The Whitesands boulder bed

 

Two smoothed and rounded giant erratics resting on the interglacial rock platform at Whitesands South in Pembrokeshire.  Like many other boulders resting on the rock platform at this location, they appear to have been in position, affected by wave washing, prior to the deposition of the materials that lie around and on top of them.  In this respect they bear direct comparison with the famous Saunton pink granite erratic and the giant erratic at Baggy Point, which both appear to have been sealed beneath sandrock and slope breccia before being exposed by coastal processes in the current interglacial.

The most logical explanation of these boulders is that they are "lag" features derived from pre-Ipswichian glacial deposits -- isolated following the removal of finer matrix materials.  Following the Ipswichian interglacial, they were covered by, and incorporated into Early and Mid Devensian slope breccias (sometimes cemented) and sandrock, and then later overridden by the Irish Sea ice which laid down the Irish Sea till and its related ice wastage products.   While these sediments accumulated, the position of the coastline was far away, to the west.

The "free" erratic boulders on the rock platforms around the Bristol Channel coasts could be of many different ages, but I see no evidence which might lead to them being attributed to low sea-level stillstands during MIS 3 or MIS 4.