The Race Underground: Boston, New York, and the Incredible Rivalry That Built America’s First Subway by Doug Most is an excellent book. I can’t imagine how much lumber and steel when into building these two subways. Below is an excerpt about Manhattan.
FOR AN ISLAND THAT STRETCHED only seven miles long, Manhattan presented an unusual array of engineering nightmares. The softer rock had a grainy texture, almost like sugar, and was known as dolomitic marble. It was mostly at the island’s northern end. The more solid, challenging rock, Manhattan schist, was dangerous to bore into because it could fracture more easily and collapse on workers. It was at the southern portion. The island’s widest point stretches only about two miles, from river to river, near 125th Street, but just north of that it narrows quickly like a soda bottle. And though it appears flat to pedestrians, most of the city’s terrain is actually quite rolling and rocky, especially between Twenty-Third Street and the northern tip of Central Park. North of 110th Street the island has a steep rise in elevation that tops out at 268 feet above sea level, Manhattan’s peak, in Inwood near Fort Washington Avenue and 185th Street. In this northern portion, there were two hurdles for major digging. The bedrock of Manhattan has two major cracks, or fault lines, one at 125th Street and the other further north near what is now Fort Tryon Park, that have existed for millions of years. Although gravel, sand, and silt deposits have mostly filled them in, the faults weaken the structure of the bedrock hundreds of feet beneath the sea.