A steephead valley, steephead or blind valley is a deep, narrow, flat bottomed valley with an abrupt ending. Such valleys arise in limestone or karst landscapes, where a layer of permeable rock lies above an impermeable substrate such as marl. They are created by a stream flowing within the permeable rock and eroding it from within, until the rock above collapses opening up a steep narrow valley which is then further eroded by the stream running across the impermeable valley floor.