Skelfhill Pen

‘On my hills the moon-beams play.
From Craik-cross to Skelfhill-pen,
By every rill, in every glen,
Merry elves their morris pacing…’ Sir Walter Scott

Skelfhill Pen from the flank of Grey Pen

Skelfhill Pen from the SW flank of Grey Pen
The long, dyke-like intrusion of phonolite outcrops along the ridge from the lower NE end (mid-left in the photograph) to the north-facing crags visible below the summit.

Skelfhill Pen is quite popular amongst hill walkers on account of its situation, distinctive shape, trig point, cairn and fine views of the surrounding countryside. A walk to the hill is sometimes by way of Knowes Head and Greatmoor Hill but we parked in the space down the hill from the track to Skelfhill Farm, followed this track to Skelfhill Pen, and returned via Grey Pen.

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

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

The igneous rock here is interesting on account of it being so rich in sodium and low in silica; characteristic traits of rock derived from magma intruded in areas subject to extensional forces and rifting.
We sampled from four locations on the hill (marked on the map with circles). The rock on the little north-eastern ridge to the summit is quite a hard pink-brown, fine-grained trachytic rock with a groundmass of sanidine microlites enclosing small crystals of aegirine augite, nepheline and riebeckite. Some of the rock that has been altered to a deeper pink contains more haematite and is very weak.
The rock in the crags below the summit is less trachytic, much darker (almost black in places) and very hard. There is a gradual transition from the pink to the dark rock.
The BGS records the general rock type as phonolite to phonolitic trachyte, the latter containing less nepheline than the former.

QAPF diagram Phonolite

The rock on the ridge to the north-east of the summit

The trachytic structure of the rock, in which the sanidine laths are aligned along lines of flow in the magma, is very well defined here.
Rachel McRobert describes its mineral content as, ‘Aegerine-augite in rectangular microlites, or grass-green micro-ophitic patches. Acmite in microlites and laths, pleochroic from brown toyellow, and subordinate in amount. Riebeckite in deep blue sub-ophitic patches or tiny flakes. Nepheline, very abundant in small idiomorphic crystals withsquare or hexagonal sections and magnetite, in octahedra.’
Acmite and aegirine are now considered to be the same mineral belonging to the pyroxene group, acmite being a sodium-rich variety of aegirine. They were originally believed to belong to separate groups, acmite to the amphiboles and aegirine to the pyroxenes.
‘Acmite’ is no longer a credited mineral name but in the descriptions of the photomicrographs that follow, we have found it helpful to use the name to distinguish between the two forms of pyroxene visible in the rock at Skelfhill Pen.

Phonolite, Skelfhill Pen NT444031 RL . (3.8mm across)

Phonolite, Skelfhill Pen NT444031
Prepared hand specimen viewed in reflected light.

Phonolite, Skelfhill Pen. Thin section viewed in plane polarised light.

Phonolite, Skelfhill Pen
Thin section viewed in plane polarised light.

Phonolite ,Skelfhill Pen. Section viewed with crossed polarising filters.

Phonolite, Skelfhill Pen
The same thin section viewed with crossed polarising filters.

Trachytic texture in phonolite, Skelfhill Pen. section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Trachytic texture in phonolite, Skelfhill Pen
Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Phonolite, Skelfhill Pen. Sanidine, nepheline, acmite and riebeckite. Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Sanidine, nepheline, acmite and riebeckite in the same thin section
Viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Phonolite, Skelfhill Pen. Sanidine, nepheline, acmite and riebeckite. Section viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

The sanidine, nepheline, acmite and riebeckite viewed with crossed polarising filters

Acmite and riebeckite pleochroism. Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Pleochroism in acmite and riebeckite
With the polariser set to polarise light in the vertical plane, the acmite is a green-brown colour and the riebeckite is a pale grey (FoV 0.5 x 0.3 mm)

Acmite and riebeckite pleochroism The blue light-brown to blue pleochroism of the riebeckite and the brown to green pleochroism of the acmite are visible here. Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Acmite and riebeckite pleochroism
With the polariser set horizontally, the grey to blue pleochroism of the riebeckite and the green-brown to green pleochroism of the acmite are clearly visible. Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Phonolite, Skelfhill Pen NT443031. Prepared hand specimen viewed with in reflected light (measures 44mm across) NT443031

Phonolite, Skelfhill Pen NT443031
Prepared hand specimen viewed with in reflected light (measures 44mm across)

Phonolite, Skelfhill Pen. Thin section viewed in plane polarised light.

Thin section from the same specimen viewed in plane polarised light

Phonolite, Skelfhill Pen. Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Feldspar phenocryst in phonolite. Section viewed in plane polarised light. (FoV 4.6 x 3.0 mm)

Feldspar phenocryst in phonolite
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Feldspar phenocryst in phonolite. Section viewed with crossed polarising filters. (FoV 4.6 x 3.0 mm)

Feldspar phenocryst in phonolite
Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Riebeckite with acmite viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Riebeckite with acmite
Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Aegirine-augite pyroxene in phonolite at Skelfhill Pen. Section viewed in plane polarised light. (FoV 1.2 x 0.8 mm)

Aegirine-augite pyroxene in phonolite at Skelfhill Pen
Section viewed in plane polarised light. (FoV 1.2 x 0.8 mm)

Aegirine-augite in phonolite at Skelfhill Pen. Section viewed with crossed polarising filters. (FoV 1.2 x 0.8 mm)

The same area viewed with crossed polarising filters

Altered phonolite, Skelfhill Pen NT443031. Prepared hand specimen viewed with in reflected light (measures 43mm across)

Altered phonolite, Skelfhill Pen NT443031
Prepared hand specimen viewed in reflected light (measures 43mm across)

Altered phonolite, Skelfhill Pen. Thin section viewed in plane polarised light.

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

Altered phonolite, Skelfhill Pen. Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Feldspar in altered phonolite from Skelfhill Pen viewed in plane polarised light. There are many holes in this section, some existing in the original rock and some as a consequence of thinning because the rock is so weak. (FoV 4.6 x 3.0 mm)

Feldspar in altered phonolite from Skelfhill Pen viewed in plane polarised light.
There are many holes in this section, some existing in the original rock and some as a consequence of thinning because the rock is so weak. (FoV 4.6 x 3.0 mm)

Feldspar in altered phonolite from Skelfhill Pen viewed with crossed polarising filters. There are many holes in this section, some existing in the original rock and some as a consequence of thinning because the rock is so weak. (FoV 4.6 x 3.0 mm)

The altered phonolite viewed with crossed polarising filters
There are many holes in this section, some existing in the original (FoV 4.6 x 3.0 mm)

Haematite and other alteration products in the same sample of altered phonolite. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Haematite and other alteration products in the same sample of altered phonolite
Along with the haematite there is an unidentified green, non-pleochroic, moderate RI alteration product seen bottom left. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The north-facing crags below the summit

The north-facing phonolite crags at Skelfhill Pen NT 442 031

The north-facing phonolite crags at Skelfhill Pen NT 442 031

The rock in the crags below the summit is much darker and harder than elsewhere on the Skelfhill Pen and is thought to be the least altered. Thin sections reveal less flow structure in the sanidine and nepheline groundmass and a lack of riebeckite, at least in the samples we took. Individual grains of aegirine-augite are more abundant here than in the more trachytic rock but still occurring in close association with the acmite and nepheline . It is tempting to interpret the differences in the two rock types as evidence of their origins in two related, but different magmas.

Phonolite, Skelfhill Pen NT442031. Prepared hand specimen viewed with in reflected light (measures 46mm across)

Phonolite, Skelfhill Pen Crags NT 442 031
Prepared hand specimen viewed with in reflected light (measures 46mm across)

Phonolite, Skelfhill Pen NT442031. Thin section viewed in plane polarised light (measures 46mm across)

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

Phonolite, Skelfhill Pen NT442031. Thin section viewed with crossed polarising filters

The thin section viewed with crossed polarising filters

Phonolite, Skelfhill Pen. The mafic minerals are concentrated in the less turbid patches of rock. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Phonolite, Skelfhill Pen
The mafic minerals are concentrated in the less turbid patches of rock. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Nepheline and acmite in phonolite. The clear nepheline crystals have a typically stubby, rectangular appearance. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Nepheline and acmite in phonolite
The clear nepheline crystals have a typically stubby, rectangular appearance. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Nepheline and acmite in phonolite. 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)

Aegirine-augite with acmite in phonolite. In plane polarised light, the aegirine-augite has higher relief,  is more strongly pleochroic and has a less intense colour than the acmite (FoV 1.2 x 0.8 mm)

Aegirine-augite with acmite in phonolite
In plane polarised light, the aegirine-augite has higher relief, is more strongly pleochroic and has a less intense colour than the acmite (FoV 1.2 x 0.8 mm)

Aegirine-augite with acmite in phonolite. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

With crossed polarising filters the visible interference colours of the aegirine-augite are more visible and clearly distinguish it from the acmite
(FoV 1.2 x 0.8 mm)

Phonolite, Skelfhill Pen NT442031. Prepared hand specimen viewed in reflected light (measures 45mm across)

Phonolite, Skelfhill Pen NT 442 031
Prepared hand specimen viewed in reflected light (measures 45mm across)

Phonolite, Skelfhill Pen. Thin section from the same specimen viewed in plane polarised light

Thin section from the same specimen viewed in plane polarised light

Phonolite, Skelfhill Pen Thin section viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Acmite and haematite in phonolite. Again the pyroxene is more concentrated in the less turbid patches whereas the haematite is more concentrated in the more turbid regions. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Acmite and haematite in phonolite
Again the pyroxene is more concentrated in the less turbid patches whereas the haematite is more concentrated in the more turbid regions. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

One of many skeletal flakes of acmite with haematite in the phonolite

One of many skeletal flakes of acmite with haematite in the phonolite’s feldspar and nepheline groundmass
Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Skeletal flakes of acmite with haematite in phonolite. Section viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

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

Acmite in a very thin section of the phonolite viewed with crossed polarising filters. When the section is very thin the interference colours of the acmite are visible. (FoV 0.5 x 0.3 mm)

Acmite in a very thin section of the phonolite viewed with crossed polarising filters
When the section is very thin the interference colours of the acmite are visible. (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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