You’ll notice from the picture below that the line of rocks above the falls runs roughly parallel to the lines of the right side cliff face indicating that they were forced up In the same upsurge of the earth’s surface millions of years ago.
I’ve used dark grey thread (eg Anchor 400) to suggest wet boulders and used even darker grey to try to describe the facets of the rock. I’ve used black to delineate the crevices. Again it is just an impression that we need as light on wet Boulder can give a different impression each time you look at it.
I’m not sure if the large, seemingly single Boulder in the middle of the river is part of this this ‘Highlands-forming’ event or whether it has fallen from the hillside above like the smaller rocks to the right of it in the photograph. It appears to be of the same colour and density so I’ve used dark greys again and black to help give some shape to it and also because it is almost always wet with spray. I use black machine sewing thread because, for some reason, it’s not as intensely black as embroidery cotton and even then I use it sparingly as on this scale black can easily overshadow everything else.