This basaltic outcrop near Bergama , Turkey, is an example of spheroidal weathering . With this type of erosion the basalt's subsurface is already fractured by joints or cracks. Blocks, defined by the joints , become rounded as the concentric layers forming the block surfaces are separated. The shape and size of boulders such as this one are determined by the spacing of joints in the intact rock mass. Spheroidal weathering is common in basalt and granite and is also found in dolerites andesite and some sandstones . Photo taken on August 10, 2013.