180 



CAPE COD GEOLOGY 



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crumble rather than to fold. For this reason folding is less common in the 

 Herod gravel than in the underlying Gardiners clay and Jacob sand, and the 

 sharp folds seen in those formations are not found in the Herod. The Herod was 

 probably disturbed by the ice in two ways — first, by its push and drag, which 

 would be especially effective where an escarpment in the gravel was exposed 

 to the advancing ice; second, by the weight of the overriding ice, which caused 

 unequal settling of both the Herod and the underlying clayey beds. In other 

 words, much of the Herod gravel was exposed to both lateral and vertical pres- 

 sure, to which it adapted itself as best it could in view of its loose and friable 

 nature and its relations to the underlying formations. The Herod gravel that 

 lies under the outwash gravel which covers the great plain area has probably 

 been somewhat disturbed, for there is no reason to suppose that the Montauk 

 ice sheet stopped before reaching this region. However, the ice probably did 

 not encounter a rough surface, and it therefore probably overrode the Herod 

 member without causing much deformation. 



Source of materials. — The immediate source of most of the material of the 

 Herod gravel is probably the older Pleistocene or earlier deposits of the same 

 region and the region now occupied by Vineyard Sound. The original source 

 must have been the mainland north of Marthas Vineyard. The granitic and 

 gneissic pebbles, which are relatively fresh, may have come directly from the 

 mainland. 



Relation to other deposits. — At some places the Herod gravel grades down 

 into the Jacob sand; at others it changes rather abruptly from coarse sand 

 above to fine sand below. It therefore rests conformably on the Jacob sand and 

 follows the undulations of its surface. Where the Jacob sand does not lie below 

 it the Herod gravel may rest directly on the Gardiners clay or it may lie uncon- 

 formably on earlier deposits, for the Herod gravel is a more extensive deposit 

 than the Jacob or the Gardiners. The upper contact of the Herod gravel with 

 the Montauk till is usually less disturbed and less irregular than would be 

 expected. At some places there is an apparent conformity between the two 

 beds, the unconformity seen at other places being due to erosion by the ice 

 before it began to form deposits. In places the Montauk ice removed the Herod 

 gravel entirely and the till rests on older beds. Where the Herod is in contact 

 with the Montauk till the contact is sharp. At places where the Montauk ice 

 did not deposit any till the upper surface of the Herod is in contact with the 

 Hempstead gravel. At such places there must be an unconformity between the 

 two, although it cannot be detected. 



