Underground flow of water is more common than surface run-off in limestone areas. Why?
In limestone areas, underground flow of water is more common than surface run-off because limestone rocks are permeable, thinly bedded and highly jointed and cracked. This allows water to percolate well through the rocks rather than flowing over the surface.
The chemical process of solution is very active in limestones which are rich in calcium carbonate. Surface water dissolves the limestone, forming sinkholes and swallow holes that allow water to enter underground. After going down vertically to some depth, water flows horizontally through bedding planes, joints or through the materials themselves.
The continuous dissolution of limestone along bedding planes creates caves and underground passages, further facilitating the underground movement of water.
Explanation
The textbook explains that water percolates well when rocks are permeable, thinly bedded and highly jointed. In limestone areas, the chemical process of solution is active - surface water and groundwater dissolve calcium carbonate, creating sinkholes and swallow holes. Water percolates down through cracks and joints, then moves horizontally along bedding planes. This creates caves and underground passages, making underground flow dominant over surface runoff in karst topography.