86 THE ILLINOIS GLACIAL LOBE. 



is very nearly west to east, The prominences on the rock ledge appear to 

 sustain the eastward rather than the westward direction of movement, but 

 they are so slight that some distrust of this interpretation is felt. The 

 harmony with the westward-bearing stria? and lack of harmony with the 

 eastward would be in favor of their classification with the former. It 

 will be noted that two exposures north of Burlington are eccentric in 

 showing bearings S. only 15° and 33° E. All the stria? in the vicinity of 

 Burlington are found at elevations 100 feet or more above the level of the 

 Mississippi, or at about the general level of the higher portions of the rock 

 surface. 



The glacial origin of certain strise at Alton, Illinois, has been called in 

 question by Prof. J. E. Todd. 1 The strise first seen at that city occur 

 beneath a culvert on Piasa street, in a small valley near the round house of 

 the Chicago and Alton Railway, at a level perhaps 30 feet lower than the 

 higher part of the rock ledges of the immediate vicinity. Todd has expressed 

 the view, in the paper just cited, that these stria? were produced by a hori- 

 zontal slipping of rock ledges, and that they are now exposed through the 

 removal of ledges which once covered them. Numerous exposures of slick- 

 ensides and slight faults appear in that vicinity, showing that it is a district 

 where disturbances have occurred. He recognized, as has the writer, that 

 the stria? bear a strong resemblance to glacial stria?, but as glacial stria? and 

 slickensides are both produced by the movement of rock upon rock, he 

 urged that the resemblance to glacial stria? does not forbid their being con- 

 sidered slickensides. Since the appearance of Todd's paper the writer has 

 reexamined this ledge and found satisfactory evidence that the stria? are not 

 slickensides. The striation occurs on three different layers of surface rock. 

 Now, if it was produced by rock ledge slipping upon rock ledge, we should 

 expect the striation to occur between these layers as well as on their exposed 

 surfaces, but an examination showed that the striation is confined to the 

 exposed surfaces, and that the striating agency must have been such as 

 could affect only the exposed surfaces. 



Furthermore, following the Chicago and Alton Railway track north- 

 ward from this exposure, two other striated surfaces were found at levels 



1 Striae and slickensides at Alton, Illinois, by J.E.Todd: Proc. Am. Assoc. Adv. Sci., Vol. XL, 1891, 

 pp. 254, 255. 



