19,750 square miles--an area about ten times that of the present lake. At the time of its maximum extension its depth was 1,000 feet, while Great Salt Lake ranges only from fifteen to fifty feet in depth. The pass through which the discharge finally took place is at Red Rock, on the Utah and Northern Railroad, at the head of Cache Valley on the south and the lower part of Marsh Creek Valley on the north. During the long period preceding and accompanying the gradual rise of water in the Utah basin to the level of the highest terrace, Marsh Creek (the upper portion of which comes from the mountains on the east and turns at right angles) had been at work depositing a delta of loose material in the col which separates the two valleys. This deposit rested upon a stratum of limestone at the bottom of the pass, and covered it with sand, clay, and gravel to a depth of 375 feet. Thus, when the water was approaching its upper level, the only barrier to prevent its escape was this unstable accumulation of loose material upon top of the rock. It would have required, therefore, no prophet's eye to predict that the way was preparing for a tremendous _débâcle_. [Illustration: Fig. 61.--Map of the Quaternary Lakes. Bonneville and Lahontan (after Gilbert and Russell).] The critical point at length was reached. After remaining nearly at the elevation of the pass for a considerable period, during which the 1,000-foot shore-line was formed, the crisis came when the water began to flow northward towards Snake River. Once begun in such loose material, the channel rapidly enlarged until soon a stream equal to Niagara, and at times probably much larger, was pouring northward through the valley heretofore occupied by the insignificant rivulets of Marsh Creek and the Port Neuf. It is impossible to tell how rapidly the loose barrier wore away, but there is abundant evidence in the valley below that not only the present channel of the lower part of Marsh Creek, but the whole bottom of the valley for a mile or more in width, was for a considerable time covered by a rapid stream from ten to twenty feet in depth, and descending at the rate of thirteen feet to the mile.