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.

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.

Volcanic Tuff near Louisbourg Lighthouse – Part 3

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

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.]

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

Rock colour, pattern, and texture in Main-a-Dieu sequence volcanics near Louisbourg

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

Volcanic Tuff near Louisbourg Lighthouse – Part 2

 

Coastal exposure of volcanic tuff rocks in Cape Breton

The volcanic ash deposits or tuff found in coastal rocks around Louisbourg Lighthouse in Cape Breton show subtle colour banding. Originally, ash from volcanic eruptions fell into lakes occupying the valleys around the volcanoes, and accumulated in horizontal layers, each representing an individual eruption event. The colours of the ash layers differed slightly according to the content and the temperature. When ash remained very hot on its journey through the air from the volcanic vent, the particles often melted together on landing, forming welded tuff. Welded tuff has a purple colour instead of the more normal shades of grey. We can see the layers as colour bands because we now see the layers of consolidated ash in cross-section. The layers were originally deposited in horizontal beds in water. Over the great period of time that has elapsed since deposition (575 million years) earth movements have brought the layers into an almost vertical orientation so that they are now viewed end on.

The textures are varied but in a quiet way with combinations of different sized fragments and changes of hue in the finer ash and small pyroclastic rock pieces. One of the images below shows an example of a volcanic bomb. This was in the first instance a glob of molten lava that was spewed from the vent along with the ash, becoming rounded in shape as it fell through the air, and then landing and forming a depression in the soft ash surface. Subsequent ash falls buried it.

[We stayed at the most excellent Louisbourg Harbour Inn while we explored this part of Cape Breton Island.]

Volcanic bomb embedded in tuff

Detail of texture in volcanic tuff rock dating from the Neoproterozoic 575 million years ago

Texture of tuff - a rock made of volcanic ash

Texture of tuff - a rock made of volcanic ash

Volcanic Tuff near Louisbourg Lighthouse – Part 1

Rocks composed of volcanic ash (tuff) near Louisbourg Lighthouse in Cape Breton, Nova Scotia.

The rocks around Louisbourg Lighthouse on Cape Breton Island in Nova Scotia belong to the Lighthouse Point Member of the Main-a-Dieu sequence (formerly assigned to the Fouchu Group). They were deposited 575 million years ago, following a series of explosive volcanic eruptions in a subduction zone at the edge of a continental plate. They are the youngest rocks from the Coastal Belt and represent the end of an era of violent volcanic eruptions in this part of Avalonia. The pyroclastic deposits around Louisbourg Lighthouse are made of compacted pumiceous ash in ignimbritic units, and the resulting rock is called tuff. Layers within the tuff, distinguished by varying shades of grey, represent a series of separate eruption events. The grey layers transition into a purple layer of welded tuff where the ash and debris remained hot enough to melt the individual particles together as they landed.

[We stayed at the most excellent Louisbourg Harbour Inn while we explored this part of Cape Breton Island.]

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

Seatown Beach Boulders

As you walk east along the shore at Seatown in Dorset, you reach Ridge Cliff from which numerous boulders have fallen over the years, and accumulated across the beach and into the water. What is most interesting is the great variety of shapes, colours, textures, and compositions. They represent all the different strata that make up the 80 metre high cliffs.

Seatown Shattered Eype Clay

The 80 metre high cliffs on the east shore at Seatown in Dorset along the Jurassic Coast are subject to land slips and rock falls. Large lumps of shattered blue-grey clay are common on the beach. They come from cascades of Eype Clay Member material that forms the lower part of the cliff exposures.