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.

Limestone Walls

Dry stone walls on The Burren

The stark landscape of The Burren in County Clare, Ireland, comprises fields of Carboniferous Limestone rock pavement with sparse and specialised vegetation divided by walls of precariously balanced rocks. Moss and lichens cling to the limestone boulders and bent thorn trees miraculously survive with their roots among the stones.

Seatown Rock with Piddock Holes

A contributory factor in the erosion of the beach rock at Seatown in Dorset is the burrowing activity of the marine bivalve mollusc called the piddock. Low on the shore millions of holes in the soft calcareous mudstones are evidence for the burrows made by Pholas dactylus. The holes are almost circular in shape reaching up to two centimetres in diameter,  and can occur as a scatter or as dense populations wherever the rock remains wet between the tides. They seem to prefer the darker layers rather than the alternating light layers – although they are found in both. 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.

Where successive generations of this boring mollusc have colonised the strata, the mudstone has been reduced to an irregular honeycombed mass. Most of the holes seem unoccupied and small pieces of orange-coloured gravel have filled them. In some burrows the empty white shells of the piddock can still be seen. Some of the burrows are undoubtedly still occupied but I did not have an opportunity to locate any for photographs since the area was only exposed for half an hour. The shells of the living animals may not have been visible because they tend to lie deep within the burrow but the living specimens can often be detected by the fact that their siphons extend from the shell to the surface and these periodically squirt out water during low-tide.

Pebbles and beach stones which have neat circular holes in them are frequently wave-washed and beach-tumbled pieces of rock that have broken away from intertidal rock layers that have been riddled with burrows made by rock-boring molluscs such as piddocks, in the way shown in these photographs from Seatown in Dorset, England, along the World Heritage Jurassic Coast.

Volcanic Tuff near Louisbourg Lighthouse – Part 5

Not all the rock exposed at Louisbourg Lighthouse is composed of tuff. Molten lavas intruded the tuff at later stages forming harder bands of igneous rock with a contrasting greenish colour and distinct fracture patterns. The textures of the two kinds of rock are very different.

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