First Avenue. The ancient creek apparently followed the course of a
valley in the rock, the valley having become filled to a considerable
depth with very fine quicksand. This concurrence of depressions in the
rock surface with the watercourse shown on Viele's map was noted in so
many places and the difficulties of construction were so serious at
these places, that a section of the map showing the old topography along
and adjacent to the station and tunnel lines is reproduced in Plate IX.

[Illustration: PLATE XI.--Long Island Shaft. Lines _A_ and _B_]

The unfavorable conditions developed at Fifth Avenue affected both the
construction of the tunnels and the maintenance of adjacent buildings.
It would be necessary to construct the tunnels in open cut for a large
part of the way westward, causing serious inconvenience to the public;
the buildings were mostly of the older class, founded in earth, but
there were several modern high buildings with foundations in the same
material; some of these had been built since the tunnels were planned.
In view of these added risks and the increased cost of construction, the
value of the three-track construction was reconsidered, and two
important changes were made in the plans. The first of these was to
continue the twin tunnel westward to Sixth Avenue in 32d Street, and to
a point 180 ft. west of Sixth Avenue in 33d Street; the twin tunnel
being 9-1/2 ft. less in height than the three-track tunnel and 9 ft.
narrower, the change reduced the difficulties considerably. Where the
three-track tunnel was thus eliminated, there was no longer objection to
a steeper grade, so that, going eastward from the station, a grade of
0.8% in 33d Street and 0.9% in 32d Street was substituted for the
original 0.4% grade. From the west line of Fifth Avenue eastward short
sections with descending grades of 0.3% connect with the original 1.5%
grade near Madison Avenue. The effect of these two changes--type of