Forms of Weathering on a Single Sandstone Outcrop in South Valley Park

The little sandstone eco-community in the photo above is an example of two different kinds of weathering.  The moss creates a form of chemical weathering, as the acid in the moss dissolves minerals in the sandstone.  The roots of the miniature tree, meanwhile, are a form of physical or organic weathering, its roots reaching down through slender cracks in the rock and expanding them. 

In the picture (above) we see moss, plant, and water weathering, and also erosion in the seasonal transport stream running down a narrow seasonal rill in the sandstone outcrop. The water penetrates through cracks in rocks, further contributing to the slow disintegration of the rock.

The photo above is a good example of mechanical weathering.  It is hard to discern from the picture, but the roots of this tree extend down into a joint in the outcropping.  Also, the action of frost wedging has eroded the sharp edges and corners of the outcrop, making the sandstone appear spheroidal. 

In the photo above we see a form of mechanical weathering called frost action or frost shattering.  The melted snow seeps into cracks and joints in the rock.  Then when the water freezes its mass expands, jarring the cracks a little wider.  Thus over time sections of rock split off into smaller rocks or gravel.  

The whitish portion of the sandstone is due to a form of mechanical rock decay called salt weathering.  The salt can come from many sources, including pollution and rising ground water.  Salt weathering tends to occur in arid climates.  
The decay is caused by the salt crystals precipitating in pore spaces in the rock, thus exerting pressure on the rock and causing mineral breakdown.   

This patch of green lichen is a form of both mechanical weathering and chemical weathering.  The former is caused by microscopic rootlets pushing down through grains in the rock; once there they swell and shrink, enlarging the gaps.  The latter is caused by the humic acids in the lichen breaking down the rock into the beginnings of soil.