Weathered sandstone at Purakaunui Bay, The Catlins, Otago

When rocks are exposed to the elements they undergo a process called weathering, where the action of water, heat and chemical reactions gradually act to break down the rock structure. Many rocks contain tiny cracks or weaknesses along which water can penetrate, and this is often how weathering processes get started -- water seeps into the cracks, carrying dissolved oxygen that oxidises the surrounding rock, weakening its structure and making it more susceptible to other processes such as cycles of heating/cooling or wetting/drying. These stresses weaken the rock further, gradually opening up any weaknesses to allow more water to penetrate, causing still more weathering which eventually breaks the rock mass down into smaller pieces.

The sandstone in this photograph is in the early stages of weathering: a rectangular network of cracks in the rock has allowed water to penetrate which has caused iron in the rock to oxidise (rust) around the cracks, creating a striking pattern of intersecting lines. The surface of the rock is also exposed to salt water now and then which causes salt crystals to form in the top layer of rock, and these crystals gradually cause the rock surface to disintegrate. Salt water may pool in hollows in the rock and because the iron oxide concentrating around cracks may be quite hard and resistant to erosion, salt weathering may happen preferentially where the rock is softer -- such as in the areas away from any cracks. In this way a regular pattern of small eroded-out hollows can form on a rock surface over time, a process called 'honeycomb weathering'.