AGE OF MISSOURI RIVER VALLEY 299 



boulders, which lies just above river level, is at least 12 to 14 feet thick 

 and extends along the water's edge for a distance of 100 yards, while 

 scattered boulders and ferruginous gravel occur at intervals for another 

 200 yards. Overlying the boulders are 15 feet of gravel. While some of 

 the boulders of this deposit may have been brought here by floating ice, 

 it is probable that most of the deposit was left here by the pre-Wisconsin 

 ice-sheet when it advanced south of the river. The finer materials of the 

 drift, if they were ever present, have been carried away, leaving the gravel 

 and boulders. 



Pleistocene Yalley of Missouri and Yellowstone Rivers 



But while the Missouri Eiver probably occupied its present valley for 

 a considerable time prior to the Glacial period, the ice-sheet, when it 

 invaded the region, blocked the valleys of both the Missouri and Yellow- 

 stone rivers and also the preglacial valley of the Little Missouri, forcing 

 these streams to seek new channels. Lakes were formed in the valleys of 

 the Yellowstone and Little Missouri rivers, the water rising until it over- 

 flowed the divide between the latter and the Knife Eiver south of the 

 Killdeer Mountains. The combined waters of the three rivers flowed east 

 across Dunn County and southeast across Morton to the mouth of the 

 Cannon Ball Eiver. The valley thus formed crosses the divide between 

 the Knife and Heart rivers and also that between the Heart and Cannon 

 Ball. The length of this Pleistocene valley of the Yellowstone and Mis- 

 souri rivers from the head of the Knife to the mouth of the Cannon Ball 

 is 155 miles. It is followed for 30 miles by the Northern Pacific Eailroad 

 between Almont and Hebron, this portion of the valley being today occu- 

 pied by Curlew Creek (figure 1, plate 14). The Heart Eiver follows the 

 valley for 6 to 8 miles below the mouth of Curlew Creek, and the broad 

 depression continues its southeasterly course through the divide to the 

 Cannon Ball, being followed for many miles by Louse Creek, a tributary 

 of the Cannon Ball. 



Two broad valleys connect the Knife Eiver Valley with that of Curlew 

 Creek. One enters the latter valley between 3 and 4 miles below Glen 

 Ullin and is followed by the northward flowing Elm Creek throughout a 

 portion of its extent. Between the latter and the tributary of Curlew 

 Creek the valley bottom is occupied in part by a hay marsh. The other 

 valley, which joins that of Curlew Creek just below Hebron, is known as 

 Farmers Valley and extends to the head of Deep Creek, a tributary of 

 Knife Eiver. 



