The shoal, forming another Sarn, is shown on an historic chart extending some 750m offshore. Admiralty Sailing Directions note that 'there is a shoal patch of foul ground, the Clarach, stretching off for nearly half a mile from the high water line as the foot of the cliffs?' Small coasters and boats there is a depth of 2 feet over it at low-water springs, small coasters and boats passing between Aberystwyth and Aberdovey with a falling tide should not shut in the north end of the low cliff at Altwen upon the outer part of Pen Cwingen. When the whole of the rocks of Pen Cwningen are covered, a vessel drawing 6 feet may safely bring the low part of the cliff at Altwen in line with Castle point.'

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, pg40
Historic Admiralty Chart 1486_A1C, RCAHMW digital collections sourced from the UK Hydrographic Office and first published in 1843

Maritime Officer, RCAHMW, September 2014.