DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 935 



the crisis came when the water began to flow northward 

 towards Snake Eiver. Once begun in such loose material, 

 the channel rapidly enlarged until soon a stream equal to 

 Niagara, and at times probably much larger, was pour- 

 ing 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 bar- 

 rier 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. 



The continuance of this flood was dependent upon the 

 amount of water to be discharged, which, as we have seen, 

 was that contained in an area of 20,000 square miles, with 

 a depth of 375 feet. A stream of the size of Niagara 

 would occupy about twenty-five years in the discharge of 

 such a mass, and this may fairly be taken as a measure of 

 the time through which it lasted. When the loose mate- 

 rial lying above the strata of limestone in Red Rock Pass 

 had been washed away, the lake then continued at that 

 level for an indefinite period, with an overflow regulated 

 by the annual precipitation of the drainage basin. This 

 stage of the lake, during which it occupied 13,000 square 

 miles and was 625 feet above its present level, is also 

 marked by an extensive and persistent shore-line all 

 around the basiu. But, finally, the balance again turned 

 when the evaporation exceeded the precipitation, and the 

 vast body of water has since dwindled to its present insig- 

 nificant dimensions. 



My own interest in this discovery of Mr. Gilbert is en- 

 hanced by the explanation it gives of a phenomenon in 

 the Snake River Valley which I was unable to solve when 

 on the ground in 1890. The present railroad town of 



