61 
The Central Area. 
This area embraces all that part of Nantasket east of the rail- 
road and south of Atlantic and Centre Hills. The geological 
relations of the central area to the preceding or coastal area can 
be most easily traced in Willow Ledge Hill, which belongs in 
part to each. The summit and main mass of this elevation is a 
nearly square block of melaphyr and tuff. These rocks have 
been described in the preceding section, and the limits of this 
block are sharply defined on the map by the bounding fault- 
lines. The similar block of conglomerate on the west lies at a 
lower level, and its outerops are separated from the volcanic 
block by a small artificial pond. 
Both of these blocks are separated from the narrow ridge of 
conglomerate between them and Hull Street by the well-marked 
east-west depression occupied by Spring Valley Road. This 
depression is an evident fault-line, for the beds of conglomerate 
are so nearly horizontal that they must abut directly against the 
melaphyr; and the contact is undoubtedly oblique to the strike 
of the conglomerate. As indicated on the map, this is one of 
the great faults of the Nantasket region, traversing the entire 
area, from Strait's Pond to Weir River Day. Few of the dis- 
locations of this district are more obvious, and none are of 
greater structural importance. East of Valley Beach Avenue 
it is marked by the well-defined escarpment terminating the 
melaphyr of Centre Hill on the south, the conglomerate lying 
at а lower level here, under a meadow ; while to the westward 
it coincides with the similar escarpment along the north side of 
Melaphyr Plateau, and, crossing the bay, defines approximately 
the abrupt northern shore of the western area. This line of 
fracture is thus cl arly the natural boundary line between the 
Coastal and central areas. But when we pass to the considera- 
tion of the direction and amount of the displacement along this 
line, and the stratigraphic relations of the two areas, a much 
more difficult problem confronts us. The escarpments, which 
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