This episode was filmed in Salcombe, Devon (near Agatha Christie's hometown of Torquay), rather than on the actual Cornish Coast where the story is set.
His attention was specially directed to the study of the igneous and metamorphic rocks in Alpine regions and in various parts of England (e.g.: the Lizard in Cornwall, at Salcombe and in the Charnwood Forest), Wales and the Scottish Highlands.