878 STUDIES FOR STUDENTS 



first, in glaciated regions the ice mantle which has carried away 

 blocks from the lee side of elevations lying in its path ; and, 

 second, in regions of severe climate, the frost, which has separ- 

 ated blocks and allowed them to roll down steep slopes. A 

 special cause of scarp formation may be the undermining by 

 solution of beds of calcareous rock, which has allowed the over- 

 lying beds to fracture and fall. In all cases, however, some 

 earlier structure planes must be assumed along which the block 

 has separated, and these planes may have been either planes of 

 jointing, or faulting, or foliation, or of original bedding. Con- 

 sidering each scarp by itself, therefore, and quite independent of 

 the system of the region, it is more likely to represent a fault in 

 proportion: first, as it does not face in the direction in which 

 the ice mantle moved over the area; and, second, as it is not 

 parallel to a plane of fissility or of bedding (not parallel to the 

 strike). 



Study of the southern New England area has shown that 

 cliffs of considerable extent which adhere to a uniform trend 

 often for long distances usually do so, not along a single plane, 

 but by a series of minor zigzags the elements of which are joints 

 or faults of other series combined with the one which corre- 

 sponds in direction with the general trend of the cliff. 1 Fig. 4 



will illustrate the manner in which it is 

 ^^ accomplished. Such a composite cliff 

 FlG ' 4 ' is shown in Fig. 5. Figs. 1 and 10 



must be regarded as showing the same type of structure, but in 

 the next higher orders of magnitude, respectively, within a com- 

 posite series. This observation of the composite nature of lines 

 of dislocation is regarded as of much significance in showing the 

 genetic connection of faults and joints, as well as in reading the 

 geological structure of an area. 



10. Fault gorges. — In many places, and especially throughout 

 New England, are found deep clefts in the harder rock which 

 are locally known as "purgatories," " devil's dens," " ice-glens," 

 " ice-gorges," etc. The walls of these clefts are nearly or quite 



l Vide Branner, "Geology in its Relation to Topography," Trans. Am. Soc. Civ. 

 Engineers, Vol. XXIX (1898), p. 63. 



