What is it?
A gorge is a deep narrow valley with steep;almost vertical sides. The river flows down resistant rocks and it erodes vertically faster than the sides of the valley can be worn away.

How it is developed:

A gorge is developed when a waterfall retreats upstreams. As the river flows to the edge of the resistant rock of limestone, it increases velocity as it plunges vertically down, excavating a plunge pool at the base. Armed with rocks debris, the backsplash of the turbulent water at the base of the waterfall undercuts the cliff face of less resistant sand and shale found below the limestone. As the cliff face is being eroded, the overhanging cap rock loses support and collapse.