'basal blue clay 373 



of " blue clay " 21 feet in thickness. No samples of this clay have been 

 seen, but the fact that it rests on the granite instead of on a thick series 

 of glacial gravels, as does the only known Pleistocene clay of the region, 

 points to its probable Cretaceous age. If so, it doubtless accumulated 

 soon after the uplift and seaward tilting, which has been described as 

 beginning in early Cretaceous times, and which increased the gradient 

 of the streams, with the result that they rapidly attacked the weathered 

 surface of the granite, carrying the resulting materials to the sea when 

 they were deposited as the clays and sands so characteristic of that time. 



As originally deposited the Cretaceous beds have usually been con- 

 sidered to have formed a belt along the southern New England coast 

 overlapping the Connecticut and Rhode Island shores. Erosion, how- 

 ever, subsequently so reduced these beds that the only remnants probably 

 remaining in early Pleistocene times were those in the western third of 

 Long island and possibly those on western Marthas Vineyard. On Block 

 island the Cretaceous beds were almost certainly below sealevel until 

 uplifted by the folding to be subsequently described, and it is not im- 

 possible that such was also the case at Marthas Vineyard, although it is 

 more probable that the Cretaceous at this point stood somewhat above 

 sealevel even before the folding. 



Mannetto and Jameco gravels. — In the record of the Ferguson well (page 

 372) the materials from the surface to a depth of 260 feet, or to about 240 

 feet below sealevel, are given as " gravel, boulders, and sand." In it 

 there is no mention of the thick, tough clays which occur to a thickness 

 of perhaps 100 feet in the clay pit, and which are found above sealevel at 

 other points on the island. As no driller could fail to recognize this, clay, 

 it is evident that it is cut out at the point where the well is located, 

 probably having been eroded by ice action, as explained on page 370. 

 The " boulders," except those in the thin surface layer of Wisconsin 

 till, can come only from the much older till known as the Montauk 

 drift, which probably has a thickness of not over 100 feet. This would 

 leave about 160 feet of gravels and sands between the old drift and the 

 basal blue clay (Cretaceous?), to which must be added the unknown 

 thickness of gravels removed by the same erosion that cut out the thick, 

 dark-colored clays. This is a considerably greater thickness than is nor- 

 mally found on Long or Block island, where the corresponding gravels 

 belong to a single formation — the Jameco — which on Block island, the 

 nearest point at which they are exposed, have a thickness of not over 50 

 feet. Even if twice this thickness was present on Fishers island, there 

 would still be about 50 feet of materials remaining unassigned. 



This interval may in part be made up of Cretaceous sands and grav- 

 els, but as these are commonly distinctly different from the glacial mate- 



