36 GEOLOGICAL SURVEY OF THE TERRITORIES. 



very large portion of the foot-hills and terraces, as well as the immediate 

 bottom of the river, is literally paved with rounded bowlders, so much 

 so as to render almost worthless, except for grazing, much land which 

 would otherwise be excellent for farming. Along the bottoms also are 

 old river-beds 50 to 100 feet above the present bed. These old channels 

 are covered thickly with the rounded bowlders, and walls of them are 

 piled up on either side. These are results of very modern date. They 

 must represent the latest period of the draining of the lake-basins. We 

 believe that the temperature was much lower than at present ; that the 

 surrounding mountains that form the drainage into these lakes were 

 covered with vast masses of snow and ice which, at certain seasons of 

 the year, became detached and floated down in the form of huge icebergs. 

 All these forces raa^- have been in action from the sources of the Yellow- 

 stone and Missouri down to the plains below, and thus the huge icebergs, 

 loaded with immense quantities of bowlders, floated over the valleys, 

 dropping their contents here and there, as we find them at the present 

 time. Most of the bowlders in this valley could have been moved along 

 by the action of swift torrents of water alone, but not even the strongest 

 mountain-torrent of which we have any knowledge now could have 

 moved great numbers of the huge granitic rocks which we find high njy 

 on the foot-hills or terraces, at least ten or fifteen miles from their original 

 position. No forces now in operation, even if we were to suppose that the 

 melting of the spring-snows would raise the river so as to overflow all 

 the lowlands, could have transported these bowlders. As it is, they 

 cover an area several miles in width, quite above the reach of the river- 

 waters at their highest stage. 



Another point I may allude to here again in this connection, and that 

 is the more modern outflow of basalt which is seen in several localities 

 in the valley. It is probable that the basalt spread all over the valley 

 at one time in the form of a cap, and that it has been swept away in the 

 process of erosion. On the west side, about two miles below Boteler's 

 ranch, there is an extensive remnant of this basalt cap, which has been 

 so smoot-hed by the passage of ice over it that the surface is glazed. 

 There are also numerous small grooves or scratches. The surface is 

 covered thickly with rounded granite bowlders, but one of them was 

 worthy c^ special note from its size, which measured 12 feet in height 

 and 20 feet in diameter. It is perfectly massive, composed of a coarse 

 aggregate of quartz, feldspar, with small bits of mica. The basalt has 

 a tendency to break into imperfect pentagonal columns. Underneath 

 it are 100 to 200 feet of what I have called Plio<iene deposits, but 

 they are composed very largely of rounded pebbles and bowlders, with 

 light-gray marly clay. On the east side of the Yellowstone, about two 

 miles above the ranch, the river has cut a vertical section through the 

 marly clays and sandstones 100 feet or more, with 50 to 80 feet of loose 

 drift bowlders and pebbles, the whole capped with 20 to 30 feet of basalt. 

 This outflow of igneous matter was among the latest events. ]N"early all 

 the lake-bafsins have this basaltic cap to a greater or less extent, and the 

 evidence indicates that the outflow was synchronous. In the Snake 

 Eiver Basin this cap covers an area of fifty to eighty miles in width and 

 several hundred miles in length, and quite large streams sink beneath 

 it and flow into Snake Eiver. The geological relations of this cap or bed 

 of basalt are about the same wherever it occurs, pointing to a common 

 cause as well as time. 



I believe that it occurred before the waters subsided, so that we may 

 trace a portion of its history at least. The lake-deposits are certainly of 

 very moderate date, at least as late, and perhaps later, than Pliocene. 



