While the main ridge of high land, that lay on the left of the line in Westchester county, controlled the general location, numerous spurs of ridges, of various forms and extent, bounded by deep ravines, rendered it impossible to avoid deep cutting and frequent tunnelling, through ridges, and heavy rilling in the valleys; leaving a very limited proportion of the line in favorable cutting. On the island of New York, with the exception of about one mile from Harlem river, there was no leading ridge to control or guide the location ; which rendered it more difficult to decide on the conflicting claims of different routes.

There are on the line sixteen tunnels, varying in length from 160 to 1 263 feet, making an aggregate length of 6841 feet. The height of the ridges above the grade level at the tunnels ranges from 25 to 75 feet. There are in Westchester county twenty-five streams crossing the line of aqueduct, that are from 12 to 70 feet below the grade line, and from 25 to 83 feet below the top covering of the aqueduct. The most prominent of the valleys, are Loun sherry's, Indian brook, Sing Sing kill, Mill river, Jewel's brook, and Saw Mill river the foundations of ;

which are in no case less than 40 feet below the grade line, or 53 feet below the top covering of the aqueduct. Besides those above mentioned, there are numerous brooks and valleys of less depth, requiring culverts, and artificial foundations to support the aqueduct. The level of mean tide at Harlem river is 120.66 feet below the grade line of the aqueduct, of which a more particular description will be given hereafter. On New York Island there are several deep and abrupt valleys, along the precipitous rocky hill that bounds the southerly shore of Harlem river.