NM4788 : Valtos Sandstone

taken 5 years ago, near to Cleadale, Highland, Great Britain

This is 1 of 2 images, with title Valtos Sandstone in this square
Valtos Sandstone
Valtos Sandstone
A low cliff of sansdtone, studded with smallish concretions.
Valtos Sandstone: Concretions and Dykes
The rocks at the northern extremity of the Isle of Eigg are sandstones of Jurassic age (the time when dinosaurs were the dominant animals on Earth), named after the outcrop of the same rock at Valtos in Skye.

The sandstones were laid down by rivers flowing into extensive lagoons. Fossils found in thin limestone beds within the Valtos sandstone suggest that these lagoons were almost fresh water until the sea returned about 166 million years ago.

A striking feature of the Valtos Sandstone is the vast number of concretions found both in the sandstone and, detached by erosion from the bedrock, littered along the shore between Laig Beach and the Singing Sands.

Concretions form when calcium carbonate, for example shell sand, is dissolved by water, percolates through a rock, and is precipitated as calcite. The calcite cements the sand grains together, making the rock much harder and more reistant than the surrounding beds, so that when these beds are eroded the concretions are left intact. It is thought that concretions form round nuclei of calcium carbonate, for example a fragment of shell, and they take about 3 million years to grow to a diameter of 50 centimetres - though many of them are much bigger than this so must have taken significantly longer.

About 60 million years ago, during the period of volcanic activity that accompanied the splitting of the Earth's crust that formed the Atlantic Ocean, and hence separated America from Europe and Africa, a swarm of basalt dykes intruded the sandstone.

The basalt magma was very hot, and where it came in contact with the sandstone it baked the sandstone on both sides, so that the sandstone nearest the dykes became more resistant to erosion. Over time the basalts and the softer, unbaked sandstone further from the dykes were eroded away, leaving double ridges of hard-baked sandstone standing proud above the remaining rocks.

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Grid Square
NM4788, 63 images   (more nearby )
Photographer
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Date Taken
Sunday, 27 April, 2014   (more nearby)
Submitted
Monday, 5 May, 2014
Geographical Context
Coastal  Geological interest  Islands 
Subject Location
OSGB36: geotagged! NM 4717 8878 [10m precision]
WGS84: 56:55.2656N 6:9.3687W
Camera Location
OSGB36: geotagged! NM 4716 8875
View Direction
North-northeast (about 22 degrees)
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