The rhyolitic headland on which the castle is built is strong and not easily eroded. The cliffs to each side, however, are less resistant, being made up of glacial drift, layers of boulders, stones, clay and silt which were laid down during the last ice age. Sea walls were already in existence at the time of the first Ordnance Survey map in 1891, and the west shore sea wall had been extended and groynes built by 1913. Extensive remedial work was completed in 1965, and the defences were again strengthened in 1974 and 1985. In 1995 work was started on improving the defences along "The Esplanade", followed in 1997 by further work to replace the crumbling gabions below "Lôn Felin". Submerged forests occur in a number of places off the Cardigan Bay coastline, including Criccieth; these are deposits of peat, soil and tree remains and appear to be post-glacial coastal lagoons and estuaries, which have been flooded by rising sea levels.