Black Cap is part of a north-south-trending ridge known as the Green Hills. The next summit to the north on the ridge (towards Kearsarge North) is Hurricane Mountain, and to the southwest is Middle Mountain. Black Cap is located entirely in the watershed of the Saco River, which flows to the Gulf of Maine (Atlantic Ocean) near Saco, Maine. Tributaries of the Saco that flow off the mountain include Artist Brook to the west, Mason Brook to the south, White Lot Brook to the southeast, and Weeks Brook to the east.