Geologically, Plymouth has a mixture of limestone, Devonian slate, granite and Middle Devonian limestone. Plymouth Sound, Shores and Cliffs is a Site of Special Scientific Interest, because of its geology. The bulk of the city is built upon Upper Devonian slates and shales and the headlands at the entrance to Plymouth Sound are formed of Lower Devonian slates, which can withstand the power of the sea.
Along with shales, what is most of the city built on?
Upper Devonian slates