Oyster shells on hills north of Lake Alexandrina

GEOLOGY is concerned with the physical structure of the earth and also its history. Geologists typically divide the earth’s history into epoch’s with the most recent referred to as the Holocene and having lasted about 10,000 years – so far. Before this was the Pleistocene.

Its interesting that while The Basin Plan has as a central objective flushing salt from the Murray Darling much of the Lower Murray is underlain with marine sediments.

When I showed a geologist an early map of Lake Alexandrina drawn by John Arrowsmith in 1838, he was less interested in the water quality as marked on the map, than the presence of oyster shells on the hills.

oyster hills

He commented:

“I have just noticed the delightful fact that the map that you reproduce carries the inscription ‘oyster shells cover these hills’ for the Murray banks north of Lake Alexandrina.

This refers to very well known shallow marine deposits of Plio-Pleistocene age (say 5-2 Ma) that occur widely in this part of the basin. These oyster shellbeds are underlain by older marine strata back to early Miocene and older in age.

There is no doubt that for the majority of its recent geological history the lower Murray Valley has represented a marine and estuarine gulf, which, being progressively fed with sediment during the Pleistocene climatic oscillations, had shrunk to become a restricted estuarine system at its lower end by the Late Pleistocene.

Some may think that this comment is irrelevant, and in a sense they would be right – insofar as the immediate roots of the presently perceived social-political problem lie in the Holocene history only. But wouldn’t it be wonderful if government Ministers had some understanding of deep history and also history.”

Map of Lake Alexandrina drawn in 1838 by John Arrowsmith based on water quality report from the British explorer Charles Sturt. The map indicates that the lake contained salt water, brackish water and freshwater consistent with it being part of an estuary. The complete map can be downloaded from http://nla.gov.au/nla.map-rm2633

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One Response to “Oyster shells on hills north of Lake Alexandrina”

  1. david

    I like your commentary on the linkages between the Lower Murray today and its geological history. I always find it amusing that our early European explorers were convinced they would find an “Inland Sea” . If only they had arrived a million or so years earlier & they would have found a huge body of water covering the area we now know as the Murray Basin,

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