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In the Windows area, you can see many stages of arch formation. Look closely. Some arches are hard to see because of rock walls behind them. 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. 1) A series of uplifts and collapses caused severe cracking in the 300 foot layer of buried Entrada Sandstone. 2) When overlying rock layers eroded away, the Entrada was exposed to weathering. Cracks slowly widened and parallel rock walls, called fins, were formed. 3) Rainwater continually dissolves the 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 falls out of a fin that an opening is formed. 4) These holes continue to erode, and in time, the same forces of weathering that created arches will destroy them.