BERKEY, GEOLOGY OF SOUTHERN MANHATTAN ISLAND 257 
point where solid core could be secured is drift cover. Yet a little critical 
examination of materials will show that in some places the drill penetrates 
a thick layer of residuary (decayed) matter belonging to the formation 
below the drift. Occasionally such disintegration is so complete that core 
cannot be saved for a depth of 50 to 100 feet after leaving all traces of the 
drift cover. Recent explorations and those now in progress confirm this 
view. In addition, it is clear that such decay is more pronounced on the 
limestone belts and along known crush-zones, and these are sometimes very 
narrow. 
To increase still more the uncertainty and imperfection of many records, 
it is not at all unusual to strike large bowlders in drilling which give every 
apparent behavior of rock ledge. Where the only thing sought is depth to 
bed rock, therefore, many such finds are reported as rock floor. Some of 
these show, jyv the material recovered from them, that the rock is not of local 
type, but this is usually overlooked because of lack of critical knowledge 
of the variations allowable in the local formations. 
It is the writer’s opinion, therefore, that it is impossible to construct a 
map showing the topography of the rock floor of southern Manhattan. 
The bulletin by Professor William Herbert Hobbs 1 is a good attempt at such 
a study, but it is certain that the contour lines of southern Manhattan are 
very different south of Twenty-third Street. The map is of great service, 
however, for the larger number of records of depth to rock given than hereto¬ 
fore and for the handy form in which they are available for combination 
with accumulating data of more detailed and more accurate character. 
Tabulated Records of Borings in Southern Manhattan Island 
The information combined in the tables which follow has been accumu¬ 
lated in a systematic inspection of every drill core that could be seen from 
southern Manhattan and the other side of East River. Most of the material 
is the property of the various New York City departments, the Public 
Service Commission and large transportation companies as noted below. 
Scattering groups of cores represent private operations for deep Avells, or 
plunger elevators, or tests of foundations or railroad tunnels. Many of 
these have been tabulated as to location and depth to rock by Hobbs, 2 but 
in his discussion the type of rock in each case was accepted as originally 
identified at the time the borings were made. Some of these are old, made 
1 U. S. Geological Survey, Bull. 270. Plate I. 1905. 
2 Op. cit. 
