for the sides of such excavations wash down rapidly, and a muddy-red surface clay or loam will often run down over a bed of lighter colored clay beneath so as to completely hide it from view. If the cut is deep and freshly made, the depth of weathering can frequently be determined.

Springs. In many cases the presence of clay is shown by the occurrence of one or more springs issuing from the same level along some hill slope. These are caused by surface waters seeping down from the surface (Fig. 18), until they reach the top

Formation of spring due to ground water following a clay layer.

of some impervious clay stratum, which they then follow to the face of the bank where they issue. The presence of springs, however, cannot be used as a positive indication of clay, for a bed . of cemented iron sand, or even dense silt, may produce the same effect (Fig. 19).

Spring

Loose sand

Cemented sand

Fig. 19.

Formation of a spring due to a layer of cemented sand.

Ponds. In many regions covered by glacial drift, pools of water are often retained in depressions because of the presence

A fertile farming country.

below of a water-tight bed of clay (Fig. 20). It does not necessarily indicate a thick deposit, for a very thin layer may often hold up a considerable body of 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. (Pl. 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 information 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, however, 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

an auger furnish a more satisfactory and rapid means of determining the thickness of the clay deposit away from the outcrop. An auger for this purpose (Fig. 21) can be made easily and cheaply by welding a one and one-half inch, or two-inch carpenter's auger to a piece of three-fourths inch gas pipe. The latter is cut into short sections and by means of a T joint, a

Fig. 21.

Auger used for boring clay.

handle can be screwed to the upper length. When the bore hole is started only one section is used, but additional ones are added as the hole increases in depth. In very wet, sandy clays augers are of little value as the bore hole washes in and fills up as soon as the auger is withdrawn. The only way to prevent this is to drive a pipe down in the bore hole, keeping the auger slightly in advance of the lower end of the pipe. It is of the highest importance that a sufficient number of borings should be made to determine the extent and thickness of the clay deposit.