High Force waterfall is formed in the upper course of
the River Tees, where a layer of hard rock (Whinstone)
overlies soft rock (Carboniferous Limestone). The force
of the falling water (hydraulic action) has eroded the
softer rock to form a plunge pool and helped to wear
away the backwall forming an overhang. Eventually the
overhang will become too heavy and will fall into the
plunge pool causing the waterfall to erode towards the