Eroded Gypsum at Knossos

Once upon a time the ancient palace at Knossos on the island of Crete was made of smooth white blocks of gypsum with polished surfaces that gleamed in the sunlight. The effect of weathering over the centuries has stripped away the surface of the building blocks and created rough textures and patterns of sharp edged furrows where acid rain has dissolved the stone as it runs over and down the masonry. The gypsum blocks mimic a phenomenon called rillenkarren found on a larger scale in limestone landscapes all over the world. These erosion patterns in the landscape are known as karst topography. I previously photographed an example of karst topography with rillenkarren in the Queensland outback in Australia near the old mining town of Chillagoe.

A Stone Wall in Rethymnon

Apologies for the quality of these pictures taken way back in 2009 with my first digital camera. I just came across the images as I was sorting my photo collection. I took them on a holiday to Rethymnon on the island of Crete in the Mediterranean. At the time, I thought the strange patterns in the stone walls of the old Venetian fortress were part of the rocks themselves. Locally the bedrock is described as crystalline limestone. Looking at the pattern and texture now, I am not so certain. It looks more like something that has been caused by the weathering process. I am thinking that maybe over the centuries since the construction of the wall, the limestone has dissolved in rain water and the calcium has recrystallized in this way between the large and small rocks that make up the wall. This has in a way reinforced the wall by further binding the elements together. I don’t know whether the walls were originally built as dry stone or whether they incorporated mortar. The crystal formation seems to be acting as a mortar now.

I have seen something similar to this phenomenon on my beloved Rhossili Beach on the Gower Peninsula in South Wales. I will fish out some photos for comparison.

Fractured 1-3

Natural fracture patterns in Jurassic rocks at Seatown in Dorset, England, re-coloured with digital wizardry.

Rock Texture & Pattern at Dog’s Bay

The rocks at Dog’s Bay in Connemara, Ireland, are part of the Galway Batholith. In particular they are composed of the Errisbeg Townland Granite riven by faults and many dikes containing other younger intrusive igneous rocks. The juxtaposition of the different rock types is a phenomenon marked by contrasting, colours, textures, and patterns. The whole rocky terrain has been levelled off and smoothed by ice sheets and reflects many glaciation features.

Feely, M. Leake, B.E., Baxter, S. Hunt, J. and Mohr, P. A Geological Guide to the Granites of the Galway Batholith, Connemara, western Ireland.Geological Survey of Ireland, 2006.

Volcanic Tuff near Louisbourg Lighthouse – Part 4

More rock textures from the compacted ash in tuff deposited from explosive volcanic eruptions during the Neoproterozoic period at Louisbourg in Cape Breton, Nova Scotia, Canada.

Seatown Dissected Mudstone Layers

Coastal mudstone layers eroding into long fingers of rock separated by narrow sinuous channels

The rocks low on the beach at Seatown in Dorset are wearing away in a most peculiar fashion. In an earlier post I showed narrow sinuous channels cutting their way down through the mudstone between tide levels and I wondered how they had formed and what had influenced their shape. These are my thoughts and speculations about the processes contributing to these formations.

At one point along this stretch of shore, the narrow winding channels can be seen dissecting the rock layers into a number of adjoined parallel bars  (a bit like the fingers of a Kit-Kat). So what might be going on?

I have noticed by looking at the cliffs along the western and eastern shores at Seatown that there seems to be a propensity for this kind of medium to naturally form polygonal cracks or fractures once exposed to air and losing moisture. Below are three random examples of fracture patterns in cliff materials.

I think that the same phenomenon is a feature of the exposed layers of mudstone bedrock that outcrop inter-tidally. It is just possible to see the faint lines of these natural cracks in some of the close-up photographs below. Most of these original cracks are obscured because they have become the preferred location for Polydora bristle worms to occupy and create burrows. Although only a few millimetres across, the holes made by burrow-making activity have weakened the fracture lines, widened and extended them. At the same time as this bio-erosion activity is going on, continual swash and backwash by waves, and attrition by rolling gravel and pebbles, has smoothed and lowered the surface by physical destructive processes. (Chemical erosion plays a part too but will need a lot more explanation another time.)

As the combined physical and bio-erosion processes continue, the depressions where the worms burrow increase in size and can join up to form channels.

Once a channel is open, water and hard transported materials like rocks, pebbles gravel, and sand, can move rapidly through the channel in an upshore direction with each wave that breaks on the beach; and in a seaward direction as the water drains back down the shore. This physical action accelerates the erosion of the channels which speedily become deeper and wider to such an extent that they can carve the rock into distinct blocks. Smaller channels can form diagonally, at an angle to the shoreline, as they follow the conjoined outlines of the burrow-filled rock fractures. However the main force of the waves on the beach is perpendicular to the shoreline. This means that the channels formed by chance in that orientation are the ones that are most affected and enhanced by the swash and backwash of the waves.

The images also show 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. The uppermost layers being weakened by various erosional processes that have effectively divided them up into strips, erode and break away more easily on the shoreward edge parallel to the shore.

There are more contributing factors to rock erosion on the coastline than I have been able to talk about here and I will explore them further in later posts.

All the photos are shown again below and you can click on any thumbnail to see a larger version of the image in a gallery format

Patterns in Pyroclastic Breccia near Louisbourg

Angular rock fragments embedded in a volcanic ash matrix from a pyroclastic flow in Cape Breton Island

The entire coastline north and south of Louisbourg on Cape Breton Island in Nova Scotia, Canada, is composed of Neoproterozoic volcanic rocks dating back 575 million years. A few hundred metres north of the Louisbourg Lighthouse along the Trail to Morning Star Cove and Gun Landing Cove, lies an area of seashore that offers the chance to take a close-up look at the compositions and natural patterns in rock made of pyroclastic breccia.

Pyroclastic literally means ‘fire-broken’ and is used to describe volcanic rocks made up of fragmented pieces that are normally the result of an explosive volcanic event. Clasts are pieces of broken down rock. According to the Oxford Dictionary of Earth Sciences edited by Michael Allaby (ISBN 978-0-19-921194-4) “breccia is a coarse clastic sedimentary rock, the constituent clasts of which are angular. Breccia literally means rubble and implies a rock deposited very close to the source area. The term may also be applied to angular volcanic rocks from a volcanic vent.

Rock of a similar type of origin, although not identical, has been used by the sculptor Emily Young in the creation of the carved heads that were recently on display on Neo Bankside in London. Stillness Born of History II is described as being made of “onyx with volcanic pyroclastic brecchia”.

Stillness Born of History II at Neo Bankside

Carved stone head by Emily Young displayed at Neo Bankside in LondonBeautifully textured and patterned onyx with volcanic pyroclastic breccia has been used by the famous sculptor Emily Young to create this fabulous head called Stillness Born of History II displayed (courtesy of Bowman Sculpture) at Neo Bankside in London, England, just south of the Tate Modern Gallery. Pyroclastic breccia is composed of fine-grained volcanic ash, pumice, and rock fragments larger than 2.5 inches (63.5 mm). When the fragments are smaller than this, the rock is called tuff.

Flints & Ironstones at South Beach Studland

Beach stones derived from the chalk cliffs at South Beach, Studland, Dorset, England.

The upper part of the sandy beach at South Beach, Studland in Dorset is littered with pebbles and stones of many colours and interesting patterns and textures. They are mostly flint and ironstone that has weathered out of the chalk that forms impressive cliffs from here to Old Harry Rocks and the Foreland or Handfast Point in the distance.

Click on any image to enlarge and view in a gallery.

REFERENCES

Barton, CM, Woods, MA, Bristow, CR, Newell, AJ, Weathead, RK, Evans, DJ, Kirby, GA, Warrington, G, Riding, JB, Freshney, EC, Highley, DE, Lott, GK, Forster, A, and Gibson, A. 2011. Geology of south Dorset and south-east Devon and its World Heritage Coast. Special Memoir of the British Geological Survey. Sheets 328, 341/342, 342/343, and parts of 326/340, 327, 329 and 339 (England and Wales), 9–100.

Cope, JCW, 2012 Geology of the Dorset Coast, Geologists’ Association Guide No. 22, Guide Series Editor SB Marriott, The Geologists’ Association, 191-194.

Swanage Solid and Drift Geology (map), British Geological Survey (Natural Environment Research Council) 1:50,000 Series, England and wales Sheets 342 (East) and part of 343

Joggins Rock Textures 15

Ripples from water currents over fine sediments deposited in the Carboniferous Period preserved on the lower surface of boulders from a rock fall

Joggins Rock Textures 15a – Ripples from water movement over fine sediments deposited in the Carboniferous Period, preserved in boulders from a rock fall at Coal Mine Point (also known as Hard Scrabble Point) at Joggins Fossil Cliffs, Nova Scotia, Canada.

Ripples preserved from water currents over fine sediments in the Carboniferous Period on the lower surface of boulders from a rock fall

Joggins Rock Textures 15b – Ripples preserved in stone.

Ripples preserved from water currents over fine sediments in the Carboniferous Period on the lower surface of boulders from a rock fall

Joggins Rock Textures 15c – Ripples preserved in stone.