484 J.E.TODD THE CHANNEL OF THE MISSOURI RIVER 



its surface very much. Its bottom is composed of low kuoUs elongated 

 with the axis of the valley, and usually with abundant boulders, espe- 

 cially upon the summit, as if a stream bed had been unoccupied for a 

 long time and erosion had removed most of the fine material. This 

 channel passes over the divide between Stone Creek and a branch of 

 Virgin Creek which leads north-northwest, east of La Plant. The bot- 

 tom of this channel east of La Plant is at an altitude of about 2,000 feet. 



Of course, one outlet corresponds with the present course of the Mis- 

 souri Eiver. There are points near Welland, east of the Missouri River, 

 which rise to more than 1,900 feet, or 500 feet higher than the Missouri. 



Another channel, which probably had served as an outlet before the 

 glacier had covered it in its advance westward, has a general south-south- 

 east course, passing a little east of the northwest corner of towai 120 

 north, range 76 west. It extends for 7 or 8 miles, from the breaks of 

 Swan Lake Creek to the valley of the Little Cheyenne. 



The order in which these outlets served was, first, the eastern 

 one, leading into the Little Cheyenne; second, the Missouri as now 

 located ; and, third, the western one, which probably was the main one in 

 the earlier and higher stages of the lake. It had probably cut down 100 

 to 200 feet before the recession of the ice uncovered the present course of 

 the Missouri, which has been occupied ever since. The maximum extent 

 of the ice was in the formation of the Altamont moraine, of which por- 

 tions toward the south are found in the Artichoke Hills and Sully Buttes. 



The edge of the ice must have been very near the present river, prob- 

 ably covering, but not extending at any time to, -the third, or Stone Creek, 

 outlet. The region between the present course of the Missouri and the 

 old Stone Creek outlet is driftless, so far as has yet been discovered, at 

 least 10 or 15 miles north from the Cheyenne Agency. This area does 

 not seem to be much higher than the region farther north; and, yet we 

 must conclude that this area, at the time of the highest stage of Lake 

 Arikaree, must have been 100 feet higher than at present. It is not 

 difficult to suppose that there has been time sufficient for the carrying 

 away of such an amount of material by erosion, for the Pierre formation 

 occupies the whole height of six or seven hundred feet and there are 

 numerous deep ravines running southeast parallel to Stone Creek. The 

 Pierre formation is almost pure clay and very subject to landslides on 

 steep slopes, especially wlien the base is softened or eroded by running 

 water. The whole surface of the bluffs of the Missouri River opposite 

 the Cheyenne Agency, for a height of 250 to 300 feet, is covered with 

 sliding masses 50 to 150 feet wide. 



