Geologically, the Cave Hill is at the southern end of the Antrim Plateau, which was largely formed about 65 million years ago (yes, just at the end of the age of the dinosaurs) by great lava eruptions that continued for millions of years. The basalt rock which resulted was worn down by subsequent ice ages and weathering into the smoother formations seen today.
There are a number of sites of archaeological interest on the Cave Hill, dating back to the stone age. These include several ringforts (raths) and a lake dwelling (crannog) at Hazelwood. McArt's Fort is on the highest point which is popularly (but inaccurately) known as Napoleon's Nose. It is likely that this was used for ceremonial rather than defence purposes and it appears to date from about three thousand years ago.