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Category Archives: ROCK
Natural Rock Patterns: Lithologic Series 1-12
Patterns of iron-stained calcite deposited on the surface of quarried limestone on the Isle of Portland – which is part of the UK’s Jurassic Coast.
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Septarian Nodules at Ringstead
What is a septarian nodule? Well, basically, it is a big boulder containing a three dimensional jig-saw of smaller angular pieces of the same rock – and all the pieces are bound together with white crystalline calcite. I can’t do better than to quote the definition given in The Oxford Dictionary of Earth Science:
A concretion, roughly spheroidal in shape, usually of clay ironstone, and characterised by an internal structure of angular blocks separated by radiating mineral-filled blocks. The mineral filling the cracks is usually calcite. The structure results from the formation of a hard exterior to the nodule due to the development of an aluminous gell on the exterior, followed by dehydration of the colloidal mass in the interior, leading to cracking and subsequent infilling of the radiating pattern of cracks.
The British Regional Geology Series for the area indicates that the Ringstead Waxy Clays, which are virtually at the top of the Corallian Beds of the Upper Jurassic strata, comprise about 5 metres of clay with thin seams of clay ironstone that are nodular in places. It seems very possible that the septarian nodules are from this source. The Ringstead Waxy Clay is also the deposit in which numerous fossil oysters, Deltoideum (Liostrea) delta, are found [mentioned elsewhere in Jessica's Nature Blog and also on the sister site Oysters etc.]
References
Oxford Dictionary of Earth Sciences, Edited by Michael Allaby, Oxford University Press, first published 1990, third edition 2008, ISBN 978-0-19-921194-4
The Hampshire Basin and adjoining areas, R. V. Melville and E. C. Freshney (1982), British Regional Geology Series, Fourth Edition, Institute of Geological Sciences, HMSO, ISBN 0-11-884203-x.
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Rocks at Caswell Bay
Caswell Bay in Gower features a classic sequence of different rock types within the broader category of Carboniferous Limestone. As you walk in an easterly direction from the café and car park at the top of the beach, towards the sea with the rock outcrops on your left, you walk past a series of spectacular rock formations with marked stratification and jointing, weathering and erosion patterns, faults, thrusts and folds. A repeated sequence of Caswell Bay Mudstone, Caninia Oolite, Laminosa Dolomite, Crinoidal Limetones, and Seminula Oolite.
It is not a straightforward series because of the synclinal and anticlinal folding and thrusts – so I am still trying to fathom out which rock is which! Nevertheless, artistically and photographically there was much to enjoy and this Posting presents a range of the natural patterns and structures in the limestone. Some of the more interesting rock patterns have been photographed close-up and were shown in an earlier Posting Caswell Rock Patterns & Textures.
One of the sources of information I am using to try and understand the geology at Caswell Bay and to identify the rocks that I am photographing is the on-line Geological Society Field Guide to Caswell Bay.
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Budleigh Salterton Pebbles Part 4
Click here for more pictures and Posts on Pebbles at Budleigh Salterton in Jessica’s Nature Blog.
COPYRIGHT JESSICA WINDER 2013
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Cliffs at Mewslade Bay
The cliffs at Mewslade Bay on the Gower Peninsula are spectacular. They are formed from High Tor Limestone from the Carboniferous Period. You can see a range of the wonderful shapes of the tall peaks and cliffs that fringe the bay in the gallery below. Click on any picture to enlarge it and to see the photographs as a slide show.
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Black Rock Limestone at Rhossili
Details of the natural patterns, colours, shapes, and textures in cliff strata at Rhossili Bay on the Gower Peninsula. These rocks belong to the Black Rock Limestone Sub-Group of Carboniferous period strata (Tournasian, Courceyan, Pembroke Limestone Group). They are typically limestones that are dark grey, thin to thick bedded, bioclastic and dolomitic in the upper part.
The black colouration in these close-up images is superficial and created by an encrusting bio-film, probably of black lichen but maybe a cyano-bacterial film. The green colours are caused by a coating of microscopic algae. The bright red, yellow, and orange patches are areas where rock has recently broken off to reveal limestone containing iron compounds. I took these photographs because I found the abstract compositions pleasing – natural geological abstract art.
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Rocky Oregon Sea Shore at Yachats
The seashore at Yachats in Oregon is quite unlike any shore I’ve seen before. Rounded pillows of rock form extensive platforms on the beach. Landwards, the rocks are mostly uncolonised by marine invertebrates and algae but the lower shore is covered by a profusion of seashore creatures and some hardy species of seaweed. Low-growing dense mats of golden green algae, that coat the rocks in isolated patches, glow vibrantly in the late afternoon sun.
The North West Pacific Coast of America at Yachats is pounded relentlessly by high waves that crash against the rocks giving rise to 7 metre high plumes of sea spray even on a summer’s day – higher in winter or windy weather. Wave watching here is a great tourist attraction because it can be so spectacular. It is a great draw to visitors throughout the year – especially when stormy weather washes agates ashore. Local hotels and restaurants accommodate people who love to watch the Pacific Ocean waves – with strategically placed windows and balconies and hot tubs for winter watchers.
Smooth rock platforms extend north and south as far as the eye can see. The size and scale of the scenery is difficult to convey but some images below show buildings built on the rocks just above high tide level to put the views into perspective. Despite the obvious hardness of the rock, the power of the waves has worked at small joints and cracks to produce an erosion surface with numerous shallow pools and deep gullies which are important habitats for flora and fauna.
There did not seem to be much in the way of flotsam and jetsam when I visited. No strand line of seaweed or seashells. No rubbish. No small stuff washed ashore. Driftwood high and dry on the rocks consisted of whole trees stripped bare, worn smooth, and bleached by sun and salt.
A sea mist or fog is a frequent occurrence at Yachats. On a clear day, a bank of fog always seems to hover and threaten from the horizon. When the mist rolls inshore, it can make an August day seem like mid November. On a sunny day, the haze filters the sunlight reaching the rocky beach and creates wonderful atmospheric effects in which the seaweeds glow a brighter hue contrasting vividly with the dull rocks.
Holiday homes and hotels perch on the coastline – normally just out of reach of the waves. They must be very robustly-built to withstand the onslaught of the sea and weather. I dread to think what the house insurance must be like – especially as this is also an area liable to damage from tsunamis. There are signs on viewpoint benches and roadsides.
The sandy soils above the rocky shore support an abundance of flowering plants at Yachats. This flat tract of land is bordered by an area of pines and swampy pools where black bears roam, marking territory by claw marks on tree bark, and scat with plum stones. Cougars descend to the shore at night from the mountains that loom over Yachats – so trail users need to be wary.
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High Tide at Mewslade Fault Gully 1
Video clip showing high tide waves surging into the fault gully at Mewslade Bay, Gower, South Wales – showing the regular High Tor Limestone strata of Thurba Head in the background, with the fault breccia below it on the water’s edge and to the bottom left of the picture.
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Fault Gully Rocks at Mewslade Bay
As you arrive at the narrow gully that forms the entrance to Mewslade Bay, you cannot help but notice just how different the rocks are from those that outcrop on the hills around. Up on Thurba Head to the left and to the high cliffs and outcrops on the right, thick grey layers of High Tor Limestone from the Carboniferous period are clearly visible. However, down in the gully, there is nothing but a jumble of assorted broken rocks of all kinds of origin, which in some places have been cemented together with white crystalline calcite.
The reason for the unusual geology at this point is an ancient rupturing and tearing of the rocks – faulting. The steep-sided dry valley that leads from Pitton village down to Mewslade Bay follows the line of a fault. In fact, two faults converge here. The fault line itself extends inland and North-north-east across first the High Tor Limestone which appears as cliffs along the South Gower shore in this locality, then passes across Hunts Bay Oolite (the next in the succession of Carboniferous Limestone strata), and possibly other rocks in the series such as Gully Oolite. Further north, around and beyond Pitton and Middleton, superficial drift deposits of glacial till, like gravelly clay, mostly cover and obscure the solid geology below. Beneath this layer of till deposit lies the solid Old Red Devonian Sandstone rock.
Faulting occurs as brittle rocks crack when they can no longer bend and fold to accommodate earth movements. Once the rocks break, there can be major movements of the strata on each side of the crack. One side may move up or down in relation to the other. The two sides may slide and tear against each other horizontally. The friction of the rocks grinding one side on the other often breaks up the rock into smaller pieces. Compared with the solid rock strata, the area of smaller fragments is very water permeable.
It is thought that the entrance to Mewslade Bay is a fault gully, which has probably been eroded out by glacial meltwater during a period of permafrost when an ice sheet covered most of the Gower Peninsula. The various rock types represented in the gully structure would have been derived from both the Carboniferous Limestone and the Old Red Devonian Sandstone geology fractured by the fault and carried down towards the sea by water. The name for this type of deposit is Fault Breccia when the pieces are large, and Fault Gouge when they are much smaller. Secondary minerals such as calcite can precipitate from groundwater circulating around the loose fragments, filling the empty spaces or voids and cementing rock together.
Low down, nearest the ground, the fault breccia looks like tutti-frutti ice cream with rock fragments coloured pink, red, brown, yellow, and grey embedded in a crystalline calcite. Higher up, the fragments are consolidated – but without the all-encompassing white matrix.
Gallery of fault breccia rocks in the fault gully at Mewslade Bay
Calcite occurs in different forms. There are neatly formed crystal clusters as veins or in pockets. Most are basically white but some are stained orange with iron. Superficially, the crystals are often coated with green or brown algal bio-films. In some places, especially in tunnels and low down in the deposit, where the the rocks are washed by the sea and abraded by debris at high tide, the calcite is worn down to a smooth surface with just an outline pattern of the crystals and cleavage.
Slide show of crystal calcite from the fault breccia at Mewslade Bay
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