By Lance Hodgins
We take for granted the peninsula on which we live and play, but we never stop and think of the forces of nature, over millions of years, that formed it.
“A peninsula is a piece of land that is almost entirely surrounded by water but is connected to the mainland on one side.” (National Geographic)
The aborigines might well have witnessed the formation of the Mornington Peninsula. Those camped in the vicinity of today’s Somerville and Tyabb 20,000 years ago would have been aware of water on only two sides – to the south (Bass Strait) and to the west (Port Phillip). In the east the sun rose on the far side of a broad fertile river valley several miles wide.
The first earthquake would have been both bewildering and catastrophic. In one shuddering movement, the valley floor was lowered 30 metres and in rushed the sea. After a period of calm, a series of smaller movements lowered the valley by another 30 metres, and the waters of Western Port were here to stay – the peninsula land mass was now surrounded by water on three sides.
The streams which flowed into Western Port tell the story. With each lowering of the valley, they cut deeper and left a series of steps in their floors and terraces on their banks. As the sea rushed in, the old valley’s river system became the shipping channels of a new bay.
Geologists talk about this period as being of “recent” times. The story really began several million years earlier.
An underwater birth
450 million years ago the eastern third of Australia lay under the sea in a large depression called the Tasman Syncline. As sediment and dead marine life settled on the ocean floor it was compressed into horizontal layers by the weight of water above it. The mud and sands were compacted into shales and sandstones, called sedimentary rocks, containing many fossils.
The earth’s surface is like an egg shell, broken into several pieces. Pressure from the interior drives these portions – called “plates” – in various directions. The Australian plate, which was once connected to Antarctica, is today the fastest moving of them all, travelling northwards at the rate of 7 cm a year – or two metres every 25 years!
These same internal forces also created pressure on the horizontal layers of sedimentary rock, and buckled them into ridges and valleys, like a corrugated iron sheet.
These tilted sedimentary rocks can be seen in many roadside cuttings around the Peninsula, especially in the Mt Martha district.
The making of Arthurs Seat
Take a large leap forward in time to 150 million years ago. Despite what many people think, Arthurs Seat is not an old volcano, although it is of “volcanic” origin. Molten rock is called lava only when it reaches the surface – and, in this case, it did not.
Magma from a large underground molten rock chamber pushed up into the horizontal layers of sedimentary rock. Although this heated and chemically changed them into much harder rocks, the old sedimentary rocks were more easily eroded leaving the harder granite dome exposed. Arthurs Seat remains a prominent feature today, standing at over 300m above sea level.
Other domes, Mt Martha (163m), Mt Eliza (156m) and Olivers Hill (55m), are all of similar granitic rock and made in the same way. Their location all in a row suggests that they developed at points of weakness along a line in the earth’s crust.
Let the lava flow
About 40 million years ago, lava did flow on the surface but this came from outside our region well to the north east of us. This lava was thin and fairly fluid and oozed gently and quietly from numerous fissures in the earth’s surface. It flowed down the synclines (valleys) on each side of our ridge where it flooded the broad river plains and buried the existing soils.
These two lava flows came together near Cape Schanck and their combined weight caused the land there to sink.
Over the years there were many separate lava flows. These can be seen in the cliffs at Cape Schanck and Flinders where each flow is separated by thinner layers of ash and soil.
These softer soil layers eroded faster and wave action has left distinctive coastal features: rock platforms at sea level, vertical cliffs, and the occasional special feature of resistant rock such as Pulpit Rock and Elephant Rock.
Building up the ridge
Over time, the buckling up of our ridge (anticline) developed weaknesses and cracks began to appear, called “fault lines”. The main ones were along the ridge line in the same north-easterly direction. The Balcombe fault lies along the central axis of the fold, with Selwyn fault to the west and Devilbend fault on its eastern side.
Since these fault lines are weaknesses in the rock layers, vertical movements occurred along them from time to time and the land between them was either raised or lowered. These movements would have been accompanied by rumblings and sizable earthquakes.
Two main blocks were thrust up between these fault lines.
Bald Hill was the first to be raised – between the Devilbend and Balcombe faults – about 120 metres, leaving it only 60 metres less than the nearby Arthurs Seat.
The Arthurs Seat Block, between Selwyn and Balcombe faults, was elevated some time later. There were three distinct movements – 90m, 60m and then 30m – which pushed the granitic dome of Arthurs Seat to 314 metres, the highest point on the peninsula.
There were two important consequences of these uplifts.
Firstly, before it was raised, the Arthurs Seat block was partly covered by sands blown in from the ocean. This unproductive land was never cleared for farming, and “Greens Bush” remains our largest area of bushland today.
Secondly, as these blocks rose higher they created a steeper fall to the sea and streams were energised to cut more deeply onto their courses. The longest stream on the peninsula, Main Creek, runs through a deep valley between two main blocks.
The inland sea
The old lava flows had largely filled the broad valleys on either side of the peninsula ridge. Streams slowly cut new channels and the two main river systems joined up south of Cape Schanck and ran for 50 miles across a broad plain before reaching the open sea somewhere between Cape Otway and King Island.
About 25 million years ago these broad basalt valleys were flooded by a significant and long-lasting rise in sea level, called the Miocene Sea. When this sea receded, it left behind some interesting features – for instance, sea fossils in the sandy limestone on top of the cliffs at Flinders and Cape Schanck.
Other sediments known as Baxter Sandstones were left in the shallow waters further to the north. Limestone and marine clays were also deposited on the edge of this vast sea, especially on the Port Phillip side. These deposits were later easily eroded along the present day beach coastlines of Mount Martha, Mornington and Mount Eliza.
Port Philip Gets that Sinking Feeling
Fifteen million years ago, a massive subsidence occurred between the Selwyn fault and the Bellarine fault near Queenscliff. These faults formed a “hinge fault” with a fulcrum in the north, and a widening towards the south where the land was lowered by nearly 500 metres. The result was a huge inlet – narrow in the north and widening into Bass Strait where the vast plain and the old river mouths were submerged.
Today, Selwyn Fault can be seen as a long cliff stretching from Dromana to Cape Schanck. North of Arthurs Seat it lies a couple of kilometres out in the bay, the coast having eroded back to the more resistant rocks of Mount Martha, Mt Eliza and Olivers Hill.
The making of a Bay
One million years ago, as the subsidence of the Port Phillip sunkland continued, large quantities of sand from the Pt Lonsdale side were carried towards us by westerly winds and ocean currents.
This material attached itself to the Mornington Peninsula along the cliff face of Selwyn fault and eventually built up a new land mass of over 100 square kilometres, later named the Nepean Peninsula. This blocked off the southern part of the broad inlet and created the bay we call Port Phillip.
This extensive bay bar is far from a simple sand dune. It is a complex depositional feature created by changes in sea level during several small ice ages. With each glacial period, water was held in the ice caps and the sea level fell – only to rise again when the ice melted and waters warmed. With each rise and fall of the oceans the dune material was periodically heaped up, exposed, and rearranged.
Older dunes were covered by newer ones and their shells were compressed and cemented into hard dune limestone – a resource later mined and smelted by the early settlers of the Sorrento district, or cut into blocks for their houses.
The surface dunes of today, however, are quite young – and very moveable if not bound together by vegetation. In places, the wind has created rounded hummocks as in the “Cups” region of the renowned golf course landscapes near Cape Schanck.
This dune material doesn’t hold water and there are few streams. In some places the bay tides would scour out a depression which, if sealed by a sand bar, would develop into a marshy vegetation area, such as the Tootgarook Swamp.
At long last a true peninsula!
Fast forward to “today”, or strictly speaking, 20,000 years ago, and the movements along the Tyabb Fault which formed Western Port. Not only did this create a true peninsula, with water on three sides, but a region with its own distinctive personality.
The Western Port sunkland was too recent and too shallow to experience the rise and fall of the sea with the various ice ages and there was no complex bay bar like the Nepean Peninsula. The closest was a beautiful hooked sand spit called Sandy Point stretching eastwards along the coast from Somers near Cerberus.
The lowering of Western Port brought a considerable amount of silt flowing in from the Koo Wee Rup Swamp to the north. The tides, hindered by French Island, pushed it back and forth and created the extensive tidal mud flats and unique mangrove ecosystems found north of Stony Point.
Peninsula Essence January 2026