The first institute building opened in 1906 at the foot of 66th Street, followed in 1910 by other structures. Exact sequencing of the ponderous retaining walls along the drive is difficult to determine, but the one from 64th to 65th Street was not there in 1911, when The New York Times reported that John D. Rockefeller Jr., had been summoned to court because a 30-foot cliff had collapsed onto Exterior Street.

Rockefeller agreed to remedy the situation with a wall that remains as the southernmost section, the one with fabulously craggy stonework and giant iron doorways. The stones in the walls are set in a random pattern, and those used for the rocky arches over the windows, called voussoirs, are made up of several skinny sections, which somehow gives them a threatening air, suggesting witch’s fingers.

The wall may have been designed by York & Sawyer, known for banks as well as several hospitals, or by Shepley, Rutan & Coolidge; both firms were working for Rockefeller University at that time. York & Sawyer was known for massive stonework, and the Shepley firm was the successor to Henry Hobson Richardson, who made a specialty of craggy rock-faced facades. This may be wishful thinking: perhaps the rocky arches are from the hand of an anonymous engineer.

The retaining walls gave Exterior Street a secluded atmosphere, and in 1929 The New York Times profiled “one of the city’s least known thoroughfares,” with “Sleepy Exterior Street at Last Comes to Life” as the headline. “Here, within easy hailing distance, pass the boats bound upstream. Churning tugs with romantic names scuttle past, followed by black, mysterious looking craft. A rowboat maneuvered by a boy and two girls noses along the edge of the stream.”