Dunion Hill

The abandoned quarry at Dunion Hill is located 3km west from Jedburgh on the B6358 to Denholm road. There is a lay-by by the locked quarry gates with access to the track up to the quarry via a low fence to the right of the gate.

Map showing locations, sample points and rock types around Dunion Hill
Key Dunion to Hill Map

Dunion Hill is the remains of a Carboniferous volcano that became plugged by microgabbro. Quarrying has removed much of the upper part of the plug but has created the opportunity for us to get inside the oval structure and enjoy the scale of the intrusion. Good exposures lie all around.
The quarrying also removed a good deal of a large hill fort fort that once enclosed an area of between 6 and 6.5 hectares (60,000 – 65,000 m2) – a size comparable with the ‘oppida’, high status fortifiedlate Iron Age settlements, of the Tweed Basin. Apparently, the site started as a small fort on the summit and later expanded in stages.

At the centre of the intrusion at Dunion Hill NT 625 190

At the centre of the intrusion at Dunion Hill NT625190
Looking towards the north-east end and the way in to the quarry

The microgabbro at Dunion Hill is intermediate between the Jedburgh and Markle types i.e. it is a porphyritic olivine microgabbro with large (Markle) and small (Jedburgh) labradorite feldspar and olivine phenocrysts. The feldspar phenocrysts are more numerous than the olivine.

QAP diagram showing the quartz, alkali feldspar, plagioclase composition of microgabbro relative to other intrusive rocks

Microgabbro QAP diagram
Microgabbro, Dunion Hill. Prepared hand specimen viewed in reflected light. (Section measures 38mm across)

Markle/Jedburgh type microgabbro, Dunion Hill.
Prepared hand specimen viewed in reflected light. (Section measures 38mm across)

Microgabbro, Dunion Hill. Section viewed in plane polarised light. (Section measures 38mm across)

A thin section from the same sample viewed in plane polarised light
(Section measures 38mm across)

Microgabbro, Dunion Hill. Section viewed with crossed polarising filters. (Section measures 38mm across)

The same section viewed with crossed polarising filters
Alignment of the plagioclase phenocrysts, typical of the Jedburgh type of microgabbro, is easy to see.

Plagioclase phenocryst in microgabbro, Dunion Hill. Section viewed in plane polarised light.(FoV 4.6 x 3.0 mm)

The largest plagioclase phenocryst in this thin section of microgabbro
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase phenocryst in microgabbro, Dunion Hill. Section viewed with crossed polarising filters.(FoV 4.6 x 3.0 mm)

The same area of the section viewed with crossed polarising filters
Small, high-birefringent crystals of olivine and pyroxene are visible (FoV 4.6 x 3.0 mm)

Dunion Hill Quarry looking south-west

Dunion Hill quarry looking to the south-west end

A second prepared hand specimen of the Dunion Hill microgabbro. Sample viewed in reflected light.

A second prepared hand specimen of the Dunion Hill microgabbro
Sample viewed in reflected light.

Microgabbro, Dunion Hill Quarry. Section viewed in plane polarised light. (Section measures 40mm across)

A second section of Dunion Hill microgabbro viewed in plane polarised light
(Section measures 40mm across)

Microgabbro, Dunion Hill Quarry Section viewed with crossed polarising filters. (Section measures 40mm across)

The same thin section viewed with crossed polarising filters

Plagioclase phenocrysts in plagioclase, olivine, pyroxene and opaques groundmass. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Plagioclase phenocrysts in the feldspar, olivine, pyroxene and iron-titanium oxides groundmass.
Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Partially altered olivine in microgabbro, Dunion Hill. Section viewed in plane polarised light. (FoV 1.2 x 0.8 mm)

Partially altered olivine in the same thin section
Section viewed in plane polarised light. (FoV 1.2 x 0.8 mm)

Partially altered olivine in microgabbro, Dunion Hill. Section viewed with crossed polarising filters. (FoV 1.2 x 0.8 mm)

The same olivine crystals viewed with crossed polarising filters

Yellow ochre coloured olivine alteration with sparse biotite in microgabbro, Dunion Hill. Section viewed in plane polarised light (FoV 2.3 x 1.5mm)

Yellow ochre coloured olivine alteration with sparse biotite in microgabbro, Dunion Hill
Section viewed in plane polarised light (FoV 2.3 x 1.5mm)

Yellow ochre olivine alteration with sparse biotite in microgabbro, Dunion Hill. Section viewed with crossed polarising filters. (FoV 2.3 x 1.5 mm)

The same alteration material viewed with crossed polarising filters

Carbonitised microgabbro  at Dunion Hill. Section viewed in plane polarised light. (Section measures 40mm across)

Carbonitised microgabbro at Dunion Hill
Section viewed in plane polarised light. (Section measures 40mm across)

Carbonitised microgabbro  at Dunion Hill. Section viewed with crossed polarising filters. (Section measures 40mm across)

The same thin section viewed with crossed polarising filters

Carbonitised microgabbro  at Dunion Hill. Section viewed with crossed polarising filters. (FoV 4.6 x 3.0mm))

Highly carbonitised microgabbro at Dunion Hill
Section viewed with crossed polarising filters (FoV 4.6 x 3.0mm)

Altered phenocryst in carbonitised microgabbro  resulting in haematite, Dunion Hill. Section viewed in plane polarised light.(FoV 1.2 x 0.8 mm)

Altered phenocryst in the same carbonitised microgabbro resulting in haematite
Section viewed in plane polarised light.(FoV 1.2 x 0.8 mm)

Altered phenocryst in carbonitised microgabbro  resulting in haematite, Dunion Hill. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same altered phenocryst viewed with crossed polarising filters

Exposed host sandstone to the left against the intruded microgabbro  at Dunion Hill

Exposed host sandstone (left) against the intruded microgabbro plug (right) at Dunion Hill

Chilled microgabbro, Dunion Hill Quarry NT625190 Prepared hand sample viewed in reflected light. (Sample measures 41mm across)

Chilled microgabbro close to the contact with the sandstone
There are more vesicles in this sample taken from the edge of the intrusion.Prepared hand sample viewed in reflected light (sample measures 41mm across)

“Big Blast” in the Borders
Here’s a newspaper article we found in the archives of the Glasgow Herald that records the public viewing of a mighty explosion at the Dunion Hill quarry inJanuary 1938.

Report of quarrying at Dunion Hill, Glasgow Herald 22 January 1938

References

MacAdam A.D., Clarkson E.N.K., Stone, P. (editors) 1993 Scottish Borders geology: an excursion guide. Edinburgh, Scottish Academic Press.

Rideout, J S. 1987 Excavation of an Iron-Age fort at The Dunion, Roxburghshire‘, Proc Soc Antiq Scot, vol. 117, p 361-2.

Glasgow Herald 22 January 1938
https://news.google.com/newspapers?nid=GGgVawPscysC&dat=19380122&printsec=frontpage&hl=en

BGS online Geology of Britain http://mapapps.bgs.ac.uk/geologyofbritain/home.html

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

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