Both Great and Little Mell Fells are unique among the fells of the Lake District by being composed of the Mell Fell Conglomerate, a sedimentary rock formed from deposits of sand and gravel in alluvial fans and braided river channels in a desert environment. The rock contains no fossils. The stones in the conglomerate came from the erosion of both the Borrowdale Volcanic Group and the Windermere Supergroup. The reddish-coloured rock appears to date from the late Devonian Period, sometime around 375 million years ago. The erosion of this conglomerate has formed the smooth and rounded outlines that are distinctive of the two Mell Fells. While there are no crags on Great Mell Fell, boulders of the conglomerate may be seen in places, especially on the north slope, and it may also be seen in the two stream beds.