The passage between Sarn Wallog and Sarn Gynfelin is shown on an historic chart with a line of bearing to Aberystwyth Castle. The water depths shown are 2-2 1/4 fathoms. Admiralty Sailing Directions dating to 1870 note that at distance of rather more than 1 1/4 miles from land the shoal 'deepens to 2 and 2 1/2 fathoms, forming an inner swatchway nearly half a mile wide. The mark for running through this channel is the monument on Pen-y-Dinas in line with the main part of the ruin of Aberystwyth castle S 1/4 W'.

Sources include:
Admiralty, 1870, Sailing Directions for the West Coast of England from Milford Haven to the Mull of Galloway including the Isle of Man, pg41
Historic Admiralty Chart 1411_A5, RCAHMW digital collections sourced from the UK Hydrographic Office and first published in 1842

Maritime Officer, RCAHMW, August 2014.