Grey Pen

Grey Pen from the track out of Skelfhill Farm

Grey Pen from the track out of Skelfhill Farm
Approaching from this direction, Grey Pen obscures Skelfhill Pen

We took in Grey Pen on the same excursion as the adjacent Skelfhill Pen, parking in space off the road, down the hill from the track through Skelfhill Farm to the base of the hill.

Map and Key showing locations and rock types in the locality of Grey Pen and Skelfhill Pen

Map showing locations, rock types and sample points around Grey Pen
Key to map showing locations and rock types around Skelfhill Pen

McRobert thought Grey Pen may have been the old plugged up main vent of the volcano.She described the rock as, ‘ a very fine-grained, typical non-porphyritic aegerine-trachyte. The sanidine, partly turbid, occurs in good laths showing splendid fluxion-structure; aegerine, quite subordinate, and magnetite dust make up the rest.’ adding, ‘ Nepheline was tested for, but proved absent.’
The flow texture in the ground mass really is well defined with striking waves of feldspar laths flowing around the many inclusions in the rock.

QAPF diagram showing the composition of trachytes relative to other extrusive rock types

QAPF diagram Trachytes
Trachyte, Grey Pen NT 444 034. Prepared hand specimen viewed in reflected light (41mm across)

Trachyte, Grey Pen NT 444 034
Prepared hand specimen viewed in reflected light (41mm across)

Trachyte, Grey Pen. Thin section viewed in plane polarised light

Trachyte, Grey Pen
Thin section viewed in plane polarised light.

Trachyte, Grey Pen. Thin section viewed with crossed polarising filters.

Trachyte, Grey Pen
The same thin section viewed with crossed polarising filters.

Trachytic texture in trachyte, Grey Pen. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Trachytic texture in trachyte, Grey Pen
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Trachytic texture in trachyte, Grey Pen. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Trachytic texture in trachyte, Grey Pen
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The rock on Grey Pen is quite shattered, much of it falling away in large and small plates. It is heavily altered so that all our samples contained quantities of haematite along with carbonate and chloritic minerals. The inclusions range from microscopic fragments to pieces clearly visible with the naked eye and when polished, the rock is visually very interesting.
One inclusion has a group of stumpy, rectangular crystals edged wth iron oxide. They are altered plagioclase, highly carbonated but still showing the remnants of polysynthetic twinning.

Trachyte, Grey Pen NT 445 035

Trachyte outcrops and scree, Grey Pen NT 445 035

rachyte, Grey Pen NT445035. Prepared hand specimen viewed with in reflected light (measures 40mm across)

Trachyte, Grey Pen NT 445 035
Prepared hand specimen viewed with in reflected light (measures 40mm across)

Xenolith? in trachyte, Grey Pen. Thin section viewed in plane polarised light

Thin section from the same specimen viewed in plane polarised light

Xenolith in trachyte, Grey Pen. Thin section viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Alteration at the edge of the inclusion in the trachyte. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Alteration at the edge of the inclusion in the trachyte
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered inclusion in trachyte. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Altered inclusion in trachyte
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Altered inclusion in trachyte. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same viewed with crossed polarising filters

Inclusion in trachyte, Grey Pen NT445035. Prepared hand specimen viewed in reflected light (measures 44mm across)

Inclusion in trachyte, Grey Pen NT 445 035
Prepared hand specimen viewed in reflected light (measures 44mm across)

Trachyte, Grey Pen. Thin viewed in plane polarised light

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

Inclusion in trachyte, Grey Pen. Thin section viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Inclusion containing plagioclase in altered trachyte Section viewed in plane polarised light(FoV 4.6 x 3.0 mm)

Inclusion containing plagioclase in altered trachyte
Section viewed in plane polarised light(FoV 4.6 x 3.0 mm)

Inclusion containing plagioclase in altered trachyte Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area of the section viewed with crossed polarising filters

Inclusion containing plagioclase in altered trachyte Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Inclusion containing plagioclase in altered trachyte
A closer look at the inclusion viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Inclusion containing plagioclase in altered trachyte Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

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

Trachyte, Grey Pen NT449036.Prepared hand specimen viewed in reflected light (measures 44mm across)

Trachyte, Grey Pen NT 449 036
Prepared hand specimen viewed in reflected light (measures 44mm across)

Trachyte, Grey Pen. Thin section viewed in plane polarised light (measures 46mm across)

A thin section from the same specimen of trachyte
Thin section viewed in plane polarised light

Trachyte, Grey Pen. Thin section viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Iron oxides, carbonates and chlorite in trachyte. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Iron oxides, carbonates and chlorite in trachyte
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Iron oxides, carbonates and chlorite in trachyte Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

A closer view of the iron oxides, carbonates and a little chlorite in the same section of trachyte from Grey Pen

Iron oxides, carbonates and chlorite in trachyte Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area of the section viewed with crossed polarising filters

The trachytic rock looks much the same in all the outcrops around the hill – the views change considerably.

On the way back to the car we picked up a sample of the Silurian greywacke, the host rock for the Grey Pen and Skelfhill Pen intrusions. In thin section there does seem to be some alignment of the larger crystals highlighted by the white mica that we see in our sample.

View towards Rubers Law from Grey Hill

View towards Rubers Law from Grey Hill

Riccarton Group greywacke, Grey Pen NT449036. Prepared hand specimen viewed in reflected light (measures 42mm across)

Riccarton Group greywacke, Grey Pen NT 449 036
Prepared hand specimen viewed in reflected light (measures 42mm across)

Greywacke, Grey Pen. Thin section viewed in plane polarised light

Thin section from the same specimen of greywacke viewed in plane polarised light

Greywacke, Grey Pen. Section viewed with crossed polarising filters

The same thin sectiongo greywacke viewed with crossed polarising filters

Mica orientated in the same plane as other longer crystals in carbonate ground. Section viewed with crossed polarising filters (FoV 5.0 x 3.0 mm)

Greywacke, below Grey Pen
There are mica crystals orientated in the same plane as other longer crystals in the carbonate ground.Section viewed with crossed polarising filters (FoV 5.0 x 3.0 mm)

Mica crystals orientated in the same plane as other longer crystals in carbonate ground. Section viewed with crossed polarising filters (FoV 5.0 x 3.0 mm)

The same area of the greywacke section viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

References

McRobert R.W. Igneous rocks of Teviot and Liddesdale Transactions of the Edinburgh Geological Society, 11, 86-103, 1920, https://doi.org/10.1144/transed.11.1.86

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

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