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.

Tribulation Drift

Abstract natural patterns of coral sand and plant debris on the drift line at Cape Tribulation beach

I like these abstract natural patterns that I photographed several years ago on the beach at Cape Tribulation in Northern Queensland, Australia. I have seen similar nearer to home, on Studland Beach in Dorset, England. Complex dendritic or branching drainage channels, where water has flowed down the shore with the ebbing tide, cut through a surface layer of much-comminuted dark brown flotsam plant debris, leaving designs of white contrasting coral sand.

Abstract natural patterns of coral sand and plant debris on the drift line at Cape Tribulation beach

Abstract natural patterns of coral sand and plant debris on the drift line at Cape Tribulation beach

Abstract natural patterns of coral sand and plant debris on the drift line at Cape Tribulation beach

Abstract natural patterns of coral sand and plant debris on the drift line at Cape Tribulation beach

General view of Cape tribulation beach with driftline of fine particle plant debris

Queensland Seaweed – Turbinaria ornatum

Spiny Tops seaweed on coral washed ashore on sandy beach.

This is the Spiny Tops seaweed, Turbinaria ornatum, photographed at Cape Tribulation in Queensland, Australia. Superficially, it looks like the seaweed Sargassum crassifolium featured in a previous Posting. However, if you look closely, you can see that here the ‘fronds’ are much thicker and rounder. They are actually the expanded ends of small branchlets. They look a bit like small cookies that have been cut out of thin dough with a serrated-edge cookie cutter.

A. B. Cribb in his book Seaweeds of Queensland, A naturalist’s guide (ISBN 0-9595607-1-8) says about this species that:

The axis bears closely placed top-shaped branchlets with rigidly spiny margins. Eventually a gas-filled cavity develops in these branchlets and the buoyancy keeps the plant erect when submerged. This species is restricted mainly to the tropics where it is common on coral reefs.

Damon Ramsey in his book Ecosystem Guides, Tropical Seashores of Australia (ISBN  978-0-9757470-6-3) tells us that in these very distinctive seaweeds:

Along the top half of the stalk many smaller branchlets grow off to form a dome shape, and each has flat, star-shaped spikes at the end. They can be common in the murkier sandy shallows, and sometimes wash up on tropical beaches.

Spiny Tops seaweed on coral washed ashore on the Queensland coast.

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Queensland Seaweed – Botryocladia leptopoda

Queensland Seaweed (1) Botryocladia leptopoda - Grape Weed [Botryocladia leptopoda (J. Agardh) Kylin] washed ashore on sandy Cape Tribulation beach, North Queensland Coast, Australia.

Queensland Seaweed (6) Botryocladia leptopoda - Close-up image of Grape Weed [Botryocladia leptopoda (J. Agardh) Kylin] washed ashore on sandy Cape Tribulation beach, North Queensland Coast, Australia. Compared with the coast of Great Britain there were relatively few seaweeds on the beaches I visited on the Queensland coast. However, the red seaweed illustrated here was one of the most unusual in appearance. I found it washed up on the sandy beach at Cape Tribulation. It resembled a mass of molluscan or fish eggs – a bit like glistening caviar.

I photographed it from every angle and concluded from the way the swollen ‘leaves’ or ‘eggs’ were attached to branching stems that it was indeed a type of marine alga. It wasn’t till I obtained a copy of the book Seaweeds of Queensland – A Naturalist’s Guide (by A. B. Cribb of the Queensland Naturalists’ Club: Handbook No. 2, 1996, ISBN 0 9595607 1 8) that I could verify its identification. In fact, it is the same species that is featured on the front cover of the book – Botryocladia leptopoda (J. Agardh) Kylin.

The specimen in my own photographs has been affected by the hot sun and most of the ‘leaves’ have dimples in them where they have begun to shrivel out of water. Apparently, the common name is Grape Weed and to quote from the above book,

the densely placed vesicles clothing the branches make this one of the most beautiful red algae in Queensland. Large specimens may reach 40 cm in length. The generic name is derived from the Greek botrys – bunch of grapes, and clados – branch.

Its habitat is mainly in the sub-tidal region in sheltered and semi-exposed areas but also occasionally in shaded pools. Intriguingly, this lovely seaweed is thought to contain chemicals with important medicinal properties. Research is being conducted into these properties in connection with the treatment of lymphatic filarial parasites.

Queensland Seaweed (2) Botryocladia leptopoda - Grape Weed [Botryocladia leptopoda (J. Agardh) Kylin] washed ashore on sandy Cape Tribulation beach, North Queensland Coast, Australia.

Queensland Seaweed (3) Botryocladia leptopoda - Grape Weed [Botryocladia leptopoda (J. Agardh) Kylin] washed ashore on sandy Cape Tribulation beach, North Queensland Coast, Australia.

Queensland Seaweed (4) Botryocladia leptopoda - Grape Weed [Botryocladia leptopoda (J. Agardh) Kylin] washed ashore on sandy Cape Tribulation beach, North Queensland Coast, Australia.

Queensland Seaweed (5) Botryocladia leptopoda - Close-up image of Grape Weed [Botryocladia leptopoda (J. Agardh) Kylin] washed ashore on sandy Cape Tribulation beach, North Queensland Coast, Australia.

COPYRIGHT JESSICA WINDER 2013

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Cay Sandstone or “Beach Rock”

Cay Sandstone or "Beach Rock" (1) - Layers of "Beach Rock" or Cay Sandstone recently formed by a natural cementation of coral and shell fragments in still shallow water at the edge of Normanby Island, one of the Frankland Islands, Queensland, Australia.

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.

Cay Sandstone or "Beach Rock" (2) - Normanby Island is linked to an adjacent small island in the Frankland group by layers of "Beach Rock" or Cay Sandstone recently formed by a natural cementation of coral and shell fragments in still shallow water in Queensland, Australia.

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