Some beaches in New York City were formed by glaciers. Others, by Robert Moses. The sparkling sands of Coney Island and the Rockaways predated him, but much of the way we experience the city’s beaches today is a result of Mr. Moses’s four-decade reign p residing over the New York park system.

It started with Jones Beach, his first public work, which was born of a personal obsession and virtually dredged into existence out of a bunch of marshy sandbars.

In the spring of 1927, Mr. Moses ordered the largest floating dredges in the United States to be brought to New York City; crews worked for months pulling up sand and piling it into dunes over 10 feet high. The workers tried to break for the winter, when ice began to set in, but Mr. Moses insisted they press on. He made them set up camp on the dredges to keep the pumps running.