DELAWARE RIVER AND BAY
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