This page contains three distinct sketches. The two uppermost drawings are very rough, and even the correct orientation is difficult to ascertain. They appear to represent a dark, rocky tunnel and therefore may depict the interior of one of the galleries cut through the mountains on the road to the Simplon Pass, for example the famous Gallery of Gondo, see folio 91 (D14320; Turner Bequest CLXXIV 90). A comparable study can be found in the Passage to the Simplon sketchbook (Tate D16919; Turner Bequest CXCIV 15a).
The details at the bottom of the page have been drawn with the sketchbook held horizontally and represent part of the landscape composition on the opposite sheet of the double-page spread, see folio 91 verso (D14321; Turner Bequest CLXXIV 90a). The subject is a view of the Ponte Alto, near the Gallery of Gondo.
For a general discussion of Turner’s journey to the Simplon Pass see the introduction to the sketchbook, and also the introduction to the Passage of the Simplon sketchbook (Tate, Turner Bequest CXCIV).