At a distance of about 440 ft. beyond the Manhattan ledge, the material
at the bottom of the face changed suddenly to one in which the layers
of clay composed probably 98% of the whole. The sand layers were not
more than 1/16 in. thick and averaged about 2 in. apart. The surface of
the clay rose gradually for a distance of 40 ft. in Tunnels _A_ and _B_,
and 100 ft. in Tunnels _C_ and _D_, when gravel and boulders appeared at
the bottom of the shield. At that time the clay composed about one-half
of the face.

The surfaces of both the clay and gravel were irregular, but they rose
gradually. After rock was encountered, the formations of gravel and clay
were roughly parallel to the rock surface.

As the surface of the rock rose they disappeared in order and were again
encountered when the shields broke out of rock on the east side of the
Blackwell's Island Reef. East of the reef a large quantity of coarse
open sand was present in the gravel formations before the clay appeared
below the top of the cutting edge. In Tunnels _C_ and _D_ this was
especially difficult to handle. It appears to be a reasonable assumption
that the layer of clay was continuous across the reef. Wherever the clay
extended above the top of the shield it reduced the escape of air
materially. It is doubtless largely due to this circumstance that the
part-rock sections in the reef were not the most difficult portions of
the work.

While sinking the lower portions of the shafts the tunnels were
excavated eastward in the solid rock for a distance of about 60 ft.,
where the rock at the top was found to be somewhat disintegrated. This
was as far as it was considered prudent to go with the full-sized