Melbourne is built predominantly on a sequence of Ordovician/Silurian sedimentary rocks and the best places to view them are in the nearby environs. Whittlesea and Sunbury lie 45 minutes to the north of Melbourne and it is in the local quarries and road cuttings where many of the rocks are visible. On the Whittlesea to Kilmore Road are numerous road cuttings in Ordovician to Silurian sediments of the Melbourne Trough. These sediments are comprised of marine mudstones, siltstones and sandstones that are sometimes fossiliferous. Some of the fossils found here include Brachiopods, Crinoids and Graptolites. Brachiopods are a type of clamshell and the fossils found here represent some of the very first Brachiopods to appear on Earth. They are very small, mostly <1cm in size. The Emu Bottom Homestead, at Sunbury, and close by at the old Bullengerook slate quarry are Ordovician black slates containing Graptolites. Graptolites are a marine organism much like some types of modern day seaweed. They once lived in deep oceanic water where they formed colonies that drifted with the ocean currents. They lived during the Ordovician and became extinct not long after. Along the Homestead Road are outcrops of siliceous white sediments composed of marine diatoms. The diatoms are microscopic organisms with a hard silica skeleton (test) that is preserved after they die. The rock is called a diatomite and is composed entirely of these tests. They can often been seen in road cuttings surrounding Melbourne. Many different types of rock occur in and around Melbourne and some of the sedimentary types are described above. The fossils associated with them provide information about their history and age.
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Brachiopod Impressions, Ordovician, Whittlesea
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Graptolites, Bullengerook
