2014

NM4789 : Leac a' Ghratain

taken 10 years ago, near to Cleadale, Eigg, Scotland

Leac a' Ghratain
Leac a' Ghratain
Did the dykes come from Rùm? Many of the dykes along this stretch of coast, like this one at Leac a' Ghratain seem to point in the direction of the Isle of Rùm, which might suggest that the basalt magma that formed them may have come from the Rùm volcanic centre. However not all of the dykes are parallel, and some of them cut across earlier dykes. See also NM4789 : Parallel Dykes.
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.


Creative Commons Licence [Some Rights Reserved]   © Copyright Anne Burgess and licensed for reuse under this Creative Commons Licence.
Geographical Context: Coastal Geological interest Islands
This photo is linked from: Automatic Clusters: · Dykes come from Rùm [2] Other Photos: · Parallel Dykes ·
1:50,000 Modern Day Landranger(TM) Map © Crown Copyright
+
+
1:50,000 Modern Day Landranger(TM) Map © Crown Copyright
TIP: Click the map for more Large scale mapping
Grid Square
NM4789, 48 images   (more nearby 🔍)
Photographer
Anne Burgess   (more nearby)
Date Taken
Sunday, 27 April, 2014   (more nearby)
Submitted
Tuesday, 6 May, 2014
Subject Location
OSGB36: geotagged! NM 4710 8989 [10m precision]
WGS84: 56:55.8600N 6:9.5040W
Camera Location
OSGB36: geotagged! NM 4724 8983
View Direction
West-northwest (about 292 degrees)
Clickable map
+
NW N NE
W Go E
SW S SE
Image classification(about): Geograph
This page has been viewed about 38 times
You are not logged in login | register