Hole-in-the-Rock is a series of openings (tafoni)eroded in a small hill composed of bare red arkosicconglomeriticsandstone. The sandstone was first formed some 6–15 million years ago from the accumulation of materials eroding from a Precambrian granite, long since eroded away. The tafoni are thought to have been eroded by water. An open, shelter-like chamber in the face of the formation communicates with the rear of the formation via a hole eroded completely through the rock. Another substantial opening exists in the "ceiling" of the chamber.
There is evidence that the Hohokam, early inhabitants of the region,[2] used and recorded the position of sunlight shining through the latter opening to mark the seasons—notably the equinoxes and the solstices, which were marked by carving a slick area (metate) in the rock. Other positions were marked with boulders.
The formation is a popular attraction in the park. The openings and main chamber near the summit are easily accessible via a smoothly ascending path that passes behind the hill. It is also possible to climb the face of the hill to reach the chamber. While popular, the trail can be dangerous.[3] The chamber provides a good view of the city of Phoenix west of the park.[4] A nearly constant wind blows through the openings in the rock.
