The access road to the park is on the old railroad bed (called right-of-way or ROW for short) for the Orange & Alexandria Railroad (now Norfolk Southern RR). Today the railroad runs on the tall trestle bridge next to the dam.
Note the old O&A Railroad right-of-way (ROW) "fills" and "cuts". A "fill" is constructed with earth removed from "cuts". "Cuts" are man-made valleys dug through natural ridges to allow a level path for the railroad. You can see the active railroad in winter toward the south.
The cache is the size of a shoe box.