Stones with holes made by Wrinkled Rock Borers (& other seashore creatures)

Beach stone with holes made by seashore creatures at Charmouth, Dorset, England.

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.

Beach Stones with White Lines & Patterns

Natural pattern in a Charmouth beach stone

Just east of the outlet of the River Char on Charmouth beach, in the area close to Raffey’s Ledge, the upper shore is strewn with many large irregular stones. Amongst these, the most noticeable are those with white patterns and lines, which on closer inspection turn out to be crystalline calcite-filled cracks in the matrix of the rock. I have been looking at these strange stones over the years and wondering what they were (see the earlier post Pebbles with white lines on Charmouth beach). Now I think I have the answer. They are the worn remnants of the inner cores of Birchi Nodules. Birchi Nodules appear high in the cliff above this section of shore and have a complex structure resulting from a series of processes in the sediments that took place millions of years ago before the sediments compacted into rock. The large ovoid or discoid Birchi Nodules can be seen scattered along a line below the more continuous stratified rocky Birchi Tabular Bed at the top of the cliff. The rest of the cliff below is mostly composed of darker thinly-bedded shales.

These remnants of the inner cores of Birchi Nodules are also a kind of septarian nodule. The stones illustrated here from Charmouth are partial remains that have been worn smooth by rolling around on the beach for a long time. Further east along the coast at Ringstead I have seen complete septarian nodules  that have freshly fallen from the cliff face of a different type of rock formation  (Septarian Nodules at Ringstead).

[I found out about Birchi Nodules from the most excellent on-line resource for the geology of the Dorset Coast written by Ian West. This is a veritable cornucopia of information but requires that you continuously scroll down the page to locate the items in which you are interested. It is well worth the effort if you really want to find out the information.]

Pebbles in a Stream

The bright sunshine created wonderful effects on the rapidly moving, crystal clear water of a chalk stream as it flowed over rounded pebbles. Light reflected from the rippled surface of the water. The stones below were covered with some sort of brownish algae that disturbed the flow and made either rainbow-coloured prisms or golden patterns of reflection.

Pebbles at Havelet

There are different combinations of colours and patterns in the pebbles of different beaches in the Channel Island of Guernsey. The assortment of pebbles in each location reflects the local geology of that area. There is a higher proportion of metamorphosed rocks compared with igneous rocks on the southern coast of the island, as seen here for Havelet which is just west of St Peter Port. Many of the rocks belong to the Southern Metamorphic Region dating back as far as 2,500 million years ago. Rocks from this region include gneisses and schists. The pebbles at Havelet can be compared with accumulations of pebbles on the north coast, such as those at Albecq, where the stones are mostly derived from younger igneous rocks such as granite, diorite and gabbro dating from about 700 million years ago in what is known as the Northern Igneous Complex.


British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernsaise, ISBN 0 9518075 2 8.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, pp 21 – 22, & 75 – 78. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0.

Seashore Goodies

I thought these colourful seashells, pebbles and seaweeds in their little bowls looked good enough to eat!

Pebbles at Langland Bay

Pebbles from Carboniferous Period rocks at Langland Bay

The pebbles at Langland Bay are all sedimentary rock as far as I can see but they include many different rock types from shores further away. Red and green Devonian sandstones, siltstones and conglomerates; light and dark bluish-grey Lower and Upper Carboniferous Period limestones (some with fossils), and shales; Namurian sandstones, grits, shales and coal measures with black and iron-bearing deposits; and no doubt the occasional erratic brought in by the ice sheets in periods of glaciation.  Consequently there is a great variety of colours, textures and patterns. Pebbles with holes made by sea creatures such as piddocks or other boring bivalved molluscs, or by marine polychaete worms and sponges are also a frequent occurrence. The pictures show the pebbles mostly in the the positions where they were found although I may have moved the odd one or two.

The stony west bank of Pennard Pill

Sea Lavender on lichen covered stones of the river bank at Three Cliffs Bay Just before the Pennard Pill watercourse takes a dramatic swing to loop around the giant sand dune to get to the shore at Threecliff (Three Cliffs) Bay on the Gower Peninsula, the right hand or west bank is composed of rough stones and then transitions into a salt marsh. [The area lies on the opposite side of the river to a more substantial and higher shingle bank that can be reached via a set of concrete stepping stones].

The low shingle surface of the right bank is relatively stable. Despite regular tidal inundations of brackish water, life clings to the limestone. Last August it was particularly attractive, covered with bright patches of yellow and black lichens, and ground-hugging clumps of partially red-stemmed plants with clusters of small pink flowers. I will have to find out what these plants are the next time I visit. I didn’t take close-ups. I had thought they might be Sea Heath (Frankenia laevis) but apparently that does not grow in this area – although it likes the same kind of habitat.  I think Sea Sandwort was also present. However, the numerous flowering stems of Sea Lavender I did recognise; and these plants were found equally spread in stony ground and on the wetter salt marsh area.