236 MAN AND THE GLACIAL PERIOD. 



Pocatello is situated just where this flood emerged from 

 the narrower valley of Marsh Creek and the Port Xeuf, 

 and spread itself out upon the broad plain of the Snake 

 River basin. The southern edge of the plain upon which 

 the city is built is a vast boulder-bed covered with a thin 

 stratum of sand and gravel. Everywhere, in sinking wells 

 and digging ditches on the vacant lots and in the streets 

 of the city, water- worn boulders of a great variety of mate- 

 rial and sometimes three or four feet in diameter are en- 

 countered. I w T as debarred from regarding this as a ter- 

 minal moraine, both by the water-worn character of the 

 boulders and by the absence of any sign of ice-action in the 

 surrounding mountains, and I was equally debarred from 

 attributing it to any ordinary stream of water, both by the 

 size of the boulders and the fact that for a mile or more 

 up the Port Neuf Valley there is an intervale, forty or fifty 

 feet below the surface at Pocatello, and occupying the 

 whole width of the valley, in which there is only gravel 

 and fine sand, through which the present Port Neuf pur- 

 sues a meandering course. The upper end of this short 

 intervale is bounded by the terminus of a basaltic stream 

 which had flowed down the valley and filled it to a consid- 

 erable depth, but had subsequently been much eroded by 

 violent water-action. 



In the light of Mr. Gilbert's discoveries, however, every- 

 thing is clear. The tremendous debacle which he has 

 brought within the range of scientific vision would nat- 

 urally produce just the condition of things which is so 

 puzzling at Pocatello. Coming down through the restrict- 

 ed channel with sufficient force to roll along boulders of 

 great size and to clear them all out from the upper portion 

 of the valley, the torrent would naturally deposit them 

 where the current was first checked, a mile below the 

 lava cliffs. The plunge of the water over these cliffs 

 would keep a short space below clear from boulders, and 

 the more moderate stream of subsequent times would fill 



