| Boulder Formation | |
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This hoodoo rock near Arch Rock shows how the great boulders of the park are formed. It's a long ridge of solid rock, rising slightly to the right. Picture it deeply covered with soil, many centuries ago. Groundwater has softened and altered the bedrock through chemical weathering. The minerals of the granite turned to clay minerals and quartz sand. This process acts strongly on joints, tending to open them up and widen them. Rocks exposed at the surface weather into blocks by a different method.
The buried ridge was eroded more deeply on its highest parts, so that the massive granite along its top was cut off into free-floating boulders. When the climate changed to today's desert conditions, the sand and clay was eroded away and the free boulder on the right side settled down where it sits today. Sometimes such a boulder is left improbably balanced atop another.
(c) Copyright 2002 Andrew Alden, licensed to About.com, Inc.
