﻿1 68 NEW YORK STATE MUSEUM 



ribs. The ribs which have been long exposed have been so corroded 

 by weathering that one might question even their glacial origin. 

 But those flutings also which are only recently uncovered have lost 

 their glaciated surface, though they may show the perfect polish of a 

 later glaciation oblique to the ribbing. This fact is important with 

 reference to the age of the ribbing. 



In plate 61 we see a typical example of the ribbed limestone, the 

 locality being the west side of a hollow several rods west of the rail- 

 road station at Threemile Bay. The ribbing is s. 45 ° w. The three 

 ribs in the foreground at the lower right corner have been strongly 

 cut and polished by an ice flow having direction s. 55 ° w. This 

 change of 10 degrees in direction is not unusual for the same ice 

 sheet, and taken alone would be weak evidence of dual glaciation. 

 The important fact here is that the later ice movement has scored 

 and polished the ribs obliquely, striking them on the east faces, 

 and the later polishing is perfectly preserved though apparently has 

 been exposed as long as other portions of the ribs where no glaci- 

 ation is visible. The ribs are being freshly uncovered by the wash 

 on the slope, but the only glaciation seen is that oblique to their di- 

 rection. The only reasonable inference is that the ribs have lost 

 the glacial surface by old age weathering and that the oblique polish 

 is from a later ice rubbing. The ribs are rough and corroded where 

 not cut by the more westerly planing, and it is certain that the lack 

 of striae and polish on the ribs can not be due to merely recent 

 weathering. 



The ribs and hollows have no fixed relation to the joints of the 

 rock. In the locality of plate 61, while the ribbing is s. 45 ° w., the 

 joints, so far as they have any dominant trend, are s. 75—85 ° w. 

 Nowhere are the joints so true and parallel as the ribbing. Only 

 occasionally do joints appear in the furrows but they commonly 

 lie boldly across the ribs and are frequently opened widely, as in 

 plate 60, where some joints are a foot wide and 10 feet deep. The 

 removal of the clays of the latest deposits from the ribbed sur- 

 face shown in plate 60 has been chiefly by subterranean drainage 

 through these open joints. It seems very unlikely that these open 

 joints were produced with their present size and form since the last 

 glaciation and beneath several feet of. the Dexter clay [pi. 58I . 

 The joints certainly are the product of atmospheric weathering 

 and solution, and it seems a safe inference that they represent a 

 time of long exposure antedating the last ice work. 



These exposed ribs east of Dexter, visible on the north side of 

 the electric line, lie in a hollow in the clay, as shown in plate 63, 



