Groundwater and karst

Groundwater percolates easily where rocks are permeable, thinly bedded and well jointed, then flows sideways along bedding planes and joints. Physical removal by groundwater does little; its main work is chemical: solution and precipitation (deposition), in rocks rich in calcium carbonate such as limestone and dolomite.

• Such landscapes are called karst topography, after the Karst region of limestone in the Balkans, beside the Adriatic Sea.

Erosional karst landforms:

• Swallow holes: small to medium, round, shallow hollows dissolved in the limestone surface.
• Sinkholes: round at the top and funnel-shaped below, from a few sq m to a hectare and from under half a metre to 30 m or more deep. Solution sinks form by solution alone; collapse sinks (also called dolines) form when the roof over a cave falls in. Solution sinks are more common. Soil-covered sinkholes can look like harmless shallow pools. Surface water often vanishes down them and flows underground, re-emerging from a cave downstream.
• Valley sinks (uvalas): long trenches where sinkholes and dolines join.
• Lapies: a maze of points, grooves and ridges as the surface is eaten away, especially along joints; they may wear into smooth limestone pavements.
• Caves: form where limestone lies between other rocks (shale, sandstone, quartzite), or in thick, massive beds. Water moves along bedding planes, dissolving long gaps. A cave open at both ends is a tunnel.

Depositional karst landforms (inside caves): calcium carbonate dissolves easily in carbonated water (rainwater with carbon dioxide) and is deposited again when the water evaporates or loses its carbon dioxide while trickling over rock.

• Stalactites hang from the roof like icicles, broad at the base and tapering.
• Stalagmites rise from the floor, built by drips falling from above; they may be columns, discs or have a small crater-like top.
• The two may fuse into columns and pillars.

Example (new): the Borra Caves in the Eastern Ghats of Andhra Pradesh are a karst cave system, with dripstone shapes formed over a very long time in limestone.