
Physical, chemical and biological, London

In the example below, a gravestone made of thinly-bedded sandstone (the type often used as paving slabs in older towns and cities) has been damaged badly. Water seeped downwards between the vertically-oriented layers at the top of the stone, froze and expanded, eventually causing the layers to split and break off. This is physical weathering by freeze-thaw action.
Meanwhile, chemical weathering is helping to break down the cementing material between the sandstone grains, aided by biological weathering (from acids released by algae and lichen on the surface), so the rock surface is gradually crumbling away.
