Valtos Sandstone: Concretions and Dykes :: Shared Description

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.

by Anne Burgess
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32 images use this description. Preview sample shown below:

NM4788 : Water Slide by Anne Burgess
NM4788 : Valtos Sandstone by Anne Burgess
NM4789 : Cleft in the Rock by Anne Burgess
NM4788 : Sandstone and Sands by Anne Burgess
NM4790 : View from Bealach Thuilm path by Anne Burgess
NM4788 : North Shore of Laig Bay by Anne Burgess
NM4789 : Natural Arch by Anne Burgess
NM4689 : Dyke in Sandstone by Anne Burgess
NM4788 : Sandstone with Concretions by Anne Burgess
NM4788 : Flat-topped Concretions by Anne Burgess
NM4788 : Dyke in Sandstone by Anne Burgess
NM4689 : Sandstone Dyke by Anne Burgess
NM4689 : Rock Pinnacle by Anne Burgess
NM4689 : Rock Overhang by Anne Burgess
NM4788 : Valtos Sandstone by Anne Burgess
NG5164 : Valtos Sandstone by Anne Burgess
NM4986 : Valtos Sandstone and a Landslide by Anne Burgess
NM4985 : Examining the Valtos Sandstone by Anne Burgess
NM4789 : Eroded Dyke by Anne Burgess
NM4789 : Leac a' Ghratain by Anne Burgess
NM4689 : Eroded Dyke by Anne Burgess
NM4689 : Half a Dyke by Anne Burgess
NM4790 : Below Blàr Mòr by Anne Burgess
NM4790 : Mushroom Rocks by Anne Burgess
NM4789 : Burn in a Dyke by Anne Burgess

... and 7 more images.

These Shared Descriptions are common to multiple images. For example, you can create a generic description for an object shown in a photo, and reuse the description on all photos of the object. All descriptions are public and shared between contributors, i.e. you can reuse a description created by others, just as they can use yours.
Created: Mon, 5 May 2014, Updated: Thu, 1 Jun 2017

The 'Shared Description' text on this page is Copyright 2014 Anne Burgess, however it is specifically licensed so that contributors can reuse it on their own images without restriction.

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