The Helmet – a tale of a salt pillar
How does a giant bump on the seabed like this form? The explanation lies in the deeper layers of the continental shelf. Long ago, during Carboniferous-Permian times (350-250 mill. Years BP), piles of salt were deposited in a long-since evaporated sea. Over time the salt deposits rose slowly but steadily to the surface, as tenacious, pillar-shaped bodies known as salt diapirs. This upwards movement occured because salt has a lower density than the overlying sedimentary rocks, which makes it float slowly but steadily upwards through denser rocks. As it rose the salt lifted and deformed the rocks and sediment layers in its path, sometimes ripping out blocks from older deep layers and bringing them up to the surface of the seabed. In the surrounding area there are many salt diapirs that come close to the surface but none which actually protrude as clearly as The Helmet. (See also: Salt diapir and gas leakage on Tiddlybanken)
Filming the Helmet from east to west
We crossed The Helmet with the video rig, acquiring video data from a 1.4 km stretch of seabed over which large variations in bottom type were observed. At first, on the eastern side of The Helmet, there was not so much life to be seen but as we moved westwards and ascended the eastern slope of The Helmet, both the geology and biology got more exciting. The bottom type which started as mud became increasingly mixed with fine gravel then coarser gravel as we moved upwards, becoming dominated by cobbles and boulders by the time we reached the top of The Helmet. As we descended the western side of The Helmet the bottom type gradually became finer ending in mud again as we reached the flat seabed.