sentences:  Most of the region is underlain by the Colinton Volcanics, composed of light grey tuff and lavas deposited in shallow water by large explosive volcanic eruptions during the Silurian period, 425 million years ago.A fault runs parallel and to the east of the Burra Road, beyond which are the even older deep sea Ordovician sandy sediments of the Pittman formation, deposited from the mouth of a very large river which drained the nearby supercontinent Gondwana.A few limestone lenses occur in younger rocks near to this fault, one of which forms the locally famous 'London Bridge' natural arch over the Burra Creek.