CLAYS OF NEW YORK 577 



The clay is chiefly blue, but where the overlying sand is wanting 

 or is of slight thickness, it is weathered to yellow, this weather- 

 ing often extending to a depth of 15 feet below the surface, and 

 to a still greater depth along the line of fissures through which 

 the water can percolate. The depth of oxidation is of course influ- 

 enced by the nature of the clay, the upper portion weathering 

 easily on account of its more sandy nature and hence looser text- 

 ure. Horizontal stratification is marked and the layers of clay 

 are separated by extremely thin laminae of sand. At some locali- 

 ties the layers of the clay are very thin and alternate with equally 

 thin layers of sandy clay. This condition is found at Haverstraw, 

 Croton, Dutchess Junction, Stonypoint, Fishkill, Cornwall, ISTew 

 Windsor, Oatskill and Port Ewen. At all of the above mentioned 

 localities except the last two, the clay is overlain by the delta de- 

 posits of rivers tributary to the Hudson, and the alternation of 

 layers may be due to variations in the flow of the rivers emptying 

 at those points, the sandy layers being deposited during period of 

 floods. The delta of Catskill creek has been found at Leeds, some 

 2 miles west of the Hudson river. -^ The delta of Rondout creek, 

 which flows into the Hudson at Port Ewen, will no doubt be 

 found by following the creek back to the ancient shore line of the 

 Hudson estuary. Isolated ice-scratched boulders are not uncom- 

 monly found in the clay. 



There is often a sharp line of division between the yellow 

 weathered portion and the blue or unweathered part of the clay. 

 The line of separation between the clay and overlying sand is also 

 quite distinct in most cases. Of the blue and the yellow clay the 

 former is the more plastic, but both effervesce readily with acid due 

 to the presence of dfc-Q>fo of carbonate of lime, and are therefore, 

 properly speaking, marly clays. The clay is underlain by a bed 

 of gravel, sand, hardpan, boulder, till or bed rock. From Albany 

 to Catskill the underlying material is a dark gray or black sand 



1 W. M. Davis. Proc. Bast. soc. nat. hist. Nov. 1892. 



