BERKEY, GEOLOGY OF SOUTHERN MANHATTAN ISLAND 251 
number of rock varieties, but in this and for the present purpose there is no 
advantage since structurally the three great formations behave as units. 
Glacial Drift 
Covering the whole series is the loose mantle, known as the Glacial Drift, 
which exhibits a complex and variable structure. All varieties from char¬ 
acteristic till to perfectly assorted sands and clays are to be found. The 
whole cover, so far as this study is concerned, serves chiefly as an obscuring 
mantle. Not only is the rock underneath hidden, but the structural rela¬ 
tions are to an equal degree obscured. 
Structural Relations 
The three large formations have each been so much modified by meta¬ 
morphism and have suffered so much disturbance in the process, standing 
in such marked contrast with any other formations of adjacent areas, that 
they are commonly thought of as a single series or simple succession of 
formations; but of this there may be some question. There is, however, 
no doubt that the Fordham Gneiss is the oldest formation known in southern 
New York, that it is in part a metamorphosed sediment arid in part made up 
of igneous intrusions cutting the older sediments in a most complex way and 
occasionally forming much more than half of the rock. There are typical 
quartzite beds and recrystallized limestone layers here and there in the 
banded Fordham. These are sufficiently clear evidence of sedimentary 
origin for those portions, but it is equally clear that other belts or areas are 
igneous, and between the two there is such variety and such extensive pene¬ 
tration, segregation, inclusion and recrystallization that it is wholly impossi¬ 
ble to draw the lines of origin very close. 
There is sufficient evidence in adjacent districts to establish the fact 
that there must be a considerable time break between this lowest member 
(the Fordham Gneiss) and the next overlying Inwood Limestone. Whether 
it is an unconformity of large value or more of the nature of an overlap is 
not determinable in this region, because of the closely crumpled and infolded 
relation that they now exhibit. The Inwood Limestone and the Manhattan 
Schist are mutually conformable, with some development of inter-bedding in 
the transition zone. Only the drift or, rarely, residuary matter from decay 
lies above these formations. 
Nothing older or lower than the Fordham Gneiss is known on Manhattan 
Island, and therefore its depth is indefinite. The Inwood Limestone may 
be regarded as between 700 and 900 feet thick at its best development, and 
the Manhattan Schist above is of great but unknown thickness. 
