208 J. B. WOODWOETH — UNCONFORMITIES OF MARTHAS VINEYARD. 



on the highest parts of the island till is entirely wanting. In general the 

 average thickness of the moraine is not greater than that of till upon the 

 rocky uplands of the interior 100 miles back from the coast. The greater 

 bulk of the materials deposited at this stage of the last ice invasion are 

 in the glacial sand-plains forming the lowlands of southeastern Marthas 

 Vineyard. 



Unconformity between last glacial moraine and all older beds on the island. — 

 The topography of the highlands of Marthas Vineyard, as pointed out b}^ 

 Shaler in 1888, is largely due to erosion before the last glacial invasion. 

 Further study of this field shows that before the ice came upon the area 



Elevations. 

 MS00 ft&over. 



m zoo to 300 ft. 



100 to ZOO ft. 

 Under 100ft 



G 'a y Head. 



Figure 2.— Sketch Map of the Island of Marthas Vineyard. 



Showing the topography of the highlands of folded Cretaceous, Neocene, and early Pleistocene 

 strata mantled with morainal debris. The heavy dotted line represents the diverted drainage of 

 the upper North Tisbury valley. 



the streams had become adjusted to the structures formed by the Gay 

 Head uplift and the subsequent mantle of Tisbury beds. Moreover, the 

 stream occupying the northern valley opening upon the plains near the 

 village of North Tisbury underwent beheading and capture at two points, 

 where Roaring and Paint Mill brooks now escape through the northern 

 ridge of the island down steep slopes into Vineyard sound. The morainal 

 materials mantle over the erosion forms thus produced, showing conclu- 

 sively that a long period of meteoric denudation intervened between the 

 deposition of the Tisbury beds and the last ice advance (see map, figure 2) # 

 This erosion epoch, which may be known as the Vineyard interval, 

 corresponds to the long interglacial epoch which has been so often 



