It took me quite a while to figure out what these structures were on Tolls Island, Just off Pelistry Bay on St Mary's, were. It turns out they are kelp pits in which dried seaweed was placed to be burnt. The soda-rich ash would then be collected and sold to be used in glass-making.
This process was introduced to the Scillies in the 1680s and became a useful way for the locals to supplement their incomes.
In their heyday there were said to be more than 100 of these pits around the Scillies although today only a dozen survive.
