Porthmeor Rocks 2

These rocks are seen on the west side of The Island or St Ives Head in Cornwall, England. They form outcrops on the west side of Porthmeor Beach. I was fascinated by the sheer complexity of the colours, patterns and textures. As far as I can make out they belong to an un-named igneous intrusion (matagabbro or metamicrogabbro) composed of silica-poor magma which was later altered by low-grade metamorphism in the Devonian period between 359 and 416 million years ago.

The rock looks very different from that nearby which was featured in an earlier post. I was particularly drawn some outcrops which had an almost dark navy blue smooth surface but the majority of the rock surfaces were blue-grey and dirty yellow shades – some with smoother almost layered textures and others textured like lumpy porridge. The patterns were complex but included ancient splits and cracks that had been infilled with other materials of contrasting colours, giving paler or darker straight lines and angled cross lines – not to mention all the complicated later fracture patterns. There seemed to be large inclusions of other rock types too; this often occurs in granites. It is incredible that rocks of the same basic type can vary so much in their appearance within a few metres.

Starfish Skeletons

Starfish, usually Asterias rubens, sometimes get washed ashore in huge numbers on Rhossili beach in Gower, South Wales. Then they disappear again, either washed back out to sea or buried in the sand. Birds peck at them but they do not seem to like to eat them very much. This April, for the first time, I saw ghostly star shapes in the dry sand high on the shore. The long buried stranded starfish were reappearing in skeletal form. You may not have realised that Asteroidean Echinoderms like the Common Starfish had a skeleton. Hidden beneath their often brightly coloured and bumpy skins, bound together by connective tissue, is a lattice network of small calcareous ossicles in the shape of rods, crosses or plates. Spines and tubercles are also part of the skeleton, sometimes separate pieces resting on the deeper dermal ossicles or as extensions of them that project to the outer surface (Barnes 1963).

In these photographs of the dessicated remnants of starfish it is possible to see many small holes in the sand around them. These holes are the places where the scavenging sand hoppers are hiding from the dry hot air and waiting from the tide to return in the cool of the day to recommence cleaning up all the vegetable and animal debris that ends up on the strand-line – like dead starfish.

Reference

Barnes, R. D. (1963) Invertebrate Zoology, R. B. Saunders Company, 526-539.

Jellyfish and Japweed at Studland

Yesterday on Knoll Beach at Studland Bay in Dorset, the two most common things washed ashore were great clumps of Japweed (Sargassum muticum) and large barrel-mouthed or dustbin-lid jellyfish (Rhizostoma octopus). There were at least a dozen jellyfish on the stretch of sand that I walked. They varied in size from about 20 – 60 cm diameter across the dome. The colours varied from crystal clear to pink and blue. They all seemed very fresh and I think maybe some of them were still alive or just expiring. As they washed to and fro in the waves, sometimes entangled in the Japweed, they turned this way and that, upright then upside down, inside and out. This species is becoming an increasing feature on south-west coasts over the last couple of summers. I first encountered these seashore creature on the Gower Peninsula in South Wales where they have long been a frequent find on the beaches. Click here to learn more about the Rhizostoma octopus jellyfish on Jessica’s Nature Blog.

Green Wheat Growing

The last couple of months have seen the wheat growing at a tremendous rate. From late April to early June the growing crop has shot up from a few inches to a couple of feet. The stalks are standing tall and the ears of wheat are developing well. The standing crop looks especially attractive when the sun shines through the leaves, and the wind sends waves across the field.

Rocks at Porthmeor Beach, St Ives

The rocks in Cornwall are certainly different from any that I’ve seen before. Generally speaking, they are mostly igneous and metamorphic in type. Often the igneous rocks themselves have been metamorphosed. The rocks shown here were seen on the beach at Porthmeor and adjacent to The Island at St Ives. They lie below a thick layer of rusty-coloured superficial deposit of glacial till. I am not absolutely certain what the bedrock is. There are a number of rock types described by the British Geological Survey in their Geology of Britain Viewer in close proximity to each other, and it is rather difficult for an amateur to decide which is which. However, this particular outcrop reminds me of basalt which I saw on the shoreline of Grand Manan in the Bay of Fundy in Canada. So, I am going to suggest that it is an un-named mafic igneous rock formed of silica-poor magma that intruded into the earth’s crust in the Devonian Period between 359 and 416 million years ago. When the magma cooled, it formed an intrusion of fine to medium crystalline rock, often as basaltic dykes and sills.

On the other hand, I could be wrong, and it might be an un-named igneous intrusion of Metagabbro and Metamicrogabbro; or even Mylor Slate Formation with Hornfelsed Slate and Hornfelsed Siltstone.

Leaves & Light

Copper beech tree with new leavesThe last rays of this evening’s sun, shining through the translucent new leaves on the copper beech tree that I see from my window, reveal transitioning shades of red and green with intricate networks of veins. In close-up, it seems that you can almost see the individual cells.