378 



M. L. FULLER — GEOLOGY OF FISHERS ISLAND 



figures 7 and 8, up to 20 feet of clay is exposed. In the section shown in 

 the latter figure the clay is 12 feet in thickness and consists of alternat- 

 ing layers of gray and chocolate silts, weathering to a somewhat darker 

 color and dipping about 45 degrees to the north. At the locality of fig- 

 ure 7 the clays are exposed to a thickness of more than 20 feet, and occur 

 in a broad trough coincident with a surface valley. In bpth localities 

 they are overlain by grayish clayey sands into which they grade. 



Clay is said to have once outcropped near Clay point, on the north shore 

 of the island, but there is now nothing but Wisconsin till to be seen in the 

 low bluffs in that vicinity. In the base of the bluff forming the headland 

 three-quarters of a mile northeast of the north end of Isabella beach 



there is exposed a few 

 feet of greenish-gray 

 clay, free from pebbles 

 and similar to parts of 

 the Gardiner, but its 

 relations could not be 

 determined (figure 9). 

 At no other points was 

 clay seen, the hills ap- 

 parently being com- 

 posed essentially of 

 sand and fine gravel. 

 Any storm, however, 

 is liable to materially 

 alter the bluff condi- 

 tions, and if the clay 

 is above sealevel it 



50' 



Figure 



-North-south Section through the Bluffs at Isabella Beach 



a, grayish sandy clay ; b, gray clayey sands ; c, clayey sand weatlv 

 ering light grayish green; d, clay with alternating gray and may be exposed at any 

 chocolate layers weathering dark. , • 



Jacob sands — Use of term. — -This term is applied to the fine gray sands 

 or the equivalents which overlie the Gardiner clay on Long island east- 

 ward to the New England islands and cape Cod. The name is taken 

 from Jacob hill, a high point on the north shore of Long island 8 miles 

 northeast of Riverhead, near which the sands are well exposed. 



Conditions of deposition. — Whether the gradual change from plastic 

 clays to the exceedingly fine, clay-like, quartz sands of the Jacob forma- 

 tion was due to an uplift, so that the water became shallow enough for 

 semi-littoral conditions to prevail, whether the change was due to the 

 deposition of the finer materials from the outwash of an advancing ice- 

 sheet, or whether it was due to a combination of the two factors can not 

 be definitely stated. The fact that the Jacob sand, like the Gardiner 

 clay, gave place in turn to coarser materials, first sand then gravel and 



