This appears to be an imaginative composition for a scene from ancient history or myth. As Finberg noted, there is a ‘classical portico’ in the middle distance, with hills beyond and possibly to the right, while a jagged leaning form may indicate coastal rocks across a bay. There appear to be seated figures among the trees in the foreground, and the overall effect is loosely comparable with that of Turner’s large oil painting The Bay of Baiae, with Apollo and the Sybil, exhibited at the Royal Academy in 1823 (Tate N00505).1
Martin Butlin and Evelyn Joll, The Paintings of J.M.W. Turner, revised ed., New Haven and London 1984, pp.139–40 no.230, pl.233 (colour).