Hexagonal weathering in Navajo Sandstone. Caused as the rock is exhumed by erosion. Since it lithified a depth under higher pressure the lower depth/exposure causes the rock to expand slightly. The polygonal patterns form to accomodate the increased volume. http://giw.utahgeology.org/giw/index.php/geosites/article/view/62
When basalt cools the resulting contraction can create hexagonal weathering. Navajo Sandstone can also show hexagonal weathering; but, it is caused by expansion of the rock as it decompresses when exposed.
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