Acklington Dyke

Location of the Acklington dyke in east Northumberland

The great length, continuous exposure and consistent petrology of the Acklington dyke have given it a special place in the Northern England/Southern Scotland suite of palaeogene dykes.
The dyke is first recorded east of the village of Acklington from where it strikes north-west and crosses the River Coquet in the fields of Acklington Park where it was once quarried. No outcrops of the dyke are visible at this location now although pieces of the dyke rock can be found in the vicinity of the linear cutting that marks the location, width and trend of the dyke.
There are signs that it was also quarried at Swarland two and a half miles further west where boulders and cobbles of dyke rock can be found in the rough ground amongst the trees.

Map showing the course of the Acklington dyke through east Northumberland with coloured circles marking sample locations

Acklington Map (East)

Petrography and Petrology

The rock’s dominant mineral is feldspar which appears in crystal form as haphazardly oriented laths that are usually of labradorite composition but can sometimes be as sodium rich as andesine. Rare samples of the dyke have been reported to contain corroded or fretted phenocrysts, sometimes occurring in glomerocrysts, of very high calcium plagioclase. Harwood and Holmes identified samples of this sort as instances of the ‘Anorthite-bearing Acklington’ type.
The augite in the rock tends to be granular or twinned and columnar while the rare hypersthene appears in stumpy crystals that have a turbid appearance due to patches of fibrous alteration products.
The generally abundant mesostasis can be a clear, colourless to sepia glass, or more commonly, a turbid devitrification product. We read that all of the rock’s alkali felspar and quartz is contained in the mesostasis along with varying amounts of plagioclase and pyroxene materials that occur in nearly the same proportions as they occur in the crystalline phases. Both glassy and devitrified types of mesostasis contain a good deal of globular and skeletal ore material. Heslop noted that where the dyke was in proximity to the Cheviot andesites, the mesostasis was more glassy.
Vesicles and chlorite lined calcite amygdaloids are few and small.

Acklington Park quarry

The disused Acklington dyke quarry at Acklington Park NU203023

The disused Acklington dyke quarry at Acklington Park NU203023

The basalt in our samples from Acklington Park quarry show many of the characteristics of the typical Acklington dyke basalt described by Holmes and Harwood.
The plagioclase appears in forms that range between small, clearly defied lathes through larger stumpy, columnar crystals,to plate-like crystals with poorly defined edges.
Most of the pyroxene is clinopyroxene in mostly granular form but also sometimes twinned, and columnar. Orthopyroxene occurs in isolated stumpy crystals in a relatively unaltered state.
The mesostasis occupies very little of the rock and the opaques are quite distinct from it, occurring as large, clearly defined skeletal shapes and plates.
We don’t find any amygdaloids in our samples.

 Acklington Dyke, prepared hand sample RL, Acklington Park Quarry NU203023

A prepared hand sample of Acklington dyke rock from Acklington Park Quarry (NU203023) viewed in reflected light

Acklington Dyke, Whole slide PPL, Acklington Park Quarry NU203023

A thin section from the same sample viewed in plane polarised light

Acklington Dyke, Whole slide viewed with crossed polarising filters, Acklington Park Quarry NU203023

The sample view with crossed polarising filters

Plagioclase. pyroxene and opaque iron oxides in Acklington dyke basalt at Acklington Park quarry. Sample viewed in plane polarised light at X10

Plagioclase. pyroxene and opaque iron oxides in Acklington dyke basalt at Acklington Park quarry
Sample viewed in plane polarised light at X10

Plagioclase. pyroxene and opaque iron oxides in Acklington dyke basalt. Sample viewed with crossed polarising filters at X10

The same area of the thin section viewed with crossed polarising filters at X10

Twinned column,  and granular clinopyroxene with plagioclase in Acklington dyke basalt. Sample viewed with crossed polarising filters at X25

Twinned columnar, and granular clinopyroxene with plagioclase in Acklington dyke basalt
Sample viewed with crossed polarising filters at X25

Orthopyroxene (light orange crystal at centre) and clinopyroxene (e.g. orange, red & blue crystal, below) in Acklington dyke basalt. Sample viewed with crossed polarising filters at X25

Orthopyroxene, the light orange crystal centre field, and clinopyroxene, the higher order interference coloured crystals, in Acklington dyke basalt.
Sample viewed with crossed polarising filters at X25

Swarland

Vicinity of Acklington Dyke, Swarland NU163025

Possible Acklington dyke quarry, Swarland NU163025

Acklington dyke boulder, Swarland NU163025

Acklington dyke boulder, Swarland NU163025

The basalt in the samples from Swarland is very similar to that in the Acklington Park rock. The appearance of the plagioclase and relatively sparse mesostasis are the same.
However, the clinopyroxene appears in granular ‘strings’ as well as in columnar and granular masses and the iron-titanium oxides appear in slightly larger skeletal forms.

Acklington Dyke, prepared hand sample RL, Swarland NU163025

A prepared hand sample from a boulder of Acklington dyke rock from Swarland (NU163025).The sample is viewed in reflected light.

Thin-section of the Acklington dyke at Swarland viewed in plane polarised light

Thin-section of the Acklington dyke at Swarland viewed in plane polarised light

Thin-section from the Acklington dyke at Swarland viewed with crossed polarising filters

The same thin-section viewed with crossed polarising filters

Pyroxene, plagioclase and opaques in Acklington dyke basalt at Swarland.  Sample viewed with crossed polarising filters at X10

Pyroxene, plagioclase and opaques in Acklington dyke basalt at Swarland
Sample viewed with crossed polarising filters at X10

Elongated pyroxene string amongst plagioclase and opaques in Acklington dyke basalt at Swarland.

Elongated pyroxene ‘string’ amongst plagioclase and opaques in Acklington dyke basalt at Swarland
Sample viewed in plane polarised lightat X25

An elongated pyroxene string amongst  plagioclase and opaques in Acklington dyke basalt at Swarland x25

The same pyroxene ‘string’ viewed with crossed polarising filters at x25

The Acklington dyke in north-west Northumberland

Map showing the course of the Acklington dyke through west Northumberland with coloured circles marking sample locations

Acklington Map (West)

Key to bedrock in the Cheviot area shown in the map

Key to the igneous rocks shown in the map

16 miles further west, another pair of tell-tale parallel ridges in the field at the summit of the hill north-west of Newton evidence yet another quarry and the continuing course of the dyke.
At Clennell, there is quite a lot of basaltic rock in and alongside the bed of the River Alwin. There has been some disagreement as to whether this is the Acklington dyke in situ, or displaced material from the dyke, or displaced material from some other rock. The sample we take has a very different appearance to other samples of the dyke which suggests that at least some of the River Alwin basalt is from elsewhere.
We find one broken boulder of Acklington dyke rock lying in the grass by the enclosures on the hill half a mile north of Alwinton that might well have had its origin in the dyke that is shown to cross this land nearby. It is very similar to the sample found further west beyond Clennell Street on the steep, eastern side of the Hosedon Burn. At this location we find the Acklington dyke outcropping from the surrounding Cheviot andesite in a number of places.
The final exposure of the dyke in the west of Northumberland is in Upper Coquetdale near Bygate where it is exposed crossing the River Coquet. Here, the dyke’s tholeiite basalt can be seen in its least altered state.

Clennell

Lebour writes, “The Acklington dyke…. as it approaches the Cheviot porphyritic mass, through which it cuts, spreads irregularly over the beds of Tuedian age in which it is encased. This is very beautifully seen along its course from a little to the South-West of Biddlestone to the banks of the Alwin, at Clennell.” Perhaps the location NT926071, shown below, marks the western end of this overflow, but as we observed, certainly not all of the basalt here is tholeiitic.

Basaltic rock in and alongside the River Alwin, Clennell NT926071

Basaltic rock in and alongside the River Alwin, Clennell NT926071

Basaltic rock, prepared hand sample RL, River Alwin, Clennell NT926071

A prepared hand sample of basaltic rock from the River Alwin, Clennell NT926071 viewed in reflected light

Basaltic rock (not from the Acklington dyke) in the River Alwin at Clennell, NT926071 (PPL)

A thin section from the same sample viewed in plane polarised light.
The lack of mesostasis and the profusion of plagioclase phenocrysts signals the rock is not from the Acklington dyke.

Basaltic rock (not from the Acklington dyke) in the River Alwin at Clennell, NT926071 (XP)

The thin section viewed with crossed polarising filters

Hosedon Burn Valley

Acklington Dyke, Hosden Burn Valley, NT917075

Boulders of Acklington dyke rock, Hosden Burn Valley, NT917075

Prepared hand specimen of Acklington dyke rock from the Hosden Burn Valley, NT917075, Viewed in reflected light.

Prepared hand specimen of Acklington dyke rock from the Hosden Burn Valley, NT917075,
Viewed in reflected light.

Thin-section of Acklington dyke rock from the Hosden Burn Valley, NT917075, Viewed in plane polarised light.

Thin-section from the same specimen viewed in plane polarised light.

Acklington dyke rock from the Hoseden Burn Valley, NT917075 viewed with crossed polarising filters

The same thin-section viewed with crossed polarising filters

The Acklington dyke basalt in the valley of the Hosedon Burn shows evidence of alteration, calcium compounds replacing some of the pyroxenes and mesostasis.
Plagioclase appears in a range of crystal forms from slender laths to small, zoned plates.
Pyroxene appears in granular patches, twinned columns that are becoming granulised, and also granular ‘strings’.
Patches of clear glass remain in the iron-stained and calcified mesostasis and iron-titanium opaque oxides are plentiful but relatively small.

Plagioclase crystals in the Acklington dyke basalt at Hosedon Valley. Sample viewed in plane polarised light at x25

Plagioclase crystals in the Acklington dyke basalt in boulders on the east side of the Hosedon Burn valley.
Sample viewed in plane polarised light at x25

Plagioclase crystals in the Acklington dyke basaltic boulders in the Hosedon Burn valley. Sample viewed with crossed polarising filters at x25

The same area viewed with crossed polarising filters at x25

Calcite in the Acklington dyke basalt in the Hosedon Burn valley. Sample viewed with crossed polarising filters at x25

Calcite in the Acklington dyke basalt in the Hosedon Burn valley
Sample viewed with crossed polarising filters at x25

Strings of pyroxene granules  in the Acklington dyke basalt in the Hosedon Burn valley. Sample viewed in plane polarised light at x25

‘Strings’ of pyroxene granules in the Acklington dyke basalt in the Hosedon Burn valley
Sample viewed in plane polarised light at x25

Strings of pyroxene granules  in the Acklington dyke basalt in the Hosedon Burn valley. Sample viewed with crossed polarising filters at x25

The same pyroxene ‘strings’ viewed with crossed polarising filters at x25

Clear glass in altered mesostasis, with secondary cacite and feldspar in basalt in the Acklington dyke in the Hosedon Burn valley. Sample viewed in plane polarised light at x10

Clear glass in altered mesostasis, with secondary cacite and feldspar in basalt in the Acklington dyke in the Hosedon Burn valley
Sample viewed in plane polarised light at x10

Clear glass in altered mesostasis, with secondary calcite and feldspar in basalt in the Acklington dyke at Hosedon Valley. Sample viewed with crossed polararising filters at x10

The same area of the thin section viewed with crossed polararising filters at x10

River Coquet at Bygate

The Acklington dyke in the River Coquet at Bygate, Upper Coquetdale

The tholeiite basalt is less altered here than in any of the other exposures or remains of the dyke in Northumberland.
The pyroxenes are, as always, fewer in numbers than the felspars. They appear both in granular and columnar forms.
The randomly orientated plagioclase laths and the very occasional plagioclase phenocrysts are particularly well defined against abrown mesostasis that is remarkably free of carbonatisation.
The iron-titanium oxides are also very distinct, appearing in larger plate and skeletal forms as well as in fine, linear forms that are crossed all along their length by shortlinear crystals.
At an even finer scale, the mesostasis is ‘etched’ by a profusion of lighter coloured linear forms. We haven’t seen this texture in any of our other samples from the dyke.

A prepared hand specimen of the Acklington dyke at Bygate, Upper Coquetdale NT 864 086

A prepared hand specimen of the Acklington dyke at Bygate, Upper Coquetdale NT 864 086
Specimen viewed in reflected light

Acklington dyke tholeiite basalt outcropping at Bygates. Thin section viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light.

Acklington dyke tholeiite basalt outcropping at Bygates. Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Columnar and granular pyroxenes together with plagioclase laths in the Acklington dyke tholeiite basalt at Bygate. Thin section viewed with crossed polarising filters at x10.

Columnar and granular pyroxenes together with plagioclase laths in the Acklington dyke tholeiite basalt at Bygate
Thin section viewed with crossed polarising filters at x10.

Well defined plagioclase laths in the Acklington dyke tholeiite basalt. Sample viewed in plane polarised light at x10.

Well defined plagioclase laths in the Acklington dyke tholeiite basalt
Sample viewed in plane polarised light at x10.

Plagioclase laths in Acklington dyke tholeiite basalt viewed with crossed polarising filters at  x10

The sample crystals viewed with crossed polarising filters at x10

A rare plagioclase phenocryst amongst plagioclase laths in the Acklington dyke tholeiite basalt. Sample viewed in plane polarised light at  x10

An uncommon plagioclase phenocryst amongst plagioclase laths in the Acklington dyke tholeiite basalt
Sample viewed in plane polarised light at x10

The same plagioclase phenocryst viewed with crossed polarising filters

The same plagioclase phenocryst viewed with crossed polarising filters

Iron-titanium oxides and linear textured mesostasis in the Acklington dyke tholeiite basalt. Sample viewed in plane polarised light at x25

Iron-titanium oxides and mesostasis in the Acklington dyke tholeiite basalt.
Sample viewed in plane polarised light at x25

Iron-titanium oxides in the finely lined mesostasis in the Acklington dyke tholeiite basalt. Sample viewed in plane polarised light at x25

Iron-titanium oxides in the finely lined mesostasis in the Acklington dyke tholeiite basalt
Sample viewed in plane polarised light at x25

References

Lebour, G A, 1878. Outlines Of The Geology Of Northumberland. M & M N W Lambert, Newcastle upon Tyne.
Teall, J J H. 1884. Peteological Notes On Some North-Of-England Dykes. The Quarterly Journal Of The Geological Society Of London, Vol. 40. Pp. 209-247.
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.
British Geological Society

No vestige of a beginning, – no prospect of an end

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