IOWA ACADEMY OF SCIENCES. 19 



dndefinite distance. Inside this inarpin the land nowhere rises higher than the 

 margin, and it is here and there sprinkled with northern boulders, often in patches, 

 especially on the higher levels. The divide between the Moreau and Grand rivers 

 has an altitude of about 2,o00 feet. Most of the surface is of Cretaceous clays, and 

 is much eroded, the alternating layers of hard and soft material, producing an 

 interesting topography, studded here and there with high, flat-topped buttes. 



The course of the marginal ridge south of the Moreau is in line with some high 

 ■clay buttes on the east side of the Missouri, just above the mouth of the Little 

 Cheyenne, which are known as Welland Buttes. They are strewn with a thin 

 layer of boulders, and are the west end of a high divide separating the Little 

 Cheyenne and Swan Lake Creek. Crossing this divide is a well preserved ancient 

 •channel, more than 400 leet above the Missouri, and there are traces of an old ter- 

 race along the Missouri, near the Welland Buttes, at about the same level. 



Putting these things together, we come with some confidence to this conclusion: 

 Fox Ridge, with its eastern extension, the Welland Buttes and the high land south- 

 west of Bowdle and west of Faulkton, once formed the divide between the Chey- 

 enne and Moreau rivers, when they flowed through to the J ames river valley. When 

 the great ice sheet came down the latter valley during the glacial period, and occu- 

 pied the outermost terminal moraine, there was lor a time a great lake formed 

 north of this Fox ridge divide. It was deep enough to float ice-floes and probably 

 bergs from the edge of the ice sheet further north. These formed a bouldery 

 beach along the margin, particularly along the southern side. Of the two outlets 

 indicated, the western one cut down more rapidly, and formed part of the course 

 •of the Missouri. As erosion proceeded the bouldery margin became a ridge, be- 

 ■cause it yielded less rapidly to degradation than the soft clays and loose sands 

 adjacent. 



For this glacial lake we propse the name Lake Arikaree, after the Indian tribe 

 whose home formerly occupied a considerable portion of its area. 



STRIATION OF ROCKS BY RIVER ICE. 



BY J. E. TODD. 



Though it IS commonly admitted by geologists, that both land-ice and floating- 

 ice are capable of striating rocks, when armed with erratics; careful discrimin- 

 ations seem to be largely ueglecttd. The question, whether river-ice was ever the 

 active agent in scratching locks, had been raised in the writer's mind several 

 years since by a few obseivaticns in Dakota. Diligent search at several seemingly 

 favorable localities had given only negative evidence, until this past season, when 

 two or three observations seem to demonstrate the fact that such is not very 

 infrequently the case. 



In this abstract theie is room but for the clearest example. 



Three miles above Grand Tower, Illinois, there is a hard even-topped stratum of 

 dark lime-stone, jutting out from the eastern bank for several yards, and dipping 

 at a slight angle toward the bank. Tlie steep face resting upon it and extending 

 further up stream is covered with large sandstone boulders. The dip of the rocks 



