70 NEW YORK STATE MUSEUM 



roughly parallel to the old shore line. Such an escarpment would 

 be specially prominent when the underlying rock was very weak. 

 The resistant Medina sandstone, overlying the weak Hudson 

 shales, forms just such a combination, and the Medina escarpment 

 is a prominent feature of the district south of the Mohawk, though 

 lying beyond the limits of the map. 



* Since the Beekmantown is more resistant than the Trenton, and 

 that more so than the Utica. erosion tends to strip the Trenton 

 from off the Beekmantown more rapidly than the latter can itself 

 be worn back, and thus to leave a bared strip, or terrace formed 

 on the upper surface of the Beekmantown. For a like reason, i\ 

 terrace tends also to develop on the upper surface of the Trenton 

 as the Utica is worn away, the level of the Trenton terrace drop- 

 ping rather abruptly to that of the Beekmantown over the low 

 escarpment formed by the edges of the retreating Trenton layers. 

 Likewise a terrace tends to form on the even pre-Cambrian sur- 

 face bared by the removal of the Beekmantown, since the former 

 rocks are vastly more resistant. 



With the district at a given elevation, wear can be carried on 

 only down to a certain level, determined by the slope necessary to 

 permit the streams to carry away their load of rock waste. The 

 soft rocks will be worn down to this level long before the hard 

 rocks reach it. But then wear ceases on the soft rocks while still 

 continuing on the hard. Therefore features of relief produced b>- 

 varying rate of wear can reach only a certain degree of accentua- 

 tion, after which the effect of erosion is to diminish their strength, 

 and, if the region persist sufficiently long at the given level, all 

 rocks, hard as well as soft, will be worn down nearly or quite to 

 the level of the stream bottoms. Renewed uplift will however 

 cause the streams to renew down cutting, again the soft rocks will 

 go down first and the hard again come to stand above their level, 

 resulting in a reproduction of the previous features, the only dif- 

 ference being that they will be shifted to the southward of their 

 position in the previous erosion cycle. The amount of relief 

 obtainable in each cycle will also depend on the amount of 

 uplift. 



