230 FREDERICK W. SARDESON 



Why the rock was crushed and folded at this place rather than 

 at any other is not evident in this exposed section. The reason 

 is explained by another case which occurs four miles farther up 

 the river's bluff, in the quarry next to Riverside Park. This case 

 is shown in Fig. 3. The section here shows the following: 



G. Glacial till and soil about 10' 



5 6. Green clay-shale 3' 



5. Gray crystalline limestone o' 6" 



4. Green clay-shale i' 



3. Gray crystalline limestone double stratum . 2' 4" 



2. Clay-shale, irregularly laminated . . . . o' 4" 



I. Dark crystalline limestone i' 4" 



B. Massive granular limestone 4' + 



Since the till in this section now forms an ancient terrace of the 

 river, it evidently has not its original thickness. Below the till 

 the same rock strata appear as in the other section (Fig. 2) and 

 therefore the parts of Fig. 3. are littered in the same order as those 

 of Fig. 2. The main stratum (5 3) appears double here because 

 of a thin irregular shaly lamina within it. 



The noteworthy feature in the section is the reversed fault which 

 appears in the top part of the main stratum (5 3). The fault is 

 clearly the result of thrust and displacement along an oblique joint. 

 The overlying strata are disturbed by lateral compression and by 

 upward movement of the faulted piece, while the underlying strata 

 are not disturbed. The phenomenon can be explained as the 

 result of glacial friction upon the stratum at a distance from the 

 point of faulting, causing the stratum to move from right to left — 

 i. e., northwest to southeast — as far as the hne at which an obhque 

 joint in the rock led to its development of a fault. The fault, as 

 seen on the surface of the rock, which has been cleared of drift in 

 the quarry, runs from northeast to southwest in a reversed curve 

 for about 40 feet. It extends indefinitely farther. 



This and the previously described case may be considered as 

 two stages of the same process of folding. Although they were 

 probably not made by the same glacier, at the same time, since 

 the strike of the one is nearly at right angles to that of the other, 

 yet they occur in the same strata under the same till-sheet. Com- 



