In yesterday’s comments someone mentioned the footbridge over Harris Gully (or the “Blue Danube” as the map has it). Here it is in a small, blurry image from 1926:
Coincidentally, this afternoon I came upon this ad in a 1950 issue of the Sallyport. It’s a lousy reproduction–the Sallyport was printed on newsprint at this time, so if you zoom in on it too closely it melts into a blur of dots–but it’s ironically the best view of the thing I’ve ever seen, as well as the only image I’ve ever found of the the gully being enclosed.
It also helps date this picture with a bit more certainty: