The striped and layered rocks that underlie the promontory at Cape Tribulation look like sedimentary strata. They were just that at one time in the past but they have been altered and partly metamorphosed into metasedimentary rocks. They belong to a group known as the Hodkingson Formation and are the result of transformations brought about as two tectonic plates of the earth’s crust collided, with one plate then sliding under the other (subduction), generating tremendous heat and pressure that altered existing sedimentary rocks, promoted magma movements below the crust (leading to intrusive igneous rocks), and stimulated surface volcanic activity (making extrusive igneous rocks).
We are used to thinking of rocks as ancient structures that have been in place for millions of years but, of course, rocks are in the continual process of being formed. An example might be the way rivers carry erosion sediments downstream to form layers on the beds of seas, lakes, and lagoons. Or erupting volcanic lava solidifying on contact with air or water. On the coastline of Queensland in Australia the most easily visible type of present-day rock formation is that of cay sandstone, commonly called “beach rock”.
Beach rock forms very rapidly. It happens in warm shallow water close to coral reefs, where the combination of heat and evaporation, an abundance of dissolved calcium from pieces of coral and seashells, and the addition of phosphates from bird guano, lead to a cementing of all the loose fragments together to form hard concretions of rock. This is such a rapid way of rock building that it is sometimes possible to see man-made objects included in the concretion – apparently soft drinks cans have been recorded. More commonly seen are pieces of coral (sometimes still coloured), sea shells, and the impressions of plant remains such as Pandanus fruits.
The photographs in this post were taken mostly on the sheltered shore of Normanby Island where the beach rock layers form an almost continuous link and low-tide walkway to the neighbouring island. Large slabs of beach rock are prone to break off from the layers and rest on the shore. In other places deep deposits of algal-covered rock have started to wear into depressions and hollows that form new habitats for marine gastropods and crustaceans.
I travelled to Normanby Island on a tour with Cruise and Dive Frankland Islands.
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Fossil corals, brachiopods, bivalves, and crinoids, from ancient seas 400 million years ago, survive in the limestone of the Chillagoe District, Queensland, Australia. An abundance of these Silurian marine invertebrate fossils are preserved in the rocky outcrops in the area and easy to spot. You can see them on the natural rough surfaces of the cliff-like sides to the karst towers or bluffs. You can also see them in the cut sections of adjacent boulders where the marble quarrymen have been exploring the commercial potential of new sites.
I didn’t see all the types of fossil known to occur here; neither can I now put a scientific name on the specimens I discovered. I noted cross and longitudinal sections of solitary corals – some quite large. There seemed to be a lot of colonial pipe corals – transverse sections of them en masse and lengthwise views of individual pipes. There were crinoid or sea lily stems too. In life, these resemble flowers but are really multi-armed animals of the starfish family which attach to the sea bed by a long jointed, flexible stalk. It is mostly small pieces, or a series of pieces, of the articulated stalk that have been preserved in the rock.
Large bivalved mollusc shells, some still joined together in the pair, were the most numerous fossils – and there were many brachiopods as well. These occurred as distinct shelly layers in the way they had been deposited in the sea-bed sediments so long ago. However, major upheavals and folding of the compacted deposits during later geological periods has resulted in these layers of fossils being re-oriented from the original horizontal to an almost vertical alignment.
Rocks and Landscapes of the Chillagoe District by W. F. Wilmott and D. L. Trezise, 1989, Queensland Department of Mines, Queensland Department of Natural Resources, Mines and Energy, ISBN 0 7345 2486 2, QNRME04050, pp 3-7, gives details of the sediment deposition off the edge of the continent between the Silurian and Devonian Periods – when the Chillagoe fossils were formed.
COPYRIGHT JESSICA WINDER 2012