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).
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.
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.
One 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).
…….or is it ants on a beetle? These delightful creatures with their bright green abdomens belong to the Weaver or Tree Ants (Oecophylla spp.). They make their nests by ingeniously folding living leaves on the tree and binding them into position with silk. The beetle was one of many of that type we saw when we were on this trip to Queensland in 2011, where most were alive and resting on walls in daytime but some had died and were being investigated by ants, hoping to find a way through the outer hard exoskeleton to juicier bits inside. The beetles were referred to as Christmas Beetles (Anoplognathus spp.) and I cannot say if this was an accurate identification but internet sources support it. They range in size from 15 – 40 mm in size and they belong to the Scarab family (over 3000 species in Australia), which also includes flower and cock chafers, and fiddle beetles.