The bedrock of the Melbourne district is of Middle Palaeozoic age belonging to the Silurian and Devonian periods (354-441 million years ago). However, these rocks are overlain in wide areas by much younger rocks, mostly of Tertiary and Quaternary age (2-65 mya). The oldest rocks of the bedrock are Early Silurian in age (441 mya). They outcrop on the west side of Melbourne in the Maribyrnong Valley around Keilor where they are called the Springfield Formation, and in the north-east around Warrandyte, where they are known as the Anderson Creek Formation. Fossils are uncommon in these formations. Younger rocks are found east of Keilor and south of Warrandyte, ranging in age from Late Silurian (418 mya) in the city area (the Melbourne Formation) to Early Devonian (384 mya) at Lilydale (the Humevale Formation).
These Palaeozoic rocks are steeply dipping sandstones and siltstones, with a small occurrence of limestone at Lilydale - the Early Devonian Lilydale Limestone. The Silurian rocks were deposited in deep water, whereas the Devonian rocks, which are highly fossiliferous, seem to have been deposited in shallower water. These Silurian and Devonian rocks were folded into a series of anticlines and synclines in the Middle Devonian during a folding event called the Tabberabberan orogeny (about 380 mya). The axes of the folds trend approximately north-north-east to south-south-west. In places the rocks are broken by faulting.
Following the folding event, the bedrock was intruded by plutonic rocks of the granite family, of Late Devonian age. These granites outcrop at Bulla, Broadmeadows, South Morang, South Yarra and Toorak, Harkaway and Mount Eliza. Some of these granites have weathered to the white clay mineral kaolinite, that has been quarried for the manufacture of earthenware. Tough contact metamorphic aureoles - hard rims of baked sedimentary rock, now consisting of quartzites and hornfels - are present around most of these granite masses. Where the granitic magma broke through to the surface, huge volcanoes erupted lavas and ash flows that now form the Macedon and Dandenong Ranges. The Dandenong Ranges were formed from acid volcanic rocks, mainly rhyodacite, that erupted when a large triangular block of sedimentary rocks bounded by faults collapsed into an underlying magma chamber. This forced magma to the surface, where it erupted as ash flows from vents along the faults.
Following the end of the Devonian period, there was a long period of weathering and erosion over 300 million years. Then, in Early Tertiary times (from 65 mya), large block-faults produced a wide shallow depression known as the Port Phillip Sunkland. In Tertiary times, the Melbourne area experienced three episodes of deposition in this depression: an Early Tertiary terrestrial phase, a Middle Tertiary marine phase and a Late Tertiary terrestrial phase.
Most of the Early Tertiary non-marine sediments, called the Werribee Formation, are known only from bore holes. They consist of sands, and in places, coal seams, that were mined at Altona in the late 19th century. Very small outcrops of this formation containing fossil leaves occur in the Moonee Ponds Creek valley at North Melbourne and Pascoe Vale.
Overlying the Werribee Formation is a thick sequence of basaltic lava, known as the Older Volcanics. These occur north-west of the city from Tullamarine, where they were quarried, to North Melbourne, and in the north from Bundoora to Greensborough. In many places these lavas are highly weathered. They are thought to have been erupted in Eocene and Oligocene time (about 30-55 mya) and may have been associated with the block faulting forming the Port Phillip Sunkland.
Dykes - intrusions of igneous rocks filling wide fissures in rocks, thought to be associated with the Older Volcanics - are visible in road cuttings through the Silurian rocks, such as on Yarra Boulevard, Studley Park, and the Eastern Freeway. Most dykes are weathered to clay and have been cemented or bricked-up to prevent erosion.
The Older Volcanics are covered in many places by marine sediments of Late Oligocene to Middle Miocene age, known as the Fyansford Formation. These are green to grey richly fossiliferous silts, in some places with calcareous lenses, or even limestone such as at Green Gully, near Keilor. Most of the outcrops of the Fyansford Formation are weathered to ironstone, such as those in the Essendon area and in the Royal Park railway cutting. At both these localities, the formation sits directly on Older Volcanic rocks, now weathered to clay. It contains abundant shelly fossils typical of shallow rocky seabeds, and now preserved as natural moulds. Unweathered outcrops of the Fyansford Formation are found only in the shore platforms at Beaumaris and at Fossil Beach, Mornington.
The Fyansford Formation is disconformably overlain (that is, there is a break in sedimentation) by a series of sedimentary rocks - sands and gravels - known as the Brighton Group. These cover extensive areas of the southern and south-eastern suburbs. At the boundary between the two is a thin nodule bed containing phosphatic concretions, and water-worn vertebrate fossils such as whale bones and sharks' teeth. This bed outcrops at the base of the cliff at Beaumaris, but has now largely been obscured by the dumping of landfill at the boat club.
The Brighton Group comprises two formations: a predominantly marine sandy fossiliferous unit, known as the Black Rock Sandstone of Late Miocene age; and the overlying Red Bluff Sands, thought to range in age from Late Miocene into the Pliocene.
The Black Rock Sandstone is well exposed in the cliffs between Point Ormond and Mentone, as well as in railway cuttings at Royal Park and South Yarra where it is highly ferruginised and difficult to distinguish from the underlying weathered Fyansford Formation.
The Red Bluff sands are composed of poorly fossiliferous yellow-brown sands, gravels, and conglomerates, that are mostly very weathered. They outcrop over a wide area of the northern, eastern, and south-eastern suburbs, in many places as hill cappings. This formation has provided much of the sand for concrete used in building construction work around Melbourne.
During the Pliocene (2-5 mya) and through into the Pleistocene (2 mya), there was substantial volcanicity, in two episodes. The first produced extensive sheets of basalt lava on the west side of Melbourne to form the Keilor and Werribee Plains. The second phase involved volcanoes to the north of Melbourne. These extruded flows that filled the valleys of the Moonee Ponds, Merri and Darebin creeks. Lava flows erupted from Hayes Hill near Mernda, flowed down the ancestral Merri Creek and Darebin Creek valleys into the Yarra Valley at Fairfield and down that valley onto the Yarra delta as far as the present site of Spencer Street bridge.
Quaternary sediments are principally represented by raised beaches and sand ridges around the coast. They are particularly well developed at Altona, where shell beds occur, and form valley fill in the Yarra River and its tributaries, and sediments of the Yarra Delta. The alluvial flats of the Yarra River upstream of the Yarra Gorge at Warrandyte are thought to have been formed as a result of grade changes following rejuvenation of the Yarra Fault, whereas the alluvial flats upstream from Fairfield and in some of the Yarra tributaries, such as Gardiners Creek, were formed as a result of the damming of the Yarra by a stream of basaltic lava flowing down the Darebin Creek valley.
Sedimentation in the Yarra Delta commenced with the Moray Street Gravels deposited in a deep valley cut into the Pliocene land surface. These sediments are non-marine, and were succeeded during a period of high sea level by marine silts and clays, the Fishermans Bend Silt, deposited in narrow estuaries of the Yarra and Maribyrnong rivers. These silts extend well up the valleys as far as Church Street, Richmond in the Yarra Valley, and as far as Avondale Heights in the Maribyrnong Valley. The Fishermans Bend Silt was followed by a period of lower sea level at which time the Mount Fraser lava flow reached the delta. This was followed by further marine deposition during a period of higher sea level, giving rise to the Coode Island Silt, a richly fossiliferous silt and silty clay. In turn this is overlain by the Port Melbourne Sand, consisting of shallow-water sands and shell beds, on which sand beach ridges and swamps were developed, forming the present land surface.
Melbourne is situated at the north end of the Port Phillip Sunkland, which is bounded on the west by the Rowsley Fault, running from north of Bacchus Marsh to near Geelong, and on the east by Selwyn Fault, running from east of Dandenong to near Cape Paterson. These faults were active from Late Tertiary into Quaternary time and there is still some slight movement on both producing earthquakes and tremors. Beaumaris Bay is situated on the downthrown side of the Beaumaris Monocline that runs north-east from Beaumaris to Clayton. The cliffs at Beaumaris are parallel with the turnover on the monocline.
The present land surface of Melbourne has evolved through the interplay of processes of erosion, faulting and volcanic activity. The high ground in the eastern suburbs represents the dissected remnants of the Nillumbik Terrain, a gently sloping erosion surface that probably formed the Middle Tertiary ground surface. The western suburbs are situated on the Werribee Plains on basaltic lava. In the north and central suburbs the Palaeozoic bedrock protrudes through the lava to produce an undulating surface. Lava flows disrupted the previous drainage and the new drainage developed a meandering course on a gently sloping surface. Streams then became entrenched into the lava producing deep gorge-like valleys. Other streams such as the Yarra became entrenched along the boundary between the lava and the softer Silurian bedrock.
The coastal features of Melbourne are formed from a variety of erosional and depositional processes. The east side of Port Phillip as far as Mordialloc is composed of cliffs undergoing active erosion. From Mordialloc south, on the downthrown side of the Beaumaris Monocline, there are extensive beach ridges that have inhibited drainage and produced swamps. The western and northern sides of Port Phillip have extensive beach ridges and salt marsh. In places the coast is prograding owing to sediment discharging from the Werribee River, Kororoit Creek and Yarra River. Much of the coastal areas have been so affected by human activity that the original patterns of erosion and deposition have been severely disrupted and the landscape greatly modified.
The principal minerals found in the Melbourne area include quartz (mainly as common reef quartz), gold, stibnite and zeolites. Specks of gold were found in Tertiary sediments around Melbourne in the 1860s and in quartz veins penetrating the Silurian bedrock in the 1850s and 1860s. The only payable reefs were at North Balwyn, Templestowe, Warrandyte, Diamond Creek and Yarrambat (mines in the latter three areas were worked until the 1940s). Stibnite (antimony sulphide) in quartz veins was mined at Ringwood from 1869 until 1892. Several members of the zeolite group (hydrated aluminium sodium calcium silicates) are found in basalts of both the Older and Newer Volcanics.
The quarries at Clifton Hill and Burnley (now worked out) have produced spectacular crystal groups of analcime, chabazite, gonnardite, natrolite, phillipsite and thomsonite, some of which are of international significance. Other well-known localities for zeolites include Brunswick (worked out), Eastern Freeway cuttings (now inaccessible) and Bundoora Quarry (now inaccessible).