BT.JOHX.] PIERRE S BASIN. 439 



municates directly to the north with the great plain of the Snake Biver 

 Basin. It has a north-south extent of about 18 miles, and at its widest 

 part, opposite the north end of the Snake Biver Mountains, an east-west 

 breadth of about 11 miles. To the south it very gradually narrows into 

 the rounded upper end of the basin, while to the north it is defined by 

 a relatively low, inconspicuous upland barrier which gives to the in- 

 cluded area a general oval outline, embracing an area of, approximately, 

 110 square miles, with a mean elevation of 6,000 feet above the sea. 

 Pierre's Biver traverses the basin nearly centrally, its principal source 

 rising high up in the Teton plateau, and is joined near the head of the 

 basin by West Pass Creek, which latter affords a natural highway com- 

 municating with Jackson's Basin on the Upper Snake. The principal 

 tributaries from the east all originate in the Alpine basins in the heart 

 of the great range ; they are the Fox, Goodfellow's, West Teton Biver, 

 Bear and Leigh's Creeks, besides several other smaller streams which 

 collect the surface drainage of the western foreland. Numerous spring- 

 fed creeks and rivulets join the main stream on its left bank after a short 

 course from the low mountain border which defines the west side of the 

 basin. 



The basin-plain has a descent of 400 to 500 feet from its head at the 

 debouchure of West Teton Pass Creek to the point where the main 

 stream begins its canon course across the volcanic-capped low upland 

 barrier which extends across the north end of the basin below Leigh's 

 Creek. A wide strip of level alluvial land lies immediately adjacent 

 the stream, the damming of which by beavers has converted the larger 

 portion of the intervale into a swampy tract, covered with luxuriant 

 meadows, dense copses of willow and ponds, through which the channel 

 can with difficulty be traced. On either hand the plain gently rises in 

 broad stretches of terrace slopes, which abut against the low volcanic 

 border of the western foreland of the Teton Mountains, and also north 

 of Spring Point in the opposite border ; but south of the latter locality 

 these benches rest upon the sedimentary beds which here form the 

 mountain foot. Within the basin area, the larger streams have built up 

 cpiite extensive detiital deposits in the form of broad tongues extending 

 out into the basin from then debouchures, the channel following their 

 crest, from which the surface very perceptibly slopes in either direction 

 into shallow troughs which in some instances are occupied by the small 

 drainage channels that rise in the foreland. 



The above features are quite marked in the case of West Teton Biver 

 and Goodfellow's Creek, the foreland Bapid Creek flowing out into the 

 intervening depression ; and, as seen from the opposite heights in the 

 Pierre's Mountains, Bear Creek and Leigh's Creek are also seen to 

 emerge upon similiar raised tongues. But in the west-side streams this 

 peculiar feature is not well, if at all, developed, the streams having cut 

 their channels through the gentle terrace slope which is here more uni- 

 form in its surface contour, or has been less modified by the addition of 

 detrital materials brought down by the streams and superimposed on 

 the more ancient terrace formations. For there can be no question as 

 to the comparative modern origin of the raised tongues alluded to, and 

 which manifestly owe their formation to the local action of the streams. 

 This is further clearly shown in the fact that the shorter and less power- 

 ful water-courses have failed to make the same impression on the local 

 topography of the basin area, their volume being inadequate for the 

 transportation and distribution of so extensive quantities of detritus. 

 The later action of the streams has eroded their beds to a depth of 10. to 

 30 feet or more in the loose materials, front which at various stages, in 



