Bothal Dyke


Very little has been written about the the Bothal dyke.
Like the Morpeth dyke, it appears on geological survey maps published prior to 2001 but it is not marked on current maps.
Holmes and Harwood wrote that the rock, ‘exhibits characters which are intermediate between those of the Salen and Brunton types.,’ indicating that it is ophitic with granular augite and elongated labradorite appearing in roughly equal measure and that a dark brown mesostasis occupies about 20% of its volume.
Later, Laurence and Jackson wrote, “The dyke formally seen at Bothal…. probably represent(s) a continuation of the West Sleekburn dyke’, a dyke visible only in the Yard and Plessey seams of the old mine workings east of Guide Post.
Using pre-2001 maps we have searched the two locations where the dyke is shown to cross water courses; the River Wansbeck at Bothal Mill and a small tributary to the river a short distance to the north-west.

Map showing the course of the dyke, locations and local rock types
The location of the Bothal Dyke
Grid referenced points show the locations of samples.

In 2015 we did find an exposure of the dyke at NZ234861, a location on the south bank of the river between the bridge and the weir opposite Bothal Mill. We noted then that he location was difficult to get to and the outcrop was often submerged by the Wansbeck. Since then, Ian Bambrick, a visitor to the website who lives close to Bothal, has been in touch to tell us that the weir close to the outcrop has been dismantled resulting in a shallower river level that makes the dyke much more visible .

Bothal dyke September 2020
Bothal dyke September 2020 – photo Ian Bambrick

Location 1. South-east bank of the river

Bothal Dyke outcropping on the south bank of the Wansbeck at NZ234861 (August 2015)
The south bank location of the outcrop of the Bothal Dyke viewed from the north bank when river levels are higher
Section from the outcrop on the south bank of the Wansbeck near Bothal Mill (NZ234861)
The section is viewed in plane polarised light is (38mm across)
The same section viewed with crossed polarising filters

The plagioclase phenocrysts in the outcropping rock of the dyke occur individually as well as in glomerocrysts. Most plagioclase phenocrysts are distinctly zoned with high calcium cores that approach bytownite in composition. These cores have a yellow to dark-orange interference colour even when the sample is thinned to less than 30 microns.
One phenocryst in the outcropping rock has a spherical array of opaque material.

Porphyritic plagioclase concentrated in one area of the sample from the dyke’s outcropping rock occurring as glomerocrysts and individual crystals.
Sample viewed in plane polarised light (FoV 4.6 x 3.0 mm)
The same area viewed with crossed polarising filters
Plagioclase phenocryst in the sample from outcropping rock showing the yellow to deep orange-brown interference colour and zoning
Section viewed with crossed polarising filters  (FoV 1.2 x 0.8 mm)

A plagioclase phenocryst in the sample from the outcropping rock that has an unusual spherical array of opaque material scattered across the twins. Section viewed in plane polarised light.

Plagioclase laths in the matrix of the outcropping Bothal dyke rock viewed with crossed polarising filters
Some of the plagioclase laths that occupy the matrix of the rock in the outcropping sample exhibit the ‘stellar arrangement’ associated with the Brunton type of tholeiite basalt.

The sub-ophitic texture exhibited by the pyroxenes and plagioclase laths in the outcropping rock of the Bothal dyke
Section viewed with crossed polarising filters

Amygdale filled with mesostasis and calcite surrounded by plagioclase laths and pyroxene grains.
Section viewed with crossed polarising filters.
In this portion of the sample, plagioclase laths are randomly distributed rather than appearing in stellar clusters.

Location 2. North bank of the river at NZ234862


On the north bank of the river, there is a lot of basalt in the vicinity of the weir where large basalt boulders have been used to repair and extend a stone platform. Some of the basalt stones and boulders may be left-overs from the construction work, but some of the more weathered and embedded rocks look as if they might be associated with the dyke. Certainly, the texture of the rock in these samples is very similar to the texture of the rock that outcrops on the south bank.

Thin sections of rock from the boulders on the north-bank of the river near the weir reveal quite similar textures to those of the outcropping basalt at location NZ234861.

Sample from a boulder on the north bank of the Wansbeck near the weir at Bothal Mill (NZ234862)
Thin-section viewed in plane polarised light (40mm across)

Plagioclase phenocrysts in rock from the north bank of the river close to Bothal weir. (NZ234862). Section viewed with crossed polarising filters 
These plagioclase phenocrysts with their zoned high-calcium cores are very similar to those of the outcropping rock downstream on the south bank. The titanium-iron content, however, does occur as more sharply defined crystals rather than being incorporated into the mesostasis.
The same group of crystals rotated to reveal their zoned extinction

Location 3. Stream bed to the north-west at NZ231863

The stream-bed in the vicinity of the mapped north-west end of the dyke

Further to the north-west, a small stream flows down a deep gully into the Wansbeck. Basalt stones and boulders, some of them quite large, can be found in the water course all the way up to the locations where the old maps show the dyke intersecting it.
To date, we have not been able to find an exposure of the dyke here, but the presence of these boulders in this fairly inaccessible place suggests there is, or was, a natural source nearby.

Thin sections of rock from the boulders in the stream bed are similar to those on the south bank further east at NZ234861.
All the rock we have found associated with the Bothal dyke is less coarsely crystalline than the sample found near the south-eastern location of the Morpeth dyke. There is also less iron-titanium opaque material, fewer amygdaloids and less green-brown interstitial material.

Basalt from a boulder in the stream-bed at the north-west location (NZ231863)
Thin-section viewed in plane polarised light (39-mmm across)

A plagioclase phenocryst in a sample from the NW location viewed with crossed polarising filters
Plagioclase in this sample tends to the composition of labradorite
Calcite in an amygdaloid in a sample from the north-west location
Section viewed with crossed polarising filters

References

Holmes, A and Harwood, H F. 1929. The Tholeiite Dikes Of The North Of England. The Mineralogical Magazine and Journal Of The Mineralogical Society, No. 124. Vol 22.

Lawrence, D J D and Jackson, I. 1990. Morpeth-Bedlington-Ashington Technical Report Wa/90/19, Geology And Land-Use Planning: Morpeth-Bedlington-Ashington Part 2. Geology. Keyworth, Nottingham: British Geological Survey.

British Geological Survey. Sheet 14, Morpeth. http://www.bgs.ac.uk/data/maps/maps


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