DELAWARE RIVER AND BAY
SCHOONER LEDGE
A few miles below the city
of Philadelphia in the river is a protrusion of rock from the
Pennsylvania shore, only two rocks, but one is narrow and runs
longitudinally with the main ship channel, 700 feet long and 300 feet
wide. The upper end is dark granite and the lower end is Illinois
rock. A buoy , panted red with white letters marked 'rock' is placed
on a direct range line of Schooner Ledge light station, and there is
23 feet of water covering it at low tide. It lies opposite Thurlow
about three or four hundred yards from shore, below Miffin Bar. The
ledge has proven treacherous to a number of vessels in the past,
knocking holes in the bottom of passing schooners.
For many years, the ship
channel was too narrow here for vessels of considerable draft to pass
at low tide.
To the government it is a
difficult engineering improvement problem. U. S. engineers have two
suggestions, blow it up as they did Hells Gate or divert tidal
forces further out n the river. Either would have been a immense
undertaking and the government never considered the improvement.
Improvement was left to the
city of Philadelphia, so in August 1896 work began by authority of
the an ordinance of city
council and Survey Bureau engineers addressed themselves to removing
the upper surface of the rock to a depth of 26 feet at low tide, the
uniform depth of the channel to make the harbor navigable to vessels
of heavy draught.
The first task was to
'chart' an elaborate system of sounding for depths by city engineers
who used the abandoned plant of Wellman Iron and Steel Company,
below Chester, as the 'rendezvous' for the engineer corps. On the
second floor of the main office building the draughtmen took up
quarters and began the preparation of charts.
Surveyors went to work with
their instruments to plot out the area of the submerged rock in
sections and found the ledge to be 2000 feet long and an average of
300 feet wide. Every foot of the area was plotted off in sections of
80 x 5 feet. After months of laborious applications, every square
foot of the rivers surface was thus known with a minuteness and
detail which seemed a marvelous achievement.
A novice would wonder how
it was possible to mark to surface of a river with a swift tide into,
geometrical blocks of unfailing precision. This is how the surveyors
did it.
On shore, a series of
poles, 80 feet apart, each marked in black and white stripes to
show height of tides in feet. The poles also gave the range of the 80
foot sections. An 80 foot float from which soundings were made was
accurately place by transit and anchored and by lateral measurement
the five foot area was marked off.
Simple when understood.
Source Wilmington Morning
News, Friday , July 29, 1898