GeoPic of the Week: Turtle Rocks at Petit Jean State Park

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“Turtle Rocks” are unique, mounded polygonal structures that resemble turtle shells. These features are found along the Arkansas River Valley in the Hartshorne Sandstone, a brown to light gray, massive, medium-grained sandstone deposited during the Pennsylvanian Period by ancient river systems. The processes that generate “turtle rocks” are not clearly understood. One explanation suggests that these features were created by a process known as spheroidal weathering, a form of chemical weathering that occurs when water percolates through the rock and between individual sand grains. These grains loosen and separate from the rock, especially along corners and edges where the most surface area is exposed, which widens the rock’s natural fractures creating a rounded, turtle-like shape. Additionally, iron is leached from the rock and precipitated at the surface creating a weathering rind known as case hardening. These two processes along with the polygonal joint pattern contribute to this weathering phenomenon.

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