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The character of the pencil work, with its careful delineation of features of the rock on which the tower stands, corresponds more closely to that of his sketchbook drawings than the generalised indications that underlie many of these large colour studies. See also Tate D41489 for another pencil study made on the spot on the back of one of these large watercolour studies, in that instance Tate D04176 (Turner Bequest LXX Y).
See A.J. Finberg, A Complete Inventory of the Drawings of the Turner Bequest, London 1909, vol.I, p.177.