Surface features of a limestone landscape
LIMESTONE PAVEMENT
Limestone Pavements are bare limestone rock surfaces composed of slabs of rock (clints), separated by variable-width vertical cracks (grikes) . These grikes have developed by weathering and enhanced solution along joints. Composed principally of calcite, limestone is vulnerable to chemical weathering by water (solution) and dissolves easily In the Yorkshire dales exposure of very extensive pavements may be partly an example of the stripping of soil by moving ice during the last glaciation period more than 10,000 years ago.
SINK HOLE ( SWALLOWHOLE )
Surface and rain water do not flow far on exposed limestone, but infiltrate rapidly into the rock and soil. Where a joint or intersection of joints has been greatly weathered or dissolved water can pass down through the limestone . A stream travelling over an impermeable rock will very quickly disappear when it has to travel over limestone. These sinkholes can be many metres deep leading down to a series of subsurface features.
SHAKEHOLE
A shakehole is a depression in the limestone landscape. In some limestone areas there is a covering of boulder clay about two or three metres thick. Shakeholes are formed where surface water washes the boulder clay down into cracks or fissures in the limestone under the boulder clay. They are usually found in groups. Cavers have to dig the boulder clay out to see if there is a pothole of any size underneath.
SPRING ( RESURGENT)
When a stream disappears it travels underground through a complex series of caves and eventually works its way down to a level of impermeable rock or until it reaches the top of the water table. The stream travels along the surface of the impermeable till it reaches the surface as a spring. Where limestone lies on top of an impermeable rock along a valley there can be several springs formed along the intersection of the two rocks. This is a spring line.
GORGE
During the meltwater phase of the last ice age underground streams eroded huge caverns as huge volumes of water travelled through the limestone. Sometimes the roofs of these caverns can collapse exposing the "underground stream" in a very steep sided gorge.
SCAR
During the last ice age huge ice sheets scraped away the soil covered spurs in many valleys in the Yorkshire Dales. Steep cliffs of bare rock were exposed. Because these scars are more liable to frost shattering and other forms of erosion they usually have scree slopes of broken rock below them.
DRY VALLEY
During the last ice age the limestone was frozen to great depths. When the ice melted it carved out valleys over the frozen rock. When the limestone thawed out the surface water was able to infiltrate down through the rocks and the dry valleys were left with no surface water.
BEDDING PLANE
Carboniferous limestone is a sedimentary rock formed in the Yorkshire Dales around 300 million years ago. These rocks were laid down on the sea bed and made up of layers of ancient corals and skeletons of sea creatures living in the shallow seas at that time. The bedding planes are the horizontal layers formed as the rocks were compressed under deposits formed above.
ERRATIC BOULDER
The ice sheet which covered the area carried with it huge boulders from as far away as Scandinavia. When the ice melted these huge boulders were left sitting on the surface. They are called erratics because the are totally different from the bed rock of the area.
