ON FOSSIL BEACHES

Rocks have lots to tell us if you speak their language.  You have to be a geologist to have a rapport with rocks, minerals, and gemstones.  Fossil shells are fairly common place and make for interesting initial collections for hikers exploring the outdoors.    But a fossil beach?  I have found some amazing ones in my travels.

A fossil beach denotes shallow water, such as found in a sea water estuarine system.  A current or tidal flow may create ripple marks over extensive areas of flat beach.  These ripple marks can be preserved when covered over by later deposits of sand and mud.  This can happen with terrestial flooding from rivers, or from regional subsidence.  It happens frequently.   The amber deposits of Guatemala are found in such sedimentary rocks of estuarine origin.

Jo standing by a fossil beach in the Adelaide Hills

But what about close to home where I live?  In the Adelaide Hills, between the suburbs of Belair and Mitcham there is a quartzite quarry with a wonderful exposure of ripple marks. It's on the steeply dipping side of the quarry wall of fine sandstone of the Kanmantoo basement rocks, and is about 400 millions old.

It is a nice afternoon scramble down hill northwards down the gully and so to Mitcham metro station.  Take the train both ways. Enjoy the city views, take a lunch. Watch out for the local wild life, the koalas, the foxes, and on hot days beware of brown snakes slinking around in the shrubbery.

The most spectacular fossil beach I have encountered is rather difficult to get to.  It forms the exposed crest of the Spero Range inland from Bathurst Harbour in South West Tasmania. It is composed of flat lying glaringly white quartzites of Pre-Cambrian Age, and looks like a ripple marked sandy beach formed just yesterday.  It invites you to strip off and dive into the water, but the tide went out maybe 800 million years ago (see Tasmanian Mines Dept Ref. below).

Dinosaur footprints in Bolivia…..being inspected by  Philippa Haselgrove,  from Adelaide.  (click to enlarge image)

Sometimes creatures have walked along these ancient beaches and have left their footprints.   Not human beings of course, for it was before their time.   Go back a 100 million years or so and you may encounter evidence of dinosaurs out for a stroll along the beach, leaving footprints of Mum or Dad and the youngsters in the sand, and the dragging furrow of their long tail.   A good example of this is in Bolivia, at a cement works quarry near Sucre. Take at taxi out to see the sights.  The steeply dipping mudstone of the quarry wall  is covered in dinosaur footprints, both large and small, made by Tyrrannosaurus rex , having feet up to 80 cm across,  and of a smaller iguanodon.    

So the rocks do speak to us and have a lot to say about how they were formed.  You may find rocks with fossil shells on the Adelaide beaches and elsewhere, which make nice ornamental pieces for display at home.  They will remind you that all the present political skulduggery is just a flash in geological time, so don't worry too much about it.    Regards from Allano.

An Adelaide sandy beach at Brighton....no dinosaurs here.

SW Tasmanian geology and Spero Range ripplemarks.
http://www.mrt.tas.gov.au/mrtdoc/dominfo/download/TR4_34_46/TR4_34_46.pdf

Old  quarry in Adelaide Hills showing ripple marks.

Foto opposite: Having a picnic with Joanna, Christobel and dog Jarrah in the old quarry showing ripple marks on the sloping walls.   The track down from the road above is rather steep so be careful.  The area is well signposted giving the history of the workings.  It's a nice outing on a summer's day.

Regards from Allano









PS: I have found some more dinosaur footprints.   These ones are preserved for tourists to look at near Mendoza in Argentina.   (Click to enlarge images)