CLAY AND ITS MODE OF OCCURRENCE. 13 



the river during periods of high water, and is consequently 

 termed the flood plain. In such times much clayey sediment is 

 added to the surface of this flood terrace, and thus a flood-plain 

 clay deposit may be built up. 



Owing to the fact that there is usually some current setting 

 along over the plain when it is overflowed, the finest sediments 

 cannot settle down, except in protected spots, and, consequently, 

 most terrace clays are rather sandy, with here and there pockets 

 of fine, plastic clay. They also frequently contain more or less 

 organic matter. Along- its inner edge the terrace may be covered 

 by a mixture of clay, sand and stones, washed down from neigh- 

 boring slopes. 



Where several terraces are found, it indicates that the stream 

 was formerly at the higher levels, and has cut down its bed, each 

 terrace representing a former flood plain. Even along the same 

 stream, however, the clays of the several terraces may vary 

 widely in their character, those of one terrace being perhaps 

 suitable for pottery, and those of a second being available only 

 for common brick and tile. , 



Drift or bowlder clays. — In that portion of the State formerly 

 covered by the continental ice sheet (PI. X), there are occasional 

 deposits of clay formed directly by the glacier. These are usually 

 tough, dense, gritty clays, often containing many stones. The 

 material deposited by the ice (till) was usually too stony and 

 sandy to serve as clay, although often known as bowlder clay. 

 Locally, however, where the ice-transported material had been 

 largely ground to a fine rock flour, the bowlder clay is plastic 

 enough and not too full of stones for use. Such deposits are 

 mostly of limited extent, impure and of little value. 



In addition to this type of clay formed directly by the ice, 

 there were clays deposited in lakes or along flood plains by the 

 streams issuing from the glacier. These were composed of 

 material derived from the ice, but- since they were deposited by 

 water, they were stratified and may properly be classed as 

 lacustrine, estuarine or flood-plain clays of glacial age. These 

 types are discussed in detail under their respective heads on the 

 preceding pages. The clays of Singac, Mountain View and 

 Hackensack are of this type. 



