Stony Ripples from Ancient Seabeds

Posted on May 19, 2013 by winderjssc

There are many strange and interesting shapes and textures in the rocks on the beach at Mewslade Bay on the Gower Peninsula in South Wales. Most of them seem to be the result of weathering and erosion but these photographs show something different, unique, for that location. They appear to be preserved (fossilised if you like) ripple marks from the ancient seabed sediments of which the rocks are composed and date very approximately to about 350 million years ago. They have a distinct patterning which is very familiar from the sand and mud of present day seashores in the same area.

The rock itself is High Tor Limestone from the Carboniferous Period. Actually, It’s a bit old fashioned now to say just Carboniferous Period. Everything has changed. To be more accurate, I should say that the High Tor Limestone Formation is part of the Pembroke Limestone Group, which originated in the Visean division of the Dinantian, which in turn is part of the Mississippian sub-division of the Carboniferous Period.

What were at one time horizontal seabed surfaces have become near vertical over many millions of years of earth movements. The now-exposed surfaces of the old bedding planes are revealed in the entrances to caves at Mewslade Bay. The photographs show them encrusted with recent colonies of living acorn barnacles and occasional limpets.

Reference

Howells, M. F. (2007) Wales, British Regional Geology, British Geological Survey, Keysworth, Nottingham, UK, ISBN 978-085272584-9, pp 112 – 125.

Red Granite 1

Posted on May 18, 2013 by winderjssc

Reblogged from Photographic Salmagundi:

Natural Rock Patterns: Lithologic Series 1-12

Posted on May 16, 2013 by winderjssc

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.

Septarian Nodules at Ringstead

Posted on May 15, 2013 by winderjssc

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.

Rocks at Caswell Bay

Posted on May 5, 2013 by winderjssc

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.

Budleigh Salterton Pebbles Part 4

Posted on April 16, 2013 by winderjssc

Click here for more pictures and Posts on Pebbles at Budleigh Salterton in Jessica’s Nature Blog.

Cliffs at Mewslade Bay

Posted on April 13, 2013 by winderjssc

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.

Black Rock Limestone at Rhossili

Posted on April 11, 2013 by winderjssc

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.

Rocky Oregon Sea Shore at Yachats

Posted on April 10, 2013 by winderjssc

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.

High Tide at Mewslade Fault Gully 1

Posted on April 6, 2013 by winderjssc

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.

Jessica's Nature Blog

natureinfocus.WordPress.com weblog

Category Archives: GEOLOGY