The wind was blowing really hard across the navy blue water surface of slacks trapped behind the shingle banks at Rochefort Point. Rochefort Point is a short walk from the Louisbourg Fortress in Cape Breton, Nova Scotia. The ripples were tight packed and narrow, travelling at speed. The water was actually a brackish brown but reflected the clear blue of the sky resulting mostly in dark blue hues. From some angles and in certain lights the sun shone through the ripples revealing the reddish colour of the water. The low standing crests of the waves were so distinct that it seemed as if the water was viscous.
Lots of serious fossil hunters go to Seatown in Dorset to find fossil ammonites that have fallen to the beach from the cliffs. The cliffs for the most part are composed of Green Ammonite Member which is part of the Charmouth Mudstone Formation laid down in the Jurassic Period. The ammonites that are most commonly found in this type of rock are Aegoceras, Androgynoceras, Liparoceras, and Oistoceras. I haven’t found any decent fossils of the type I could pick up and take home, but there are plenty of fossils and ammonite impressions to be seen lying in pieces of rock on the shingle beach where people with hammers have broken them open. These pictures show some of the specimens that I found on my last visit. I am not sure which species they represent but maybe some local geologist may be able to look at these images and tell me what they are.
The soft smooth almost liquid muds that flow down the cliffs at Seatown after rain, pool and sink into the shingle on the beach. It doesn’t take long to see amazing networks of tracks and trails on the mud surface. These are made by a myriad of small invertebrate seashore creatures like worms, snails, and sandhoppers as they walk across, burrow, and tunnel into it, foraging for food and seeking shelter from exposure. The number of distinct track marks is amazing and I have no idea which mark was made by which animal (that is a whole new project requiring the collection of some mud samples for identification of the occupants of this habitat). Large bird footprints from crows and gulls show that these areas are also good places for them to feed on the creatures in the mud.
Images can be seen in greater detail by clicking on any photograph to view in the gallery, and then clicking “View full size” below the picture.
A contributory factor in the erosion of the beach rock at Seatown in Dorset is the burrowing activity of the marine bivalve mollusc called the piddock. Low on the shore millions of holes in the soft calcareous mudstones are evidence for the burrows made by Pholas dactylus. The holes are almost circular in shape reaching up to two centimetres in diameter, and can occur as a scatter or as dense populations wherever the rock remains wet between the tides. They seem to prefer the darker layers rather than the alternating light layers – although they are found in both. The rock on the east Seatown shoreline is composed of alternating almost horizontal layers of pale (carbonate-rich and carbon-poor) mudstone, and darker (carbon–rich and carbonate-poor) mudstone from the Belemnite Marl Member of the Charmouth Mudstone Formation.
Where successive generations of this boring mollusc have colonised the strata, the mudstone has been reduced to an irregular honeycombed mass. Most of the holes seem unoccupied and small pieces of orange-coloured gravel have filled them. In some burrows the empty white shells of the piddock can still be seen. Some of the burrows are undoubtedly still occupied but I did not have an opportunity to locate any for photographs since the area was only exposed for half an hour. The shells of the living animals may not have been visible because they tend to lie deep within the burrow but the living specimens can often be detected by the fact that their siphons extend from the shell to the surface and these periodically squirt out water during low-tide.
Pebbles and beach stones which have neat circular holes in them are frequently wave-washed and beach-tumbled pieces of rock that have broken away from intertidal rock layers that have been riddled with burrows made by rock-boring molluscs such as piddocks, in the way shown in these photographs from Seatown in Dorset, England, along the World Heritage Jurassic Coast.
On a whole range of scales, there are variations to the simple layering of the tuff (which is made of volcanic ash) and constitutes swathes of faintly striped and banded rock on the shoreline at Louisbourg on Cape Breton Island in Nova Scotia. Subsequent to the deposition and consolidation of the volcanic ash into tuff rock, the build-up of great pressures from earth movements at different times during geological history has caused both minor and major fractures in the rock. Small cracks sometimes filled up with dissolved minerals that crystallised to form veins of contrasting coloured material. In other places, intrusive molten lava squeezed its way into weak areas between or across the layers forming large-scale dikes. The igneous rock type of the dikes may be a greenish colour, and often cracks upon weathering in a characteristic way giving it distinct fracture patterns that are not present in the tuff.
[We stayed at the most excellent Louisbourg Harbour Inn while we explored this part of Cape Breton Island.]