The principle material then being exported , however , was the local lias limestone , called Aberthaw tarras , which was used to make hydraulic lime , which sets under water .
Was therefore very useful for building light houses ( including the Eddystone Lighthouse ) and canal locks .
The principle material then being exported , however , was the local lias limestone , called Aberthaw tarras , which was used to make hydraulic lime , which sets under water and was therefore very useful for building light houses ( including the Eddystone Lighthouse ) and canal locks .