A gorge is formed as a result of a change in rock type at a waterfall. At the base of a waterwall the pressure and hydraulic action of the falling water causes the softer rock which is underneath  to erode forming a plunge pool. Over time this continues to erode backwards, undercutting the harder rock which is on the surface. Eventually the over hang is so great that the hard rock can no longer support its own weight and therfore collapses. This process continues and is repeated over hunderds of years causing the waterfall to cut back into the landscape, leaving a steep sided valley known as a gorge.