Geological history: from the mountains to the sea

270 million years ago mud, silt and sand was eroded from mountains along the east coast of the supercontinent, Gondwana, and deposited here in horizontal layers by ocean currents in a shallow sea. Gondwana had drifted down to the Antarctic Circle so, further south, glaciers carried rock debris from the mountains to the coast.  There, icebergs floated north and, as they melted in the shallow waters here, dropped their loads including boulders weighing 10 tonne into the mud and silt.

Although the sea was cold, marine life was abundant and fossil brachiopod and bivalve shells, sea fans, sea lilies and horn corals were sharply preserved in the mudstone, siltstone and sandstone.

Spectacular Ordovician fossil at Ulladulla Harbour
Fossils in the rock platform, north side of Ulladulla Harbour

25 million years later, as Gondwana drifted north, magma rose from a mantle hot spot forming the Milton monzonite and two other intrusions at Jervis Bay and Bawley Point.  Where all of the sedimentary strata have been removed, closer to the coast, monzonite has been exposed to weathering, forming the very fertile red to black loamy soils of the Milton district.

Since they were uplifted 60 million years ago, the horizontal sedimentary rocks of the Sydney Basin that form our hinterland have been eroding away.  Remnants of hard sandstone cap layers that have resisted erosion form the spectacular plateau landscape of the Budawang  Mountains and the iconic Pigeon House Mountain.

15 million years ago when the climate was much wetter, sand grains were cemented in the soil to form a near surface layer of silica rock or ‘silicrete’ after other more soluble minerals were leached.  Several of these silica deposits were mined here for more than 50 years until the 1950s.

Sea level fluctuated wildly over the past 1.8 million years (i.e. during the Pleistocene Ice Age) with the most recent rise of more that 120m occurring 20,000 yrs ago.  This sea level rise flooded low lying coastal land so that creeks formed numerous ‘estuarine lakes’ such as Lake Burrill, Lake Conjola, Narrawallee Inlet and Lake Tabourie, cut off by coastal sand spits.

These shallow coastal lakes have become a wonderful source of fish, shell-fish, prawns and crabs.

by Phil Smart