Crossing the continental shelf

We crossed the edge of the continental shelf. The crust of the deep ocean beds is made of mafic rock like basalt which is denser and sits deeper. The continents are made of felsic rock like granite which is less dense and sits higher. Once the hills like Skiddaw behind Wigton were on the edge of the continental shelf - mudstones deep underwater. Then we collided with Scotland which threw them up as mountains.