Hawsen Burn, Hawsen Crags & Lambden Burn

The Lambden Valley from Broadhope Hill

Lambden Valley from Broadhope Hill

Map showing the excursion route, locations and igneous rock types.

A map showing the route, locations , and igneous rock types in the vicinity of the Hawsen and Lambden Burns
Key for the excursion map

Location 1. The Hawsen Burn at NT951227

Both the Hawsen Burn and the Lambden Burn skirt a portion of the Cheviot pluton’s surface periphery and so outcropping bedrock along their two courses will tell us something about the granite/lava junction. In the lower Hawsen Valley, where it adjoins the Harthope Valley, Al Hafdh identified the occurance of his ‘Woolhope’ granite – the evolved type of granite that we have been calling our ‘High Cheviot’ type because we see a lot of it in the stones, cobbles and boulders in that locality. If we do find it outcropping here on the valley floor, perhaps we will need to jettison our geographically descriptive name and revert to the petrologically descriptive ‘Evolved Granite’.
It’s interesting that the dark, mafic ‘Marginal’ type of granite that is associated with the pluton/lava boundary in the south, south east and north, at Low Bleakhope, Dunmoor Hill and Bellyside Hill but it hasn’t been recorded in these localities.
Further up the Hawsen Burn we will be walking through a crush zone associated with the intersection of two faults and further crush zones occur along the Lambden Burn. Also in the Lambden Burn, there are more exposures that Al Hafdh recorded as ‘Woolhope’ granite: we want to see for ourselves.
On the return leg, we will detour up to Hawsen Crags where maps show a sizeable intrusion of mica-porphyry.

Outcrop of fine-grained granitic rock, Hawsen Burn (NT951227)

Hawsen Burn with Long Crags in the distance
The pluton outcrops close to the water on the left bank here.

Outcrop of fine-grained granitic rock, Hawsen Burn (NT951227)

Outcrop of fine-grained granitic rock, Hawsen Burn (NT951227)

Outcrop of fine-grained granitic rock, Hawsen Burn (NT951227)

Outcrop on the opposite bank, Hawsen Burn (NT951227)

From the car parking space by the Harthope Burn we turn up the Hawsen Burn that flows down the approximate line of contact of the pluton and the surrounding andesitic lava that has been hardened to ‘hornfels’ by the pluton’s heat . The course of the Hawsen Burn also marks the line of an inferred fault and its intersection with a second fault higher up the burn gives rise to a ‘crush zone’ where the bedrock has been repeatedly broken and cemented together with quartz.
The burn itself is full of stones, cobbles and boulders that provide an overview of the rock types that occur along its course. There are examples of red and black andesite, pink and red porphyritic and none-porphyritic granitic rock, and quartz-rich rocks often veined with red haematite and sometimes black tourmaline.

Our first location is at NT951227, one of the few places in the first section of the burn where the bedrock is exposed. The outcrops are small and low-lying on both sides of the burn where it takes an abrupt corner, perhaps channeled by the resistant rock. In situ, the rock appears pink-red and relatively medium-grained but even at the scale of this small exposure, hand samples reveal variation in its appearance.
Al Hafdh shows this area to be dominated by the fine-grained, evolved granite that he refers to as his ‘Woolhope’ type. We are interested to know if this really is the same type of rock as that which is scattered all around the higher reaches of Cheviot.

Medium-grained sub-equigranular granite , Hawsen Burn NT951227. Prepared hand specimen viewed in ordinary reflected light

Medium-grained sub-equigranular granite, Hawsen Burn, NT951227
Prepared hand specimen viewed in ordinary reflected light

Medium-grained sub-equigranular granite, Hawsen Burn (NT951227) PPL (36mm across)

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

Medium-grained sub-equigranular granite, Hawsen Burn. Thin section viewed with crossed polarising filters (36mm across)

The same thin section viewed with crossed polarising filters

Medium-grained sub-equigranular granite , Hawsen Burn NT951227. Prepared hand specimen viewed in ordinary reflected light (42mm across)

A second sample at the same outcrop viewed in reflected light (42mm across)

Medium-grained sub-equigranular granite, Hawsen Burn NT951227. Prepared hand specimen viewed in ordinary reflected light (46mm across)

A sample from the opposite bank viewed in reflected light (46mm across)

Medium-grained granitic rock, Hawsen Burn. Thin section viewed in plane polarised light (36mm across)

Medium-grained sub-equigranular granite, Hawsen Burn
Thin section viewed in plane polarised light (36mm across)

Medium-grained sub-equigranular granite, Hawsen Burn. Thin section viewed with crossed polarising filters (36mm across)

The same thin section viewed with crossed polarising filters

Alkali feldspar, quartz and iron-titanium oxides in medium-grained sub-equigranular granite, Hawsen Burn. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Alkali feldspar, quartz and iron-titanium oxides in medium-grained sub-equigranular granite, Hawsen Burn.
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Alkali feldspar, quartz and iron-titanium oxides in medium-grained sub-equigranular granite, Hawsen Burn. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

This rock does resemble that found at Woolhope crag and in the cobbles and boulders scattered around the top of Cheviot, but there are differences. The rock sampled here is similar to that at the other two locations in it’s low mafic content and its relatively high quartz content. It is also similar to that sampled at Woolhope Crag in its low plagioclase content; both lack the plagioclase phenocrysts that appear in the rock high on Cheviot. Further, all these rocks have been altered hydrothermically but much of the rock at Woolhope Crag and on upper Cheviot has been tourmalinised but tourmaline is absent at this location.
So, they look very similar in the hand but each has difference to the others. Are they the same type? Perhaps they are similar enough to refer to them as belonging to the same group in the field but perhaps not, we think, for assuming their origin in a common stock of magma. If we continue to refer to them collectively as ‘High Cheviot’ rocks we must remember along with their similarities, their range of difference.

Photomicrographs made at the same magnification comparing examples of our ‘High Cheviot’ rock

Granite, Hawsen Burn. Plagioclase phenocrysts and tourmaline are absent. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm

Granite from this location
Plagioclase phenocrysts and tourmaline are absent. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Granite at Woolhope Crag. There is little plagioclase but a lot of alkali feldspar in the rock,  chlorite, sericite and tourmaline are present.. Section viewed with crossed polarising filters FoV 4.6 x 3.0 mm)

Granite at Woolhope Crag
There is little plagioclase but a lot of alkali feldspar in the rock, chlorite, sericite and tourmaline are present. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Granite, Cheviot. Contains plagioclase phenocrysts and rods of biotite. Section viewed with crossed polarising filters(FoV 4.6 x 3.0 mm)

Granite on Cheviot
Contains plagioclase phenocrysts and rods of biotite. Section viewed with crossed polarising filters(FoV 4.6 x 3.0 mm)

Granite on Cheviot. Contains plagioclase phenocrysts, coarsely granophyric quartz, and tourmaline. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Granite on Cheviot
Contains plagioclase phenocrysts, coarsely granophyric quartz, and tourmaline. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Scotsmans Cairn, Cheviot granite. Contains plagioclase phenocrysts, coarsely granophyric quartz, and tourmaline. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Scotsman’s Cairn, Cheviot granite
Contains plagioclase phenocrysts, coarsely granophyric quartz, and tourmaline. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Location 2. Small outcrop in the stream bed at NT948228

Probable outcrop of medium-grained Central Belt rock, Hawsen Burn NT948228

Altered granitic rock, Hawsen Burn NT948228

Further up the burn, there’s another small exposure in the stream bed of what looks like bedrock. At first sight it looks like that at location 1 but in thin section it looks like something intermediate between the ‘High Cheviot ‘ and the ‘Central Belt’ rock types. There are plagioclase phenocrysts in the rock and it has a much higher mafic content. Along with the ubiquitous chlorite, there is also tourmaline. Once again, the Cheviot complex bamboozles us when we try to categorise its rocks!

Altered medium-grained porphyritic Central Belt, Hawsen Burn NT948228

A prepared sample from location 2 viewed in ordinary reflected light

Plagioclase and sanidine phenocrysts in fine grained matrix in granitic rock, Hawsen Burn. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase and sanidine phenocrysts in fine grained matrix in granitic rock, Hawsen Burn
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase and sanidine phenocrysts in fine grained matrix in granitic rock, Hawsen Burn. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Chloritised biotite and micrographic texture in medium-grained granitic rock, Hawsen Burn XP (FoV 2.3 x 1.5 mm)

Chloritised biotite and micrographic texture in the same rock
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Tourmaline in granitic rock, Hawsen Burn PPL (FoV 1.2 x 0.8 mm)

Tourmaline and chloritised biotite in the same rock
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Location 3. Craggy outcrop at NT946229

Crag with mix of fine-grained and medium-grained granitic rock, Hawsen Burn NT946229

Small crag with mix of fine-grained and medium-grained granitic rock
Hawsen Burn NT946229

Red to flesh-coloured rock is exposed in a small crag on the east bank of the burn. Thin sections reveal it is different again to the similar looking rock that we have seen downstream.
It comprises of two parts: a fine-grained quartz-feldspar mix that contains some white mica, or sericite, together with tourmaline, and a second part that is much larger-grained comprising feldspar with some beautiful perthitic texture, rounded quartz crystals and a little biotite. There are no plagioclase phenocrysts in either part.
We think this might be best understood as a dyke-like structure in which the fine-grained material has carried along fragments of an older, larger-grained rock .
We wonder if this outcrop is related to the porphyry intrusion directly above on Cold Law.

Rock with mix of fine-grained and medium-grained material, Hawsen Burn NT946229 (40mm across)

Rock with mix of fine-grained and medium-grained material
Hawsen Burn NT946229 (40mm across)

Rock with mix of fine-grained and medium-grained granitic rocks, Hawsen Burn. Thin section viewed in plane polarised light (40mm across)

Thin section from the same sample viewed in plane polarised light

Rock with mix of fine-grained and medium-grained granitic rocks, Hawsen Burn. Thin section viewed with crossed polarising filters (40mm across)

The same thin section viewed with crossed polarising filters

Mix of fine-grained and medium-grained granitic material, Hawsen Burn. Section viewed with crossed polarising filters

Detail showing the mix of fine-grained and medium-grained material
The medium-grained material appears to xenolithic.
Section viewed with crossed polarising filters.

Rounded quartz crystals, perthitic feldspars with biotite, Hawsen Burn. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Rounded quartz crystals, perthitic feldspars with biotite in the medium-grained material
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Rounded quartz crystals, perthitic feldspars with biotite, Hawsen Burn. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Fine-grained quartz-feldspar surrounding coarser-grained feldspar crystals exhibiting perthitic texture, Hawsen Burn. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Fine-grained quartz and feldspar surrounding coarser-grained feldspar crystals exhibiting perthitic texture
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Tourmaline with white mica (sericite) in the fine-grained material ,Hawsen Burn. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Tourmaline with white mica (sericite) in the fine-grained material
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Location 4.Intensely altered andesite at NT945230

Outcrop of intensely altered andesite, Hawsen Burn NT945230 ( FoV 1.5 metres)

Small outcrop of intensely altered andesite at location
The field of view is about 1.5 m.

The andesite outcropping here and upstream to the crush zone is extremely altered and weak. Colin Scuton writes there are ‘thick veins and stringers of tourmaline and lumps of massive epidote’ in this rock but we didn’t see them here. Under the microscope we see a great deal of magnetite along with limonite and haematite and the remnants of plagioclase and biotite phenocrysts in the surviving predominantly feldspar groundmass.

Intensely altered andesite, Hawsen Burn NT945230. Prepared hand specimen in ordinary reflected light (42mm across)

Intensely altered andesite, Hawsen Burn NT945230
Prepared hand specimen in ordinary reflected light (42mm across)

Intensely altered andesite, Hawsen Burn. Thin section viewed in plane polarised light (40mm across)

A thin section from the same sample viewed in plane polarised light (40mm across)

Intensely altered andesite, Hawsen Burn. Thin section viewed with crossed polarising filters (40mm across)

The same thin section viewed with crossed polarising filters

Plagioclase and biotite phenocrysts in altered andesite, Hawsen Burn. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Plagioclase and biotite phenocrysts in altered andesite at location 4
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Plagioclase and biotite phenocrysts in altered andesite, Hawsen Burn. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area of the sampleviewedwith crossed polarising filters

Location 5. Crush Zone around NT943230

In this area, two faults intersect to form a crush zone where the rocks have been repeatedly stressed and broken by mechanical action along each fault and cemented and re-cemented by quartz deposited in the process of intense hydrothermal alteration. Many of the quartz-rich rocks are rich in tourmaline, it’s black veins and stringers contrasting sharply with the white silica. These minerals are often accompanied by the more oxidised states of iron oxide – haematite and limonite that contribute rich golden oranges and reds to the rocks.

Tourmaline in a quartz vein in the crush zone on the Hawsen Burn. Prepared hand specimen in ordinary reflected light

Tourmaline in a quartz vein in the crush zone on the Hawsen Burn Crush Zone NT943230
Prepared hand specimen in ordinary reflected light. (42mm across)

Tourmaline in silica vein, Hawsen Burn. Thin section viewed in plane polarised light (28 mm across)

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

Tourmaline in silica vein, Hawsen Burn. Thin section viewed with crossed polarising filters (28 mm across)

The same thin section viewed with crossed polarising filters

Tourmaline in silica vein, Hawsen Burn. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Tourmaline in a silica vein in the crush zone
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Tourmaline in silica vein, Hawsen Burn. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same tourmaline and quartz crystals viewed with crossed polarising filters
(FoV 2.3 x 1.5 mm)

Tourmaline in silica vein, Hawsen Burn. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Tourmaline with quartz in the crush zone on the Hawsen Burn
Some of the quartz occurs as inclusions within many of the tourmaline crystals suggesting that the quartz crystallised in advance of the tourmaline. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Quartz with haematite stringers, Hawsen Burn NT944230. Prepared hand specimen in ordinary reflected light

Quartz with haematite stringers, Hawsen Burn NT944230
Prepared hand specimen in ordinary reflected light (45 mm across)

Quartz with haematite stringers, Hawsen Burn. A thin section from the same sample viewed with crossed polarising filters (30mm across)

A thin section from the same sample viewed with crossed polarising filters (30mm across)

Quartz, Hawsen Burn. Section viewed with crossed polarising filters.

Detail from the thin section above showing difference in grain-size in the quartz

Location 6. Altered andesite on lower Blackseat Hill

At this point the Hawson Burn is hedged in by small cliffs of altered andesite. It is the red, haematite-rich variety that also contains tourmaline.

Altered andesite, lower Blackseat Hill NT942230. Prepared hand specimen in ordinary reflected light (42mm)

Altered andesite, lower Blackseat Hill NT942230
Prepared hand specimen in ordinary reflected light (42mm across)

Altered andesite, lower Blackseat Hill. Thin section viewed in plane polarised light (40mm)

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

Altered andesite, lower Blackseat Hill. Thin section viewed with crossed polarising filters (40mm)

The same thin section viewed with crossed polarising filters

Altered Altered biotite and plagioclase in andesite. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered biotite and plagioclase in andesite
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered Altered biotite and plagioclase in andesite. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

A thin section from the same sample viewed with crossed polarising filters

Tourmaline and biotite in altered andesite. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Tourmaline and biotite in altered andesite
Section viewed with crossed polarising filters(FoV 1.2 x 0.8 mm)

Location 7. The Lambden Burn at the base of Preston Hill

Outcropping granite and andesite amongst the talus at the base of Preston Hill, Lambden Burn, NT922231

Outcropping granite and andesite amongst the talus at the base of Preston Hill, Lambden Burn, NT922231

The path that will take us over to location 7 runs parallel to the Hawsen Burn just 30 metres or so to the north. So, we scramble up to it and continue west, over the side of Broadhope Hill and down towards the isolated Lambden Valley. There are no outcrops up here and the path is indistinct so we make our way as best we can.
We are heading for the screes and outcrops of red rocks at the base of Preston Hill; the location of more of what Al-Hafdh identifies as evolved ‘Woolhope’ granite. We decide to avoid the path through the plantations around NT926231 because it is largely concealed by fallen trees and overgrowth. Instead, we take the farm track at NT928231 for a couple of hundred metres heading north-west and then taking the turn west at NT927233 thereby skirting the northern edge of the triangular plantation and rejoin the path at the western point of the plantation.
From here we continue for another 100 metres and then drop down to the outcrops that lie just above the Lambden Burn.
The BGS online map shows dyke-like extensions of the pluton that intrude into the andesite lavas. There is good deal of scree but amongst it are a few small outcrops of bedrock.
As is often the case in these junction zones, there is a substantial range in the appearance of the rocks over a short distance – in this case about 100 metres. And once again here at the periphery of the pluton, there is a good deal of silica veining evidencing movement in the rocks and the circulation of hydrothermal fluids.
In thin section, most of the rock turns out to be andesite with one outcrop of granitic rock that has similarities to that found on high Cheviot containing plagioclase and biotite phenocrysts.

Outcrops where the Cheviot pluton outcrops adjacent to the lavas, Lambden Burn NT922231

Outcrops where the Cheviot pluton outcrops adjacent to the lavas, Lambden Burn NT922231

Quartz-rich rock, Lambden Burn

Quartz-rich rock, Lambden Burn NT922231

Rocks at the granite-andesite junction, Lambden Burn displayed west (l) to east (r)

Rocks at the granite-andesite junction, Lambden Burn displayed west (l) to east (r)
Prepared hand specimens in ordinary reflected light

Specimen 1
The first specimen is the one and only specimen of granitic rock that we take. It has numerous plagioclase and some biotite phenocrysts in a fine to medium grained ground in which some of the quartz and feldspars have pronounced micrographic textures. All of the feldspars have been made turbid with iron oxide.
The specimen includes fragments of hornfelsed andesite that looks to have been recrystallised with many small grains of iron-titanium oxide and biotite present.

Granitic rock with fragments of andesite, Lambden Burn NT923231. Prepared hand specimen in ordinary reflected light.

Granitic rock with fragments of andesite, Lambden Burn NT923231
Prepared hand specimen in ordinary reflected light.

Granitic rock with fragments of andesite, Lambden Burn. Thin section viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light

Granitic rock with fragments of andesite, Lambden Burn. Thin section viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Granitic rock with hornfels fragments, Lambden Burn. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Granitic rock with hornfels fragments, Lambden Burn
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Granitic rock with hornfels fragments, Lambden Burn. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Granitic rock with hornfels fragments, Lambden Burn. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area at x100
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Specimen 2
The second specimen is similarly brecciated and cemented with quartz stringer although the lithic fragments are not so obviously andesite. There is no sign of biotite in them and only one very altered plagioclase phenocryst and they do contain some quartz although this could be due to alteration.
The sample also contains a very small amount of tourmaline.

Granitic/andesitic brecciated rock, Lambden Burn NT923231. Prepared hand specimen in ordinary reflected light (48mm across)

Granitic/andesitic brecciated rock, Lambden Burn NT923231
Prepared hand specimen in ordinary reflected light (48mm across)

Granitic, andesitic brecciated rock, Lambden Burn. Thin section viewed in plane polarised light (47mm across)

Thin section from the same sample viewed in plane polarised light

Granitic, andesitic brecciated rock, Lambden Burn. Thin section viewed with crossed polarising filters (47mm across)

The same thin section viewed with crossed polarising filters

Silica stringers in granitic, andesitic brecciated rock, Lambden Burn. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Silica stringers in granitic, andesitic brecciated rock, Lambden Burn
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Silica stringers in granitic, andesitic brecciated rock, Lambden Burn. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters
The section has been left slightly thick so as to reveal the quartz in the lithic fragments.

Silica veins in granitic, andesitic brecciated rock, Lambden Burn. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Silica veins in granitic, andesitic brecciated rock, Lambden Burn
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Specimen 3
More andesite that has been fractured and cemented with silica. This one has some bright golden yellow staining associated with one silica veinlet and a very nice example of zoning in a basal section of a plagioclase crystal.

Andesite outcrop on Lambden Burn below Preston Hill NT922231. Prepared hand specimen in ordinary reflected light. (55mm across)

Andesite outcrop on Lambden Burn below Preston Hill NT922231
Prepared hand specimen in ordinary reflected light. (55mm across)

Andesite, Lambden Burn. Thin section viewed in plane polarised light (50mm across)

Thin section from the same sample viewed in plane polarised light (50mm across)

Andesite, Lambden Burn. Thin section viewed with crossed polarising filters (50mm across)

The same thin section viewed with crossed polarising filters

Iron oxide staining related to silica veining in altered andesite, Lambden Burn. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Iron oxide staining related to silica veining in altered andesite, Lambden Burn
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Iron oxide staining related to silica veining in altered andesite, Lambden Burn. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Granophyric fragment in andesite, Lambden Burn. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Granophyric fragment in altered andesite, Lambden Burn
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Zoned plagioclase in andesite, Lambden Burn. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Zoned plagioclase in andesite, Lambden Burn
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Specimen 4
Fractured, silica veined, chloritised andesite.

Altered andesite, Lambden Burn NT922231

Altered andesite, Lambden Burn NT922231
Prepared hand specimen in ordinary reflected light

Feldspar phenocryst in andesite fractured by silica veinlet, Lambden Burn. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Silica veinlet cutting across a fractured feldspar phenocryst in andesite, Lambden Burn
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Silica veinlet cutting across a fractured feldspar phenocryst in andesite, Lambden Burn  Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Chloritised biotite in andesite adjacent to a silica veinlet, Lambden Burn. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Chloritised biotite in andesite adjacent to a silica veinlet, Lambden Burn
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Location 8. Below Woolhope Crag NT926224

Granitic outcrop to the left of the tributary to the Lambden Burn beneath Woolhope Crag

Granitic outcrop to the east (left on picture) of the stream running down to the Lambden Burn beneath Woolhope Crag

Having crossed from the north to the south bank of the burn, we head across open country in the direction of Woolhope Crag aiming to explore for exposures in the upper Lambden Burn.
We pick up the forestry track and head east until we arrive at the small plantation. The stones in the burn are typically red and felsic looking – similar in appearance to the evolved granite that we have already seen today. We head upstream and enjoy this delightful little woodland and, near it’s southern end, find a good exposure of bedrock that we sample.
In the hand it is surprisingly dark and thin sections reveal a profusion of dark specks that are mostly tourmaline. The rock has been intensively altered with sericite scattered throughout and large patches of white mica occur in association with silica veining.

Altered granite in the Lambden Burn below Woolhope Crag NT926224. Much of the dark material is tourmaline. Prepared hand specimen in ordinary reflected light (48mm across)

Altered granite in stream in the Lambden Burn below Woolhope Crag NT926224
Much of the dark material is tourmaline. Prepared hand specimen in ordinary reflected light (48mm across)

Altered granitic rock in the Lambden Burn below Woolhope Crag. Thin section viewed in plane polarised light (48mm across)

Thin section from the same sample viewed in plane polarised light (48mm across)

Altered granitic rock in the Lambden Burn below Woolhope Crag.Thin section viewed with crossed polarising filters (48mm across)

The same thin section viewed with crossed polarising filters

Altered granite in the Lambden Burn below Woolhope Crag. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered granite in the Lambden Burn below Woolhope Crag
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered granite in the Lambden Burn below Woolhope Crag. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Mica in silica veinlet with tourmaline in fine-grained granite, Lambden Burn. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Mica in silica veinlet with tourmaline in fine-grained granite, Lambden Burn
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Mica in silica veinlet with tourmaline in fine-grained granite, Lambden Burn. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area viewed with crossed polarising filters

Haematite in silica veinlet in fine-grained granite, Lambden Burn.Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Haematite in silica veinlet in fine-grained granite, Lambden Burn
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Location 9. Hawsen Crags NT947232

We start our return leg of the excursion by following the forest track east and then north to finally cut across rough land again to meet the track leading over to the Hawsen Valley. With the Hawsen Burn below us, we leave the path onto the track that leads directly on and up to Hawsen Crags.
The crags are andesite, both the red and the black types each containing the usual pyroxene and andesine plagioclase phenocrysts. One thin section reveals a biotite crystal included in plagioclase and also a plagioclase crystal included in biotite suggesting their crystallisation overlapped.

Hawsen Crags, Andesite. Prepared hand specimen in ordinary reflected light (45mm across)

Hawsen Crags, Andesite NT947232

Andesite Hawsen Crag. Portion of thin section viewed in plane polarised light ( 20mm across)

Thin section from the same sample viewed in plane polarised light (20mm across)

Andesite Hawsen Crag. Portion of thin section viewed with crossed polarising filters (20mm across)

The same thin section viewed with crossed polarising filters

Clinopyroxene in andesite, Hawsen Crags. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Clinopyroxene in andesite, Hawsen Crags
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Clinopyroxene in andesite, Hawsen Crags. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm

The same area viewed with crossed polarising filters

Biotite inclusion in plagioclase in andesite, Hawsen Crag. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Biotite inclusion in plagioclase in andesite, Hawsen Crag
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Plagioclase inclusion in biotite in andesite, Hawsen Crags. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Plagioclase inclusion in biotite in andesite, Hawsen Crags
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Location 10. Porphyry intrusion NT950232

We head due west in search of an outcrop of the mica-porphyry intrusion that is shown on the BGS map. In the event, we make do with a cobble of what looks like the stuff, collected from a number of them scattered around the head of Butterwell Syke.
Thin sections reveal a very quartz-rich rock, very different from the porphyry and quartz-porphyry we’ve seen elsewhere. Perhaps the high quartz content is related to the intense sericitic alteration the rock has been subjected to, but perhaps it reflects its original composition. Clearly we need to return here and search Cold Law for outcrops.
The rock has many rafts of what look like plagioclase xenocrysts or even, in some cases, small xenoliths, usually surrounded by quartz and feldspar in a granophyric texture.

Quartz-rich intrusive rock, east of Hawsen Crags NT950232

Quartz-rich intrusive rock, east of Hawsen Crags NT950232
Prepared hand specimen in ordinary reflected light (42mm across)

Quartz-rich intrusive rock, east of Hawsen Crags. Thin section viewed in plane polarised light (40mm across)

Thin section from the same sample viewed in plane polarised light (40mm across)

Quartz-rich intrusive rock, east of Hawsen Crags. Thin section viewed with crossed polarising filters (40mm across)

The same thin section viewed with crossed polarising filters

Altered xenocryst or xenolith in quartz-rich intrusion, east Hawsen crags. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered xenocryst or xenolith in quartz-rich intrusion, east Hawsen crags
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered xenocryst or xenolith in quartz-rich intrusive rock, east Hawsen crags. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Granophyric texture around plagioclase xenocryst. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Granophyric texture around plagioclase xenocryst
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Granophyric texture around plagioclase xenocryst. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Altered xenocryst in quartz-rich intrusive rock, east Hawsen crags. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered xenocryst in quartz-rich intrusive rock, east Hawsen crags
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered xenocryst in quartz rich intrusion, east Hawsen crags. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Smaller, less altered plagioclase crystal. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Smaller, less altered plagioclase crystal
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Smaller, less altered plagioclase crystal. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Sericitic alteration of xenolith. Section viewed with crossed polarising filters (FoV 2.3 x 1.5mm)

Sericitic alteration of xenolith
The biotite has been altered to white mica. Section viewed with crossed polarising filters (FoV 2.3 x 1.5mm)

Location 11. Porphyry at NT950226

Porphyry outcrop, west of Hawsen Burn NT950226

Porphyry outcrop, west of Hawsen Burn NT950226
The rock resembles a porphyry dyke but it may be an outcrop of the pluton

We follow Butterwell Syke down to the valley bottom, cross the Hawsen Burn and pick up the track on the west bank that takes us to location 11, a small outcrop that we have noticed. At first we think it is probably more of the evolved granite, certainly the BGS map shows this area to be granitic, but thin section reveals it to be more like a porphyry dyke. No dyke is shown on the map so it’s probably another variant on the evolved, less mafic, type. There are large plagioclase phenocrysts or, quite likely, xenocrysts that are almost completely sericitised as were those at location 10, and there would also have been biotite – but this has been altered to white mica (also sericite). There is a good deal of perthite and there is a little quartz that may be an alteration product as it is in close association with the tourmaline appears in the rock.

Porphyry-like rock, west of Hawsen Burn NT950226. Prepared hand specimen in ordinary reflected light (48mm across)

Porphyry-like rock, west of Hawsen Burn NT950226
Prepared hand specimen in ordinary reflected light (48mm across)

Porphyry, west of Hawsen Burn NT950226. Thin section viewed in plane polarised light (48mm across)

Thin section from the same sample viewed in plane polarised light (48mm across)

Porphyry, west of Hawsen Burn NT950226. Thin section viewed with crossed polarising filters (48mm across)

The same thin section viewed with crossed polarising filters

Plagioclase, mica and tourmaline in porphyry. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase, mica and tourmaline in porphyry
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase, mica and tourmaline in porphyry. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Plagioclase, mica and tourmaline with perthite in porphyry. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Plagioclase, mica and tourmaline with perthite in porphyry-like rock
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Plagioclase, mica and tourmaline with perthite in porphyry. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Plagioclase, mica and tourmaline in porphyry. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase, mica and tourmaline in porphyry-like rock
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase, mica and tourmaline in porphyry. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

White mica (sericite) with perthite in porphyry. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

White mica (sericite) with perthite in porphyry-like rock
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)


Return to car

Leaving location 11, we follow the track down to the road to Langleeford where we turn left and return to the car.


References

N.M. Al-Hafdh, 1985. The Alteration Petrology of the Cheviot Granite. Thesis submitted for PhD. at Newcastle University.

J. G. Mitchel, K. M. Storetvedt, D. A. Robson, M. C. Abranches, and P. R. Ineson, 1993, Evidence for Carboniferous thermochemical overprinting in the Cheviot Complex, Scottish Journal of Geology 29, (1), 55-68,

C. Scruton (ed) 1995, Northumbrian Rocks and Landscape: a field guide, Yorkshire Geological Society.

British Geological Survey,/strong> Online geology map http://mapapps.bgs.ac.uk/geologyofbritain/home.html

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

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