Groundwater

The Work of Groundwater

Groundwater is usually thought of as a resource, but here the interest is different. The focus is on what groundwater does to the land — how it erodes landmasses and shapes landforms over long periods.

Water percolates down into the ground when the rocks are permeable, thinly bedded, and highly jointed or cracked. After moving vertically to some depth, it begins to flow horizontally through bedding planes, joints, or even through the rock material itself. It is this downward and horizontal movement that causes erosion. But the physical or mechanical removal of material by moving groundwater is insignificant in shaping landforms. That is why the results of groundwater's work cannot be seen in all types of rocks.

In rocks like limestones or dolomites, which are rich in calcium carbonate, both surface water and groundwater act through the chemical processes of solution and precipitation to develop a wide variety of landforms. These two processes are active in limestones or dolomites that occur either exclusively or interbedded with other rocks. Any limestone or dolomitic region showing typical landforms produced by groundwater through solution and deposition is called Karst topography, named after the typical topography developed in the limestone rocks of the Karst region in the Balkans, adjacent to the Adriatic Sea. Karst topography is characterised by both erosional and depositional landforms.

Erosional Landforms

Swallow holes, sinkholes, lapies and limestone pavements. Small to medium-sized, round to sub-rounded shallow depressions called swallow holes form on the surface of limestones through solution. Sinkholes are very common in limestone and karst areas. A sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom, with sizes varying in area from a few square metres to a hectare, and with depth from less than half a metre to thirty metres or more.

Some sinkholes form solely through solution action — these are called solution sinks. Others may start as solution forms first, but if the bottom of a sinkhole forms the roof of a void or cave underground, it might collapse, leaving a large hole opening into a cave or void below. These are called collapse sinks. The term doline is sometimes used for collapse sinks. Solution sinks are more common than collapse sinks.

Quite often, sinkholes are covered up with a soil mantle and appear as shallow water pools. Anybody stepping over such pools would go down, just like in quicksands in deserts. Surface run-off often simply goes down swallow and sink holes, flows as underground streams, and re-emerges at a distance downstream through a cave opening.

When sinkholes and dolines join together — because of slumping of materials along their margins or due to roof collapse of caves — long, narrow to wide trenches called valley sinks or uvalas form. Gradually, most of the surface of the limestone is eaten away by these pits and trenches, leaving it extremely irregular with a maze of points, grooves and ridges called lapies. These ridges form especially due to differential solution activity along parallel to sub-parallel joints. The lapie field may eventually turn into somewhat smooth limestone pavements.

Caves. Cave formation is prominent in areas where there are alternating beds of rocks — shales, sandstones, quartzites — with limestones or dolomites in between, or in areas where limestones are dense, massive and occurring as thick beds. Water percolates down either through the materials or through cracks and joints, and moves horizontally along bedding planes. It is along these bedding planes that the limestone dissolves, and long and narrow to wide gaps called caves result. There can be a maze of caves at different elevations, depending upon the limestone beds and intervening rocks. Caves normally have an opening through which cave streams are discharged. Caves having openings at both ends are called tunnels.

Depositional Landforms …