The Inman River Catchment sits within a well-defined geomorphological feature on the southern part of Fleurieu Peninsula, a relict of an ancient glaciation, which eroded a large east-west trending bedrock depression across Fleurieu...
moreThe Inman River Catchment sits within a well-defined geomorphological feature on the southern part of Fleurieu Peninsula, a relict of an ancient glaciation, which eroded a large east-west trending bedrock depression across Fleurieu Peninsula known as the ‘Inman Trough’. The Inman Catchment contains a unique geology and distinctive landforms, some of which have been recognised as being of international scientific significance. The key land-forming processes responsible for the location and setting of the catchment were related to a glaciation of Antarctic proportions, which occurred about 300 million years ago, during Permian times, when Antarctica was welded onto southern Australia. As the ice melted great thicknesses of sediments were deposited onto the glaciated bedrock floor. These glacigene sediments are currently being eroded by running water, re-exposing parts of the ancient glacial landscape, and moulding the present landscape.
A long geological history, which established the ancient bedrock floor of the peninsula, preceded the Permian glaciation. The oldest rocks that occur on the northern boundary of the catchment are part of the 2 billion year-old core of the region. The remaining hard rocks forming the floor and sides of the catchment were originally sediments more than 24 km thick, deposited in ocean basins during Proterozoic and Cambrian times (more than 500 million years ago). These sediments were transformed by heat and pressure, and folded about 500 million years ago to form a huge mountain range of Himalayan magnitude, into the base of which granites were intruded. Over the ensuing 200 million years this mountain range was so severely eroded that during the Permian ice age the oldest, deepest rocks in the mountains were exposed at the surface.
After the retreat of the Permian ice sheet the catchment was subjected to prolonged weathering and erosion over many millions of years, producing a landscape of low relief that was intensely weathered to depths exceeding 70 m. The progressive uplift of this surface of low relief and its ongoing weathering and erosion has produced the high plateau surface on the margins of the catchment such as at Spring Mount. Throughout this early geological history Australia and Antarctica were joined together as part of the ancient super-continent Gondwana: they were the last of the continents to separate about 43 million years ago, following which there were tectonic movements and incursions of the sea.
The sea inundated parts of the southern Australian coastline several times from about 40 million years ago and tectonic activity formed the marine St Vincent and Murray Basins. Marine incursions onto Fleurieu Peninsula deposited fossiliferous limestones, remnants of which occur at high levels in the Myponga, Upper Hindmarsh Valley and Waitpinga areas. Ongoing earth movements progressively uplifted the Mount Lofty Ranges following the rifting of Australia and Antarctica, elevating the fossiliferous limestones up to 240 m above sea level as they did so. Throughout the tectonic uplift, rivers, streams and weathering processes continued to mould the landforms in the catchment.
Over the past 2.6 million years of the Pleistocene, global sea level fluctuated dramatically with sea level falling to about 125 m below present sea level and returning to near present sea level about 30 times. Only the last major sea level fluctuation (125,000 years ago) is recorded in the catchment, producing river terraces and stranded shorelines in the lower reaches of the Inman River. During the Holocene, the last 11,500 years of Earth history, alluvial sediments accumulated in channel bottoms forming floodplains and swamps.
The arrival of humans witnessed some major changes in the landscape. Over many thousands of years Aboriginal occupants actively managed the landscape, particularly through the use of fire. This had significant impacts, with plants and animals adapting to the mosaic-burning regime, which produced open parkland environments in places. These landscapes were very attractive to the first European settlers in the early 1840s but the practices that were implemented to establish settlements supported by agriculture, albeit initially on a comparatively small scale, severely disrupted the pre-existing landscape ‘equilibrium’. Clearing of vegetation, cessation of burning practices, single-furrow ploughing of land areas, and the introduction of livestock, rabbits and weeds triggered significant changes. Thus, new landforms were generated as the landscapes adapted. Some obvious expressions of change were soil nutrient depletion, various forms of erosion (slumping, gullying, stream incision, stream bank collapse) and alluviation. Changes to hydrology and water quality in the catchment were additional consequences.
The last few decades have witnessed progressive changes in landuse, with current land management strategies being focused on soil and land conservation, erosion repair through revegetation and watercourse controls, and the eradication of feral animals and plants. Thus, the agricultural regime which triggered the disruption of pre-European settlement landscapes throughout the catchment has, over more than 100 years or so, progressively changed to a new agricultural landuse regime under which the dramatic earlier erosional features have been progressively stabilised and ‘healed’.
