Minto Hills

Minto Hills from car parking at NT 563 208. The farm track leads to the smaller of the two quarries and the larger quarry is visible to the right of the two conifers at the centre of the photograph.

The Minto Hills from the car parking space at NT 563 208
The farm track leads to the smaller of the two quarries and the larger quarry is visible to the right of the two conifers at the centre of the photograph.

The twin vent tuffs of the Minto Hills lie to the north-west of Minto village with Old Red Sandstones lying between and around them. There is space for cars at the northern end of the golf course, opposite the public right of way that leads up the southern hill to an exposure of volcaniclastic rock in the smaller of two quarries.

Map showing locations, sample points and rock types around Minto Hills

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

The twin vent tuffs of the Minto Hills lie to the north-west of Minto village with Old Red Sandstones lying between and around them. There is space for cars at the northern end of the golf course, opposite the public right of way that leads up the southern hill. Following the track brings us to an exposure of volcaniclastic rock in the smaller of two quarries on the eastern side of the hill. The two locations where we collected samples are shown by circles on the map.

Agglomerate/tuff, Minto (south) Hill NT561207 Prepared hand specimen viewed in ordinary reflected light (measures 40 mm across)

Agglomerate/tuff from Minto (south) Hill
Prepared hand specimen viewed in ordinary reflected light (measures 40 mm across)

The rock we see today results from the accretion of falling matter from eruption clouds. It is grey-green and consisting of mixed-size clasts many of which are between 10 – 20 mm in size with some up to 100 mm set in a fine-grained light brown matrix of tuff. Clast types include layered devitrified glass lapilli, fragments of Silurian greywacke, Devonian sandstone, felsite and quartz. Some clasts are chloritic, suggesting a source in more mafic rock.
The softer Old Red Sandstones that surrounded the twin vents have been eroded away revealing their original oval sections
Samples from the second, larger quarry that is marked by a couple of prominent conifer trees at the south- western base of the hill, show the rock to be much the same as that in the small outcrop to the east.

The vents are those of a trachyte volcano that would have been active around 340 million years ago. Its eruptions would have been Plinian in nature i.e. very explosive with the production of great quantities of ash. It’s viscous lavas, the product of the fractional crystallisation of basaltic magma, would have built steep sides so that the volcano might well have risen to over 1000m – over four times its present height of about 220m.

Classification of pyroclastic rocks based on material fragments

Classification of pyroclastic rocks based on material fragments

Classification of pyroclastic rocks based on particle size

Classification of pyroclastic rocks based on particle size

Agglomerate/tuff - consolidated volcanic ash and lithic material that infills the volcanic vent at Minto (south) Hill NT561207. Prepared hand specimen viewed in ordinary reflected light (measures 46mm across)

Agglomerate/tuff – consolidated volcanic ash and lithic material that infills the volcanic vent at Minto (south) Hill NT561207
Prepared hand specimen viewed in ordinary reflected light (measures 46mm across)

Agglomerate/tuff, Minto (south) Hill NT561207. Section viewed in plane polarised light

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

Agglomerate/tuff, Minto (south) Hill. Section viewed with crossed polarising filters.

The same section viewed with crossed polarising filters

Lithic and crystal fragments in the tuff of South Minto Hill.

Lithic and crystal fragments in the tuff of South Minto Hill

Microcline amongst lithic fragments with chlorite, quartz, feldspar with individual crystals set in a partially cabonatised ground. Sample viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Microcline amongst lithic fragments with chlorite, quartz, feldspar with individual crystals in a partially cabonatised ground.
Sample viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Microcline amongst lithic fragments with chlorite, quartz, feldspar with individual crystals set in a partially cabonatised ground. Sample viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area of the section viewed with crossed polarising filters

Alteration mineral in a fragment in the vent   agglomerate/tuff at Minto (south) Hill. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Alteration mineral in a fragment in the vent agglomerate/tuff at Minto (south) Hill.
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Alteration mineral in a fragment in the vent  agglomerate/tuff at Minto (south) Hill. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The altered fragment viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Alteration mineral in agglomerate/tuff. Section viewed in plane polarised light with plane of polarisation parallel to the horizontal edge of the photomicrograph. (FoV 0.5 x 0.3 mm)

An area of the same lithic fragment under higher magnification
Section viewed in plane polarised light with plane of polarisation parallel to the horizontal edge of the photomicrograph (FoV 0.5 x 0.3 mm)

Alteration mineral in agglomerate/tuff. Section viewed in plane polarised light with plane of polarisation parallel to the vertical edge of the photomicrograph. (FoV 0.5 x 0.3 mm)

The same area of the section viewed in plane polarised light
Aligning the polariser parallel to the vertical edge of the photomicrograph reveals the mineral’s pleochroism (FoV 0.5 x 0.3 mm)

The same mineral  viewed with crossed polarising filters  (FoV 0.5 x 0.3 mm)

The same mineral viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

The larger quarry further south on the hill

The larger of the two quarried areas at S. Minto Hill NT 561 207

The larger of the two quarried areas at Minto (south) Hill NT 561 207

Agglomerate/Tuff - consolidated volcanic ash infilling volcanic vent, Minto (south) Hill NT561207. Prepared hand specimen viewed in ordinary reflected light (measures 46mm across)

Agglomerate/Tuff – consolidated volcanic ash infilling volcanic vent, Minto (south) Hill NT561207
Prepared hand specimen viewed in ordinary reflected light (measures 46mm across)

Agglomerate/Tuff, Minto (south) Hill NT561207. Section viewed in plane polarised light

Thin section from the same sample viewed in plane polarised light
(Section measures 46mm across)

Agglomerate/Tuff, Minto (south) Hill NT561207. Section viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Lithic and crystal fragments in Minto (south) Hill tuff. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Lithic and crystal fragments in Minto (south) Hill tuff
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Lithic and crystal fragments in Minto (south) Hill tuff. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area of tuff viewed with crossed polarising filters

Lithic and crystal fragments in Minto (south) Hill tuff. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

A closer view of the same fragments
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Lithic and crystal fragments in Minto south) Hill tuff. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Lithic and crystal fragments in Minto south) Hill tuff
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Lithic and crystal fragments in Minto south) Hill tuff. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same fragments viewed with crossed polarising filters

Lithic and crystal fragments in Minto south) Hill tuff. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Lithic and crystal fragments in Minto (south) Hill tuff
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Fragments in tuff Section viewed in reflected light with crossed polarising filters (FoV 1.2 x 0.8 mm)

Fragments in agglomerate/tuff
Section viewed in reflected light with crossed polarising filters (FoV 1.2 x 0.8 mm)

Fragments in agglomerate/tuff. Section viewed in plane polarised light (FoV 1.2 x 0.8)

The same fragments viewed in plane polarised light (FoV 1.2 x 0.8)

Fragments in agglomerate/tuff. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same fragments viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

References

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

Open University Geology Society The Minto Hills and Rubers Law Excursion – B. Upton (leader)2011http://ougs.org/eastscotland/event-reports/the-minto-hills-and-rubers-law-scottish-borders-220/

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

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