The Horton Flags (shale and greywacke, a coarsely-sorted sandstone) in the valley originated as material cascading over the edge of a Silurian continental shelf ~430 million years ago. About 100 million years later in the Carboniferous era, those deposits had lithified to become the bed of a shallow tropical sea, on which organic material was fossilised to form the overlying Great Scar limestone. Subsequent erosion cut the valley of Crummackdale, re-exposing the older rock.
The distinct boundary between the two units can be seen in the cliff of Studrigg Scar on the other side of the valley. Unfortunately, these images show it poorly; try this photo of the same unconformity expressed in Thornton Force, ~10 km away on the far side of Ingleborough (though technically, that exposure of the pre-Carboniferous basement unit is ~70 million years older).
You might have spotted Pen-y-ghent in the background.
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