8 CLAYS AND CLAY INDUSTRY. 



limit of the ice sheet during its latest advance extending from 

 Perth Amboy to< Belvidere, along - the line now marked by the 

 terminal moraine (PI. X) - 1 For this reason no' extensive deposits 

 of residual clay are to- be looked for north of this line, while south 

 of it a few small deposits are known, but they have no great 

 economic value. 



SEDIMENTARY CLAY. 



As mentioned above, residual clays rarely remain on steep 

 slopes, but are washed away by rain storms into' streams and 

 carried off by these to lower and sometimes distant areas. By 

 this means residual clays possibly of very different character 

 may be washed down into the same stream and become mixed 

 together. This process of wash and transportation can be seen 

 in any abandoned clay bank, where the clay on the slopes is 

 washed down and spread out over the bottom of the pit. 



As long as the stream maintains its velocity it will carry the 

 clay in suspension, but if its velocity be checked, so> that the 

 water becomes quiet and free from currents, the particles begin 

 to settle on the bottom, forming a clay layer of variable extent 

 and thickness. This may be added to from time to time, and to- 

 such a deposit the name of sedimentary clay is applied. All 

 sedimentary clays are stratified or made up of layers, this 

 being due to the fact that one layer of sediment is laid down on 

 top of another (PI. I, Fig. i). If there were absolutely no 

 difference in the character of the material deposited, it would 

 form one thick, homogeneous bed, but there is usually more or 

 less variation, a layer of very fine material being laid down at 

 one time, and a layer o*f coarser material on top of it, or vice 

 versa. These layers may also vary in thickness, and, since there 

 is less cohesion between unlike particles, the two layers will tend 

 to separate along their line of contact. 



As the finer material can only be deposited in quiet water, 

 and coarse material in disturbed waters, so from the character 



1 For a full account of the Glacial Deposits of the State, see Vol. V of the 

 N. J. Survey Reports. 



