 In pre @-@ historic times the rocks that now form Skerryvore were covered by the ice sheets that spread from Scotland out into the Atlantic Ocean beyond the Outer Hebrides . After the last retreat of the ice around 20 @,@ 000 years ago , sea levels were up to 400 feet ( 122 m ) lower than at present . Although the isostatic rise of land makes estimating post @-@ glacial coastlines a complex task , circa 14 @,@ 000 BP it is likely that the reef was at the south western end of a large island consisting of the modern islands of Tiree and Coll and the surrounding land . 
