Limestone is a sedimentary rock, formed approximately 350 million years ago on the bed of a tropical sea.  The horizontal layers of rock are called bedding planes.   Vertical cracks in the rock caused by huge earth movements are called joints.   Limestone is a permeable, pervious rock because it allows water to pass through it by travelling along the joints and bedding planes.

Winnats Pass is an example of a limestone gorge or dale.  At the end of the last Ice Age meltwater from the ice sheet carved out the valley in the limestone.  The water was unable to drain away through the limestone because the ground was still frozen.   As the ground thawed, the water was able to percolate into the limestone and erode as a river would,  leaving the dry valley which can be seen today.