PLEISTOCENE LOWER BOULDER BED. 205 



Atlantic slope south of New York, and fills a blank otherwise unrecorded 

 in the geological history of the New England area. 



These Pleistocene deposits may be divided into several horizons upon 

 lithological and structural grounds.* 



Lower boulder bed. — The lowest member of these Pleistocene strata is a 

 rather discontinuous boulder bed, exposed at stations 28, 31, 32, 41, 

 between 40 and 47, and again between 54 and 59 in the Gay Head 

 cliffs, the principal features of which bed were described by Shaler 

 in 1889. 



A few points not observed in 1889 may be briefly mentioned here. 

 At station 28 the boulder bed rests upon the Miocene as a distinct 

 boulder patch with one large boulder, 8 feet long, 6 feet wide, and 3 feet 

 thick, estimated to weigh four tons. This boulder has within the last 

 two years slipped out of its original position. At station 31 there is a 

 small patch of the boulder bed containing rolled masses of the osseous 

 conglomerate along with boulders of diorite. At station 41, just south 

 of the Devil's Den, the conglomerate bed is exposed in an anticline and 

 again at the bottom of a syncline, both these structures being overturned. 

 At the top of the cliff, the boulder bed is the usual conglomeration of 

 granitite, diorite, and gneiss from the mainland, along with locally de- 

 rived materials, such as fragments of the greensand. At station 44, as 

 noted in Shaler's report, there was found in this bed a small boulder 

 of peridotite from Iron Mine hill, in Cumberland, Rhode Island, 60 

 miles away in the direction of transportation of debris in the last glacial 

 epoch. 



Between stations 54 and 59 the conglomerate lies almost horizon- 

 tally on the upturned and eroded edges of sands and clays of the Creta- 

 ceous series. That this relation is true unconformity and not due to 

 overthrust at the time of the Gay Head diastrophe is shown by the 

 abundant occurrence of pebbles of the Cretaceous sands and clays in the 

 conglomerate, which is at this point thoroughly cemented by the oxide 

 of iron. 



Between stations 33 and 35, in the quadrangular fold east of the Devil's 

 Den, the base of the Pleistocene is distinctly marked by the occurence 

 of small scattered subangular boulders of granitite resting on the light 

 green silicious sands, which have in this paper been referred to the 

 Pliocene. These boulders are much decayed, so that the granitite 

 breaks up readily under slight blows of the hammer. 



The decay of the pebbles and gravels of this horizon and the ceraen- 



* A synopsis of this Pleistocene history has been presented in the Seventeenth Annual Report 

 of the U. S. Geological Survey, pt. i, pp. 25-31. 



XXX— Bull. Geol. Soc. Am., Vol. 8, 1896 



