Limestone is an unusual rock. Although it is hard and resistant to erosion it is also slightly soluble in rainwater. This characteristic feature of limestone creates special natural features and distinctive landscapes, known collectively as ‘karst’.
Limestone is made of calcium carbonate (CaCo3), which is soluble in acid. Rainwater is slightly acidic because it dissolves carbon dioxide from the atmosphere, forming a weak solution of carbonic acid. Rain therefore gradually dissolves limestone, over time forming features such as limestone pavements, natural bridges, sinkholes and caves.
Sinks and springs
When rainwater seeps down cracks in limestone it reacts with the rock and gradually dissolves it away. Over time, the cracks become wider and deeper, forming depressions known as sinkholes or shake holes. In limestone country, streams sink into the cracks and flow underground, leaving dry stream beds which only run after heavy rain. The streams reappear at the surface as springs where limestone gives way to other rock types.
Caves and caverns
North Pennine limestones are relatively thin and do not contain large cave systems like those in the Yorkshire Dales. However, there are several small caves in Weardale and Teesdale. The most significant cave system in the North Pennines is Knock Fell Caverns, Britain’s finest example of a phreatic maze cave – a cave system formed below the water table.
Limestone to lime
Lime-rich soils support good grassland and many plant species. Farmers have long recognised this and for centuries they have improved poor soils by spreading lime on the fields. Lime was made by burning limestone in limekilns, which can now be seen dotted all over the North Pennines.