Rocks at Cape Tribulation

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).

More Sand Pellets from Crab Feeding Activities

The topography of the shore near Cape Tribulation varies. At one point it is semi-vegetated with salt-tolerant shrubs. Here the substrate holds water for longer at low tide and the root systems of the plants trap finer sediments and nutrients that are washed down the beach from the nearby forest.

The different composition and consistency of the beach sediments in this place has given rise to a variation on the theme of sand ball patterns made by feeding crabs. The crabs may be the same species or a related species to Scopimera inflata which created the scatters and radiating patterns of drying-out pellets further along the beach (see the previous post). In this location, however, the enriched sand was relatively wet, which meant that the pellets coalesced into discrete linear masses that made very interesting designs on the surface of the beach.

Sand Bubbler Crab Pellets at Cape Tribulation

Small ball-shaped sand pellets produced by the feeding activities of the "Sand Bubbler" Crab

On many sandy beaches along the Queensland coast of eastern Australia there are millions of small sandy balls. Each one is just a few millimetres across. They can form extensive mats or patterns as they cover the shore at low tide. There were many decorating the sand at Cape Tribulation when I visited in 2011. Closer inspection reveals small holes in the sand, usually at the centre of radiating lines of balls. These balls are the result of the feeding activities of small sandy coloured crabs that are themselves rarely seen. The holes are their burrows. They are the “Sand Bubbler” crabs belonging to the genus Scopimera, often S. inflata. The balls are created as the crab feeds on the organic matter attached to the sand grains on the surface around its burrow, and rolls up the cleaned grains into balls before moving on to the next spot.

The patterns vary according to the moisture level of the sand and its organic content, as well as the species concerned, I would imagine. I’ll post some more pictures later showing an interesting variation on the sand ball patterns from the other end of Cape Tribulation beach.

Crabs at Port Douglas


Fiddler crab by its burrow on a muddy seashore in AustraliaOne of many fiddler type crabs (Uca spp.) found on the low-tide mud at Port Douglas in Queensland, Australia. These small colourful crustaceans with their tall stalked eyes emerge from their burrows as the tide goes out to feast on the surface biofilm of the sediments. The one in the video clip is shielding itself, and protecting its territory, with its large right claw while it daintily scoops up mud and food with its tiny left claw and pops it into its mouth. This specimen has a blue patterned carapace about an inch across (2 cm).

Fiddler crab on a muddy seashore

A Bunch of Coconuts

Coconuts on the tree at Port Douglas, Four Mile Beach.

To most of us who are unfamiliar with the natural world of the tropics, coconuts are the small brown hard hairy things that you very rarely buy in the supermarket, or maybe have tried once in a blue moon to hit on a coconut shy at the fair. A song springs to mind:

I’ve got a lovely bunch of coconuts
There they are, all standing in a row
Big ones, small ones, some as big as your head
Give them a twist a flick of the wrist
That’s what the showman said.
I’ve got a lovely bunch of coconuts
Every ball you throw will make me rich
There stands my wife, the idol of me life
Singing roll a bowl a ball a penny a pitch …..

Fred Heatherton

The reality, of course, is that the fruits of the Coconut Palm (Cocos nucifera) look very different at the point of origin, as I found out on a visit to the north Queensland palm-fringed tropical coast some years back. They do come in all sizes in their own wrapper – including a smooth outer shell, which varies from yellow, to green, to brown, according to age.

The thick fibrous layer between the outer cover and the nut itself helps the coconuts to disperse by allowing them to float and travel long distances at sea. I found coconuts on the beach with stalked or goose barnacles attached – indicating that they had been in the water for some time before washing ashore again.

Other coconuts lying on the sand have strange characteristic holes in them, gnawed by the nocturnal White-tailed Rat (Uromys caudimaculatus) that chews right through all the hard layers to get to the white flesh inside. And coconuts are still in great demand as the basis for exotic beach-side cocktails even in far-away and isolated places, as evidenced by a barrow-load of them in the shade near a bistro hidden among the trees at Cape Tribulation.

Black Parrots and Beach Almonds

Close up detail of the fibrous inner husk of a Beach Almond

I found several small, fibrous, ovoid husks washed up on the shore at Trinity Beach and Normanby Island when I visited Queensland in Australia; and I wondered what they were. Sometimes, the fibres were still covered and contained by a tough leathery blackened skin with an unusual keel extending around the perimeter of the fruit. They had obviously lain on the beach and floated in the sea for quite a time. I couldn’t work out what they were.

It was not until I was enjoying a cold drink at a beach-side cafe in Port Douglas that I found the answer. A ruckus drew my attention to a tree in which a group of black parrots were noisily squabbling over fruits. Clusters of bright green fruits of the same peculiar shape and size as the flotsam specimens were growing high on the branches and also lay scattered whole, opened, or half-eaten on the ground where the parrots had dropped them. They were Beach Almonds.

The fruits belong to a family of trees (Combretaceae) common in the tropics. The Genus Terminalia has about 200 species, and I think it likely that the fruits I found were from one of the three most common species on the beaches of northern Queensland, Terminalia catappa, T. arenicola, or T. seriocarpa. The fruits are described as being canoe-shaped and ripening from green to blue-purple.

Rocks at Port Douglas, Queensland

These are some personal observations, just thinking aloud, and part of my learning process and fascination with geology. I have a lot of questions to ask about the rocks at Port Douglas on the Queensland coast in Australia! They are marked by a GeoCache site which says they are a batholith. A batholith is formed deep under the earth’s crust where molten magma from superheated, melted rocks, cools slowly and forms granite made up of fairly large crystals. In eastern Australia the batholith formations, known collectively as the Kennedy Province, were created between 330 and 255 million years ago  – following the earlier formation of the Hodgkinson Province which was created between 440 and 360 million years ago, and into which the batholith magma eventually intruded.

I have seen a batholith before – at Peggy’s Cove in Nova Scotia – and the the rocks I photographed at Port Douglas are very different from the rocks exposed at Peggy’s Cove – at least the ones around the edges of the formation in Port Douglas, notwithstanding that the two outcrops of bedrock are in two separate continents and have been subjected to very different erosional and weathering processes. Peggy’s Cove rocks have been smoothed and polished by ice sheets (glaciated) and their surfaces remain clean in a temperate climate. At Port Douglas, on the other hand, the rocks have eroded out and weathered in the wet tropics climate which has led to different erosional characteristics and a surface obscured in many parts by black bio-film.

Batholiths are made of granite. I think I can see granite in some areas of the Port Douglas outcrop on which the Lookout stands. A lot of what I believe to be granite is covered in black biofilm (possilbly cyanobacteria and lichen) and is colonised by organisms like barnacles so that the details are obscured. However, most of the detailed close-up shots I took of the rocks, particularly those around the edges of the feature, including loose boulder lying on the waters’ edge, did not seem at all like granite to me. There are various colours, textures and features as shown in the photographs in this post. I have been wondering to myself, speculating, whether these rocks may represent the junction between the granite of the Kennedy Province batholith and the Hodgkinson Province rocks into which they intruded, showing further changes to the earlier overlying (and already much altered, stretched, compressed and vulcanised) metasedimentary rocks.

The geological map of the area describes the outcrop of bedrock in the Port Douglas environs as Larramore Metabasalt Member which is part of the Hodgkinson Province rocks. I suspect that some of my photographs may be showing these metabasaltic rocks, or metasedimentary rocks with evidence for explosive volcanic activity and volcanic intrusions. I have read Rocks, Landscapes and Resources of the Wet Tropics by Berndt Lottermoser et al (2008) published by the Geological Society of Australia, Queensland Division, and have found it very useful. However, a more relevant account of the port Douglas geology might be given in another book which I have been trying to track down: Rocks and Landscapes of the Cairns District by W. F. Willmott and P. J. Stephenson (1989) published by The Queensland Department of Mines and Energy but it is out of print. I think it might be useful in helping me answer some of the questions I’m posing.