Dunion Hill - Geology North

Dunion Hill excursion map showing sample locations and igneous rock types in the area

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 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

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

**A thin section from the same specimen viewed in plane polarised light**
(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.

**The largest plagioclase phenocryst in this thin section of microgabbro**
Section viewed in plane polarised light (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 to the south-west end**

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

**Thin section from this specimen viewed in plane polarised light**
(Section measures 40mm across)

**The same thin section viewed with crossed polarising filters**

**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 the same thin section**
Section viewed in plane polarised light. (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)

**The same alteration material viewed with crossed polarising filters**

**Carbonatised microgabbro at Dunion Hill**
Thin section viewed in plane polarised light. (Section measures 40mm across)

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

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

**The same altered phenocryst viewed with crossed polarising filters**

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

**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.

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

Dunion Hill excursion map showing sample locations and igneous rock types in the area

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

“Big Blast” in the Borders

References

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