Mill Glen, Tillicoultry - an excursion

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From: Browne, M.A.E. and Gillen, C. (editors) 2015. A geological excursion guide to the Stirling and Perth area. Edinburgh: Edinburgh Geological Society in association with NMS Enterprises Limited. Buy the book EGS.
Figure 7.1 Geological map of the area around the Mill Glen, Tillicoultry, showing localities for Excursion 7.
Plate 7.1 Locality 7.4, Castle Craigs Quarry, Tillicoultry. Historic view towards the west of the quartz-dolerite (Midland Valley Sill-complex) in West Ochil Fault plane, with Ochil Volcanic Formation strata to right.
Plate 7.2 Locality 7.6. Gorge along line of fault in the Tillicoultry Burn, and old mill dam.

Excursion 7 Mill Glen, Tillicoultry[edit]

By Howell Francis

Purpose: To examine deformed Coal Measures, quartz-dolerite of the Midland Valley Sill-complex emplaced along the West Ochil Fault, and Ochil Volcanic Formation rocks, including volcaniclastic rocks metamorphosed into hornfels in contact with an Early Devonian diorite stock.
Logistics: The excursion begins at the head of Mill Street in Tillicoultry which leads northwards from the A91. It is readily accessible by public transport from Stirling; there is car parking in the vicinity. The trail offers easy walking over a distance of about 2km and can be done in a half day.
Maps OS 1:50,000 Sheet 58 Perth & Kinross; OS 1:25,000 Sheet 366 Stirling & Ochil Hills West; BGS 1:50,000 Sheet 39E Alloa; locality map (Figure 7.1).

The visitor trail through the Mill Glen is maintained by Clackmannan Council. The paths are sometimes closed because of rock falls, as is the case at the time of final editing. Visitors should check with the Council regarding current access, and should obey safety notices. The trail crosses exposures of Lower Coal Measures with thin coal seams in the immediate hanging wall of the major West Ochil Fault. A component of the Midland Valley Sill-complex is intruded into the fault plane, demonstrating that major movements were completed by the time the dolerite crystallised. In the footwall of the fault, Lower Devonian volcaniclastic rocks can be seen, contact metamorphosed in the aureole of a diorite stock. Evidence of former mine workings for base metals can be seen. For more detail, see the memoir by Francis et al. (1970).

Locality 7.1 [NS 9140 9760] Deformed Coal Measures[edit]

Immediately through the gate, Coal Measures are discontinuously exposed over a distance of about 100m. Originally deposited in Westphalian times, in rivers, lakes and peat swamps covering delta tops and flood plains, they include sandstones, mudstones, coals and fossil soils such as seatearths and seatclays. Seams up to 1m thick were once to be seen, but both the thin coals now visible and the mudstones and seatclays adjacent to them are highly sheared in contrast to the harder, more-resistant fine-grained sandstones. All these strata are highly inclined and in places are almost certainly overturned: they strike approximately E–W along the east side of the track, but a fold plunging steeply to the west can be seen in the bed of the burn. Folds may be seen at [NS 9138 9672], where the sandstones and mudstones are highly jointed, the joints being closely spaced. The section as a whole testifies to the intermittent deformation effected by the West Ochil Fault during part of its long history.

Locality 7.2 [NS 9136 9764] Midland Valley Sill-complex: quartz-dolerite fault-intrusion[edit]

The steeply dipping hanging-wall contact between chilled, fine-grained dolerite (basalt) and locally hardened sandstone is well exposed on the edge of the path (Figure 7.1) and spheroidally weathered coarser grained quartzdolerite can be seen in the steep bank to the north of it. The footwall contact with the older volcanic rocks is not exposed here, but can be viewed from Locality 7.4.

Locality 7.3 [NS 9132 9769] Mill Glen Diorite Stock[edit]

Immediately to the north of the fault-intrusion, coarse-grained handsomely mottled pink and blue-green diorite forms the face of an old quarry extending for 25m or so up to the first footbridge. It consists of large laths of sericitised plagioclase feldspar (andesine–oligoclase rimmed by albite), with pale green sheaf-like hornblende growing out of pyroxenes, and with accessory biotite, iron minerals and interstitial potash feldspar. Along the burn, a few metres north of the bridge, the diorite makes contact with hornfelsed (heat-altered) volcaniclastic rocks, the contact marked by a vertical 0.5m-wide zone of microgranite (pink when fresh) – a fine- to medium-grained felsic igneous rock (previously named ‘aplite’) derived from residual liquids at a late stage in the crystallisation of the diorite. From its limited size, oval outcrop and vertical sides, this intrusion is interpreted as a stock, though it may join at depth with other bodies such as the Wester Kirk Craig Diorite Stock to form a larger pluton.

Locality 7.4 [NS 9120 9760] Castle Craigs Quarry, Tillicoultry: quartz-dolerite and hornfels[edit]

This disused quarry was originally worked for dolerite kerbstones and pavement setts. Although most of the quartz-dolerite fault-intrusion has long since been removed a remnant, chilled at its (northern) footwall margin and dipping off the slope of the hill at c.45–60°, can still be viewed across the quarry from the top of the steps leading up from the first footbridge (Plate 7.1). The main part of the quarry produced aggregate from metamorphosed Lower Devonian volcanic rocks traversed by veins of pink microgranite; the rocks can be examined in safety farther up the glen rather than at the dangerously precipitous and extremely hazardous faces.

Locality 7.5 [NS 9773 9125] Diorite dyke. A dyke-like offshoot from the main Mill Glen Diorite Stock crosses the burn and, where seen along the path, is about 30m wide dipping south at 70°. Its pink microgranitic margins are soft and somewhat sheared, but merge without obvious chilling into microdiorite at the centre of the dyke. High up on the opposite (east) bank of the glen, the mouth of a trial adit is visible. A small narrow overgrown track off to the right (east) at the end of a bridge leads to the adit, which is above a slightly steep bank. It follows a disturbance for 5m, then intersects a breccia containing a thin ferruginous calcite vein; this trends NNW and has been stoped to a height of 3m over a length of 12m.

Localities 7.6 [NS 9122 9790] and 7.7 [NS 9123 9810] Hornfelsed volcanic rocks[edit]

Over the next 400m, as the track crosses and recrosses the burn (Locality 7.6), it traverses a sequence of volcanic rocks lying within the metamorphic aureole of the Mill Glen Diorite Stock. The regional NNW dip remains visible, but original textures are considerably altered. Hornfelsing has given rise to hackly fracturing and the volcaniclastic components that form the bulk of the sequence have been partly recrystallised to fine-grained patchy lithologies in which the formerly sharply defined, mainly angular clasts have blurred into the groundmass. Some blurred clasts are difficult to distinguish from pink felsic patches (probably representing reconstituted amygdales) in hornfelsed lavas. Irregular veins of pink microgranite up to 0.5m wide traverse all rock types. Malachite and hematite coatings can be seen in places. The deeper linear section of the glen (Plate 7.2) has been determined by a fault which emerges in a gully on the east side of the track at Locality 7.7. Here, and elsewhere in the glen, light is generally poor and macroscopic textures can be examined most readily on wetted rock surfaces. The texture can be quite well seen just south of the second large bridge after the bridge by the dam.

Locality 7.8 [NS 9180 9820] Dykes and sills[edit]

At the confluence of the Daiglen and Gannel burns several fine-grained andesitic dykes (previously called ‘porphyrite’) can be seen; two of them feed higher sill-like intrusions. The dykes are mainly older than the diorite, but one exposed along the track south of the confluence can be traced uphill where it cuts the Wester Kirk Craig Diorite Stock. Basalt dykes are rare in the Ochils, but one of them floors the straight course of the Gannel Burn at and just below the confluence. Access to all of these minor intrusions is hazardous when the burns are in spate.

Locality 7.9 [NS 9135 9805] Hybrid diorite[edit]

From the confluence the track returns to Tillicoultry along the side of the hill. Detouring for a short distance uphill the margin of the Wester Kirk Craig Diorite can be inspected. Inside a well-marked topographic feature hybrid rocks are abundant; they consist of partly digested clasts of volcanic country rocks in a compact fine- to medium-grained microdioritic matrix.

Locality 7.10 [NS 9170 9795] Mill Glen Diorite[edit]

The NE part of the Mill Glen Diorite Stock forms massive, rounded crags of a diorite which is pinker and less coarse than the rock seen in the glen at Locality 7.3. The track traverses two marginal lobes of the stock, linked by a dyke of pink microgranite, which cuts the hornfelsed volcaniclastic rocks between the lobes but appears to merge into the diorite without chilled contacts.

Localities 7.11 [NS 9139 9783] and 7.12 [NS 9148 9777] Fractured hornfelses[edit]

Over its final zigzag descent, the track crosses brittle hornfelses which display a high incidence of N–S shatter, possibly resulting from the mechanical emplacement of the surrounding diorite stocks. At Locality 7.11, overlooking the quarry, there is a small pink microgranite dyke. At Locality 7.12 the hornfelsing locally reaches its highest grade in the form of recrystallised granular-textured patches. The track hereabouts affords good views of the quarry and the West Ochil Fault scarp.

At all times follow: The Scottish Access Codeand Code of conduct for geological field work