240 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



sas Valley the recent Tertiary beds run up to and overlap the margins 

 of the mountains. They are composed mostly of fine sands, arenaceous 

 clays, and pudding-stones, cream colored arenaceous clays, and rusty 

 yellow marls, fine sand predomiuating. These beds weather into pecu- 

 liar architectural forms, sooiewhat like the ' Bad Lands of Dakota.' In- 

 deed they are very nearly the same as the Santa Fe marls, and were 

 doubtless contemporaneous, aud dip at the same angle, three to five 

 degrees a little west of north. The tops of the hills have all been planed 

 down as if smoothed by a roller. 1 have called this group the Arkansas 

 Marls." 



Above this valley the river is in a canon, or rather a canon-like valley, 

 until we get above Twin Lakes, when we have another valley reaching 

 almost to the head of the stream. It is about sixteen to twenty miles 

 in length, and about ten miles in width at the lower end. The whole 

 valley, as far as could be determined, is underlaid by granite. At the 

 lower end, back of Weston's ranch, in the terraces, there are modern 

 Tertiary deposits, soft conglomerate sandstones. The whole valley is 

 terraced, especially on the eastern side. These terraces are covered 

 with drift, no under-lying beds being exposed until we get to the lower 

 end of the valley. On almost all the streams coming into the Arkansas 

 from the w^est above the lower valley we have moral nal benches. Be- 

 tween the two broad valleys mentioned above, the river flows through 

 a monoclinal rift in the granites and schists, the eastern side of the 

 canon remaining intact, while the west is somewhat broken down by the 

 streams coming from the mountain on that side. The canon is plainly 

 one of erosion, and in some places is from 1,000 to 1,500 feet in 

 depth. The bed of the stream is strewn with enormous boulders, in some 

 of which there are pot-holes. The river while in the caiion falls at the 

 rate of about 60 feet to the mile. The first large creek coming in from 

 the mountains above the mouth of the canon is Pine Creek. There are 

 along this creek well-defined lateral moraines. I did not, however, go up 

 the creek, but waited until I reached La Plata, which is much larger, 

 being about fourteen miles in length. The valley is about half a mile 

 in width, with beautiful meadows in the lower portion through which 

 the stream gracefully winds. On either side bordering this meadow-like 

 valley are two morainal benches extending from the edge of the mount- 

 ains to the river, a distance af about 800 to 1,000 feet in height ,* they are 

 at the edge of the mountains, gradually falling off as we approached the 

 river. I could find no evidence of any terminal moraine. The southern 

 bench is well timbered on top with iDines and aspens, while the sides of 

 both are overgrown with sage-brush. Entering the canon, which is 

 cut profoundly into the granites, and is quite picturesque, we find that 

 the rocks scattered over the bottom are rounded and polished. There 

 were very few evidences of striation and groovings on the rocks on the 

 sides, and these were all very indistinct. In the center of the caiion 

 the stream has cut a deep, narrow, secondary caiion in the granites, 

 some 50 feet below the general level of the vallej', which once formed 

 the floor over which the glacier passed. The next creek to the north is 

 Lake Creek, which is by far the most extensive branch of the Arkansas ; 

 it IS about sixteen miles in length, and flows almost due east, one branch, 

 draining the country between La Plata Mountain and Grizzly Peak, 

 while another branch, the North Fork, comes from the north, draining the 

 country southwest of Massive Mountain. Before reaching the river, the 

 creek passes through the Twin Lakes, two beautiful lakes, probably of 

 glacial origin : they are separated by morainal material. There are 

 moraines on both sides of the creek, as well as along all the main branches, 



