The river flowing down to the seashore meets with waves from the sea at Charmouth in Dorset, England. This somewhat abstract image of the natural patterns generated from the meeting of the two forces shows the freshwater continuing to flow smoothly seawards on the left of the channel (top left) while on the right it rebounds from the curving bank with the ripples moving upstream and towards the middle of the channel. The blue and white are reflected sky, and the yellow is reflection from the shingle beach.
Beach stones with holes in them excite the curiosity of most people. How did the holes get into the rock? There is no single answer but in many cases the holes in pebbles and beach stones have been made by various seashore creatures including several types of bivalve molluscs, marine worms, and sponges. The same creatures can also make holes in thick old seashells. There are several earlier posts on Jessica’s Nature Blog describing how the holes are made, by piddocks, for example Pholas dactylus, sponges such as Cliona celata, and polychaete worms like Polydora ciliata and Polydora hoplura. Frequently, there is evidence for more than one type of organism occupying the same stone.
One of the bivalve molluscs that creates holes in stones and thick oyster shells is the Wrinkled Rock Borer Hiatella arctica (Linnaeus). Like the piddock, this species can actively excavate a burrow in soft stone for shelter and protection although unlike the piddock it can attach itself by byssus threads to the outside of solid objects or in cracks and crevices. However, once embedded in the stone it can no longer exit the burrow but obtains all it needs for sustaining life via the tunnel connecting it to the outside world. Wrinkled Rock Borers are smaller than piddocks, measuring no more than 3.8 cms in length when mature. The valves of the shell are thick and robust with distinct furrows, and the leading edges exposed to view in the burrow are rough and straight edged (truncate). Tebble (1966) says that it is not possible to distinguish between the different species of Hiatella in British waters but the descriptions apply to all the species ever recorded here. Hiatella arctica is common around the British Isles from the lower regions of the shore to considerable depths…… It has a wide geographical distribution in the northern hemisphere from the Arctic south through the Atlantic, Mediterranean and Pacific, but the particular limits of its occurrence are not known. It is almost impossible to remove empty Hiatella shells from the excavated holes without breaking them.
Hiatella holes in rock can be secondarily occupied by a similar but smaller bivalved mollusc called Irus irus (Linnaeus). This grows to about 2.5 cms in length. The protruding frill-like concentric ridges on the shell can be very distorted in shape if the shell is occupying a burrow that is too small to allow normal growth. I am not able to discount the possibility that it is Irus shells occupying Hiatella burrows in some of the beach stones illustrated here. Irus (also known as Notirus irus) occurs from low in the littoral zone to a few fathoms.
Tebble, N (1966) British Bivalve Shells: A Handbook for Identification, published for the Royal Scottish Museum by HMSO, Second Edition 1976, [Hiatella p172-173 & Plate 7h; Notirus p124-125 & Plate 7g].
Hunter, W. R. (1949), The Structure and Behaviour of ‘Hiatella gallicana@ (lamarrck) and ‘H. arctica’ (L.), with special reference to the Boring Habit. Proc. Roy. Soc. Edin. B, 63 III (19): 271-289, 12 figs.
The further you walk along Weymouth pier the deeper and bluer the water – turquoise tinted. In the shallows, the sand on the sea bed makes the water appear more yellow. On this calm day, the water surface was riffled by the wind to produce patterned textures where the transient ridges were delineated by the light they caught.
The weather was very changeable but it was still a lovely spring afternoon for a walk up the hill to the barn. It is a good viewpoint up on Charlton Down, looking over the gentle rolling hills of arable farmland. I haven’t been along that path for some time and it was amazing to see the difference in the surrounding fields. The young oil-seed rape plants that I had seen as raindrop-covered seedlings last December were now hip-high and covered in clusters of faintly scented yellow flowers. The grey skies broke with the brisk breeze and clouds scudded across the blue sky, making fast-moving shadows over the rural scene. The agricultural machinery parked by the barn remain a constant while everything around changes by the moment, with the weather, and through the seasons.
These images are a study of patterns and surface texture on the shallow water over the sandy seabed at Weymouth, viewed from the promenade leading to the pier. I like the way that the waves look as if they are drawn with fine lines onto the sea with a white pencil. The clear water reveals the yellow of the sand below the waves. (If you wish, you can click on the photographs to enlarge them and see the details).
The most common fossils at Seatown on the Dorset coast are belemnites. These are bullet-shaped internal hard parts of a type of extinct cephalopod (think cuttlefish, squid and octopus). For a great deal of the length of the beach, the rock strata are hidden by debris falling down from layers above. There are lots of minor mudslides and landslips. However, as you get nearer to the western extremity of the beach, approaching Golden Cap, a continuous kerb-like, harder, and more calcareous stone layer makes an appearance. This is the Belemnite Stone that has been raised to view by a small anticlinal flexure. Below it are many layers of Belemnite Marl that can be seen in cross-section in the vertical face at the base of the cliff; and also extending out horizontally beneath the gravelly beach and exposed at low tide. They alternate light and dark layers. Fossils are abundant with belemnites predominating but ammonites are also common. The huge numbers of belemnites are thought to have resulted from mass die-offs following mating frenzies.