METHODS OF WORKING CLAY DEPOSITS. 27 



below of a water-tight bed of clay (Fig. 20). It does not neces- 

 sarily indicate a thick deposit, for a very thin layer may often 

 hold up a considerable body oi water. Such ponds may likewise 

 in rarer instances be caused by ground water seeping down from 

 higher levels, even in the absence of clay. 



Vegetation. — Clay deposits in some areas produce a different 

 type of plant growth from other soils. In Salem county a rather 

 striking contrast was noticed between the fertile farming land of 

 the Alloway clay around Woodstown, and the pine-barren land 

 with its sandy soil to the southeast. (PI. V, Figs. 1 and 2.) 



EXPLOITATION OF CLAY DEPOSITS. 



The location of a clay deposit is followed by a determination of 

 its thickness, extent, character and uses. The first two> points and 

 some facts bearing on the third are determined in the field ; the 



Fig. 20. 



Formation of a pond due to a clay bed underneath a depression. 



behavior of the clay when mixed up and burned is found out by 

 tests made in the laboratory or at some factory, and the informa- 

 tion thus obtained indicates the commercial value of the material. 

 To determine the thickness and extent of the deposit, a careful 

 examination should be made of all clay outcrops in neighboring 

 gullies, or other cuts on the property having the clay. Since, 

 however, most clay slopes wash down easily it may be necessary 

 to dig ditches from the top to the bottom of the cut or hillside in 

 order to uncover the undisturbed clay beds. In most cases, how- 

 ever, the cuts are not sufficiently close together and additional 

 means have to be taken to determine the thickness of the deposit 

 at intermediate points. Such data are sometimes obtainable from 

 wells or excavations made for deep cellars, but the information 

 thus obtained has to be taken on hearsay. Borings made with 



