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.

Seatown Mudslides 1

Seatown beach looking west showing mudslide on the cliffs

Mudslides are common on the cliffs between Lyme Regis and Eype in Dorset, England. On the western half of the beach at Seatown, the lower cliffs are composed of soft dark grey mudstones of the Green Ammonite Member, part of the Charmouth Mudstone Formation, dating from the early Jurassic period. The mudstones become more silty and sandy in the upper part of the exposure, and the top of the member is marked by the base of a micaceous sandstone known as the Three Tiers. Above this is the Dyrham Formation comprising mostly soft pale, dark or greenish grey sedimentary rocks some of which weather to brown or yellow colours (Woods 2011 pages 30 -34).

After heavy rain, water streams down the cliff face carving narrow valleys through the soft mudstone and sandstone strata, liquidising it, and causing flows of pale viscous mud that ooze over the bright orange pea gravel and pebbles on the shore. I stopped to photograph the interesting contrast of colour and texture as the two materials mixed, and it started to rain. The scattered drops pitted the mud surface and I moved in for a closer shot of the patterns. I then noticed that the mud showed shallow flow marks and very small tracks and trails. It was not until I returned home and examined the pictures that I discovered the small creatures that were actually making the trails. They were small amphipod Crustaceans, probably sandhoppers, that were bogged down and trying to break free. Presumably, they had been sheltering from exposure in the damp gravel while the tide was out and suddenly found themselves engulfed by the mud.

I found all this particularly interesting from a palaeontological point of view, or perhaps I should say an ichnological one. Trace fossils are often found in rocks and there is a constant quest to interpret marks that have become preserved in the stone, whether made by elemental conditions or by creatures. Observing events and phenomena in the present time helps in the understanding of the phenomena of the past. It is possible to imagine that the rain pits and sandhopper trails in this mudslide could themselves be preserved as future trace fossils. All that would be needed is some hot sun and drying wind, quickly followed by burial in a fresh deposit of liquid clay or mud or gravel.

Intriguingly, just a few hundred metres further along this west shore at Seatown, the rocks are absolutely riddled with millions of trace fossil burrows made by marine invertebrates like worms and crabs – but more about that later.

Mudslide on the west cliffs at Seatown, Dorset, England.

Mudslide in Green Ammonite Member mudstones at Seaton

Liquid mud from a mudslide oozing over the beach at Seatown

Patterns in liquid mud on the seashore

Patterns in liquid mud on the seashore

Patterns in liquid mud on the seashore

Patterns in liquid mud on the seashore

Rain-pitted liquid mud with trapped sandhoppers

Rain-pitted liquid mud with trapped sandhoppers

Rain-pitted liquid mud with trapped sandhoppers

Rain-pitted liquid mud with trapped sandhoppers

Rain-pitted liquid mud with trapped sandhoppers

Rain-pitted liquid mud with trapped sandhoppers

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.]

Cliffs on the west side of Seatown Beach

Cliff on the west side of Seatown beach in Dorset, England.

In my previous post about rip-rap rocks on Seatown Beach in Dorset I mentioned that the boulders had been positioned to protect the soft cliffs from erosion. This post shows the first of a series of photographs illustrating these soft eroding cliffs on the western side of the beach.The cliffs are made of Lower and Middle Lias rocks from the  Jurassic Period.

The lower part of cliff section nearest to the rip-rap, and just west of the fault valley with the River Char, is the Green Ammonite Member of the Charmouth Mudstone Formation; this is principally a medium or dark grey mudstone which can be silty or sandy in the top part. The upper area of the cliff belongs to the Dyrham Formation; this is more variable in composition and includes pale to dark grey and greenish grey micaceous mudstone with interbeds of siltstone or very fine sandstone that weathers to brown or yellow (Woods 2011). Clay, sandstone, limestone layers and concretions are much more common in the Dyrham Formation. It is difficult to see exactly what is going on with the stratification here because so much is obscured by landslips and mud slides. Small streams cut down from the cliff top, softening the mudstones and sandstones, causing areas of collapse, and carrying the muddy fluid and liquid clay down to the pebble shore. The pattern of erosion and weathering is repeated all along the beach towards Golden Cap.

As usual, I am still working out my understanding of these perplexing cliffs, trying to find key fixed points or features from which I can work outwards to unravel the stratigraphy. At first it all looks a bit of a mess but I am sure I can interpret it in the end. At least there was a great array of natural textures and patterns to capture, and many processes of accumulation and disintegration to study.


Woods, M. A. (compiler) (2011) Geology of south Dorset and south-east Devon and its World Heritage Coast: Special Memoir for 1:50,000 geological sheets 328 Dorchester, 341/342 West Fleet and Weymouth, and 342/343 Swanage, and parts of sheets 326/340 Sidmouth, 327 Bridport, 329 Bournemouth and 339 Newton Abbot. British Geological Survey, Keyworth, Nottingham. ISBN 978-085272654-9.

Rip-Rap Rocks at Seatown

View looking west at Seatown beach, Dorset, England, showing rip-rap sea defences in foreground

The cliffs are eroding in many places along the Dorset coast, particularly where the rocks are soft. This results in land slips and mud slides. It has always been going on but in recent years the erosional processes seem to have accelerated along with changing weather conditions. At Seatown on the coast near Chideock in Dorset, large boulders have been imported to protect the shore from the sea adjacent to the Anchor Inn that sits at the mouth of the River Char. I cannot name the rock types represented in the rip-rap for certain since they are not local to the area and have been chosen specifically because they are harder and more resistant than the cliffs on this beach. I am not even sure that they were quarried in Britain. However, some of them remind me a lot of Carboniferous limestone with fossils, calcite and haematite inclusions. Anyway, they are really interesting and well worth a closer look. The patterns, colours, and textures are amazing. I would like to visit again when the rocks are wet and the more subtle colour variations would be highlighted.

Rock Pattern & Texture at Kimmeridge Again

Click HERE to buy prints of these and other rock images featured on Jessica’s Nature Blog.

For information about the geology of this location please look at:

John C. W. Cope Geology of the Dorset Coast, Geologists’ Association Guide No. 22, Geologists’ Association, 2012, pp 159-167, ISBN978 0900717 61 1.

West, Ian. Kimmeridge Bay, Dorset. Geology of the Wessex Coast of Southern England. An on-line information resource.

M. A Woods (compiler) Geology of south Dorset and south-east Devon and its World Heritage Coast, British Geology Survey, NERC, 2011, pp 61 – 67, ISBN 978 085272654 9.



Carboniferous Limestone Textures at Threecliff

Rock pattern and texture in Carboniferous Limestone

A further assortment of interesting rock patterns and textures in Carboniferous Limestone strata of the cliffs on the east and west sides of Threecliff Bay on the Gower Peninsula in South Wales. Of particular interest is the honey-comb texture of erosion along the bedding planes of some of the dipping layers, and the red colouring due to iron bearing minerals of other areas,  on the east side of the bay. On the west side of the bay the rocks seem less weathered and are often encrusted with patches of black lichen.