72 . NEAV YORK STATE MUSEUM 



on one side at the surface is likely to be more resistant than that 

 on the other, and the more rapid wear in the softer rock will again 

 bring the fault into some prominence as a topographic feature, the 

 amount depending on the difference in resistance of the two rocks 

 concerned, and on the amount of uplift. 



When the faults of the district are examined with these prin- 

 ciples in mind, it is at once seen that they are at least sufficiently 

 old, so that the original scarps have been utterly obliterated as 

 topographic features ; that no recent slipping has taken place along 

 them ; and that such small show as they make in the present topo- 

 graphy is due to the rather r-ecent uplift of the region as a whole. 

 The Little Falls fault makes no considerable show in the topo- 

 graphy except along the immediate channels of the streams which 

 cross it, all of which show falls or rapids, and gorges below, in the 

 harder rocks of the upthrow side, with sudden change in the char- 

 acter of the valley as the fault line is crossed [see pi. 11]. This 

 is most impressively shown along the Mohawk, the broad, mature 

 looking valley developed in the weak Utica shales east of the fault 

 contrasting sharply with the narrow gorge in the resistant rocks 

 west of it, and the fault scarp being a prominent feature 

 when looking up the valley [see pi. 12, 13]. Away from the 

 streams, where the fault affects the topography at all, it appears 

 as a low escarpment, with gently sloping instead of steep front, 

 the slope being down the dip of the updragged Utica beds on the 

 downthrow side. Where the very resistant pre-Cambrian rocks 

 are on one side with the Utica shale on the other, the fault is 

 fairly prominent and would be more so were it not for deep drift 

 deposits on the lower side. 



The larger faults do, however, have an important indirect effect 

 on the topography. Since they run north-south, and since the 

 uplifting of the west side has given the rocks there a tilt to the 

 westward, in addition to the usual south dip, giving them a local 

 north-south strike, erosion has developed the Beekmantown and 

 Trenton escarpments and terraces there with a north-south trend, 

 at right angles to their usual direction [see pi. 14]. The pre- 

 Cambrian terrance is also well developed on the west side of the 



