CONTORTED STRATA IN THE YOREDALE ROCKS, NEAR ASHOVER. 57 



shales will be in a state of lateral compression where the covering- 

 strata are lighter than the average ; they may, however, be 

 intrinsically strong enough, especially if aided by the weight 

 of strata above, to resist this lateral push. But when the 

 rock above is removed, or they themselves are being cut into, 

 a point will eventually be reached when they will no longer 

 be able to resist ; and, as is usual, in thinly-bedded structures, 

 the laminae of which can freely slide one against another, their 

 collapse will take the form of gentle flexures, and, as the valley 

 is deepened, crumpling. The line of least strength will, of 

 course, determine the direction of these flexures, and as this 

 line in a valley is along the lowest part of it, the axes of the 

 flexures will naturally coincide with the "run" of the valley. 



If the above be the true explanation of the phenomena I have 

 described, and it seems to fit in with all observed facts of the 

 case, these Yoredale contortions are most interesting to the 

 student, in that it furnishes him with a process still going on, and 

 with an idea as to the lapse of time (not to be measured in years, 

 though) since it began. It is clear that the valley is older 

 than the contortions ; it is clear, also, that the beds of Mill- 

 stone grit, which are now more than two hundred feet above 

 the valley bottom, must have been stripped away, at all events to 

 a great extent, before the process began. So it is reasonable to 

 think that this process of rock-folding has been contemporaneous 

 with the deepening of the valley to the extent of two hundred 

 feet. 



