Entrada Sandstone—the rock in which arches are
formed—was deposited here as sand more than 150 million years ago. Over time it
was buried by new layers, hardened into rock, and shaped by the powerful forces
of erosion.
- A series of uplifts and collapses caused
severe cracking in the 300-foot layer of buried Entrada Sandstone.
- When overlying rock layers eroded away, the
Entrada was exposed to weathering. Cracks slowly widened and parallel rock
walls, called fins, were formed.
- Rainwater continually dissolves the natural
cement that holds sandstone together. This process combines with the
pressure from water freezing in tiny cracks and causes the sandstone to
flake and crumble. Eventually, enough rock ralls out of a fin that an
opening is formed.
- These holes continue to erode and, in time,
the same forces of weathering that created arches will destroy them.
