The northern boundary of the park is formed by the seismologically active Dyckman Street Fault. The fault creates a valley separating Fort Tryon Park from Inwood Hill Park to the north. In precolonial times, a Native American road ran within this valley from the present-day intersection of Broadway and Dyckman Street to a settlement on the Hudson River. The valley formerly contained an inlet named Little Sand Bay, which flowed into the Hudson River to the west. As recently as 1989, activity of this fault caused a magnitude 2 earthquake.