GLACIAL STRIDE. 85 



not only in the sandy shales of the Coal Measures but also in a large part 

 of the area immediately underlain by limestone. The Galena limestone, 

 for example, has scarcely ever afforded exposures of striation in its exten- 

 sive outcrops in Stephenson, Jo Daviess, Ogle, and Carroll counties, for 

 its surface is usually so rotten, even under the deepest deposits of till, that 

 strire would not be preserved. 



In not a few instances the ice sheet is found to have failed to remove 

 the residuary clays which overlie the bed rock, and in such instances stria;, 

 of course, were never formed. How extensively the surface ledges escaped 

 striation is not known, but the present knowledge of rock exposures within 

 the glaciated portion of the Mississippi Basin leads the writer to think that 

 a large percentage of the rock surface never was striated. 



A few of the striae on the border of the lobe merit special notice. At the - 

 city of Burlington, Iowa, striae formed both by the Illinois glacial lobe and 

 by the southern extension of the Keewatin ice sheet are present. Only one 

 instance, however, has been found in which the striae on the Iowa side are 

 certainly referable to the Illinoian ice sheet. This exposure occurs at the 

 northeast corner of the intersection of Court and Prospect streets, in the 

 north part of Burlington, and was the joint discovery of Mr. F. M. Fultz 

 and the writer. The bearing is S. 72° W., and satisfactory evidence is 

 found, from prominences on the striated ledge, that the movement was 

 westward and not eastward. Striae were discovered by the writer on the 

 east bluff of the Mississippi, opposite Burlington, which have a nearly clue 

 east-west bearing. In this exposure the evidence concerning the direction 

 of movement, whether eastward or westward, is not decisive, there being 

 no prominences on the ledge which would throw light on the direction of 

 movement. The bearing harmonizes more easily with a westward move- 

 ment than an eastward movement, and the balance of probabilities seem in 

 favor of westward movement. 



The striae due to the earlier or Keewatin ice lobe, both in Burlington 

 and at other points north of the city reported by Mr. Fultz, have a bearing 

 generally about S. 65°-70° E., their direction being about the same as that 

 of striae found by the writer near Washington, Iowa, and by Professor 

 Calvin in the vicinity of Iowa City. One locality, however, was found 

 in quarries one-half mile north of West Burlington, in which the bearing 



