A large slab of dolerite is exposed on the seashore of Cartwright Reserve. It during extensive volcanic intrusion of country rock 200-160 Mya (Jurassic Period), when the supercontinent of Gondwana began to break up. Dolerite is common in the Hobart area- and forms the hills of kunanyi, Knocklofty and Taroona.
Good examples of the characteristic spheroidal weathering in dolerite boulders can be seen here. The foreshore is also a good site to consider the fault-and-graben system of the hills and valleys around the Derwent: formed by a complex system of deep faulting during the breakup of Gondwana.
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