The "caprock" of this waterfall is a very hard layer of black shale. You will see that the first drop of the waterfall correlates to a lighter layer of green and grey shale: this layer is much softer and more susceptible to erosion, so the waterfall formed here first, then continued to erode further down into the black shale which continues below the grey/green layer.
https://www.flickr.com/photos/linden_tea/3042199248/