Spheroidal Weathering
(c) 2005 Andrew Alden, licensed to About.com, Inc. This picture is also available as a free wallpaper image. (fair use policy)
Spheroidal weathering is a chemical process that affects many bodies of solid rock or large blocks. It's also called onion-skin weathering or concentric weathering.
In this basalt outcrop, groundwater penetrates along joints and fractures, loosening and decaying the rock layer by layer. As the process progresses into a block, the surface of weathering grows more and more rounded. I believe that this process accelerated when quarrying exposed the surface.
Spheroidal weathering is similar in appearance to the exfoliation that occurs on a large scale in plutonic rocks. That process, however, is mechanical rather than chemical.
Spheroidal weathering is documented on Mars, too. The picture below was taken by the Spirit rover near Missoula crater. It suggests that the basalt flows there were subject to alteration by groundwater. Both photos are about 25 centimeters across.
NASA image, frame 2P135521029EFF3000P2450L7M1. (fair use policy)
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