12 CLAYS AND CLAY INDUSTRY. 



and this of low velocity, the finer clays will be found at a point 

 most distant from the mouth of the river. In such cases, we 

 should anticipate an increase in coarseness of the clay bed or 

 series of beds as they are followed from what was formerly the 

 old shore line up to 1 the mouth of the former river that brought 

 down the sediment. 



Estuarine clays often show sandy laminations, and are not 

 infrequently associated with shore marshes, due to' the gradual 

 filling up of the estuary and the growth of plants on the mud 

 flats thus formed. The clays of the Hackensack region are prob- 

 ably estuarine deposits, formed at the close of the Glacial period, 

 when the region west of the Palisades stood somewhat lower 

 in respect to sea level than at present. 1 



Swamp and lake clays. — Swamp and lake clays constitute a 

 third class of deposits, which have been formed in basin-shaped 

 depressions occupied by lakes or swamps. They represent a 

 common type, of variable extent and thickness, but all agree in 

 being more or less basin-shaped. They not infrequently show 

 alternating beds of clay and sand, the latter in such thin laminae 

 as to be readily overlooked, but causing the clay layers to split 

 apart easily. Many of the lake clays are directly or indirectly 

 of glacial origin, having - been laid down in basins or hollows 

 along the margin of the continental ice sheet, or else in valleys 

 that have been dammed up by the accumulation of a mass of 

 drift across them. This wall of drift serves to' obstruct the 

 drainage in the valley, thus giving rise to a lake, in which the 

 clay has been deposited. Clay beds of this type are extremely 

 abundant in all glaciated regions. They are usually surface 

 deposits, 2 often highly plastic, and more or less, impure. Their 

 chief use is for common brick and earthenware, and none of the 

 New Jersey ones have been found to be refractory. 



Fl&od plain and terrace clays. — Many rivers, especially in 

 broad valleys, are bordered by a terrace or plain, there being 

 sometimes two or more, extending like a series of shelves or 

 steps up the valley side. The lowest of these is often covered by 



1 N. J. Geol. Survey, Vol. V. Report on Glacial Geology, p. 196. 



2 Not necessarily thin. 



