Yet more natural fracture patterns in Jurassic rocks at Seatown in Dorset, England, re-coloured with digital wizardry.
More natural fracture patterns in Jurassic rocks at Seatown in Dorset, England, re-coloured with digital wizardry.
Natural fracture patterns in Jurassic rocks at Seatown in Dorset, England, re-coloured with digital wizardry.
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.]
At Eype beach in Dorset, on England’s south coast, the Jurassic cliffs comprise an unstable combination of rocks that are prone to land slips, especially in wet weather. The Eype Clay Member near the top of the cliff is mostly composed of blue-grey micaceous mudstone. The layer can be 65 metres in depth with little stratification. It seems to liquefy when subject to heavy rain and then pours down to the shore. It eventually dries out, contracts, and cracks, resulting in an infinite variety of natural fracture patterns.
These pictures show details of the colour, texture, and patterns in vertical and horizontal exposures of the rocks of the Ringstead Clay Member (also known as the Ringstead Waxy Clays) at Ringstead Bay in Dorset, England. This is in the upper part of the Corallian Formation belonging to the Jurassic Period. It comprises four different layers or strata that reach a combined thickness of up to 5 metres. The clay deposits contain bands of reddish-brown, siderite nodules (iron carbonate) that are responsible for the rusty or ferruginous colouration of the otherwise brown and grey clay. There are interesting patterns of cracks where the clay is drying out; and changes of colour tone according to the moisture content of the clay. The most obvious fossils in this particular clay group are the large, flat, almost triangular, oysters – Deltoideum (Liostrea) delta.
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Continuing the theme of patterns that decorate the rocks at Kimmeridge Bay, the circular and ovoid fractures seen here in large detached boulders on the shore made me think of noughts and crosses. They obviously result from some entirely different processes to those that produce the large and small polygons in the rock ledges.
There are fossil ammonites in these boulders as well as the fracture patterns.
Cross or star shape at the junction of several fissures in the rock layer above. And below some slightly different sub-circular fracture patterns within a layer that additionally has polygonal cracks and fissures.
For more information about Kimmeridge Bay see the Purbeck Marine Wildlife Reserve Web Site.