Fast Castle - an excursion
O.S. 1:50 000 Sheet 67 Duns and Dunbar
- 1 Introduction
- 1.1 1. Harly Darlies: Greywacke, Glacial Deposits
- 1.2 2. Dowlaw Dean: Post-Glacial Gorge
- 1.3 3. Brander: Anticline
- 1.4 4. Muscle Craigs: Syncline
- 1.5 5. Dowlaw Burnfoot: Sedimentary and Structural Features
- 1.6 6. Souter: Stack, Glacial Deposits
- 1.7 7. Big Byrips: Multiple Anticline
- 1.8 8. Hawks Heugh: Syncline
- 1.9 9. Fast Castle: Complex Anticline
- 1.10 10. The Little Rooks: Multiple Fold, Fault Breccia
- 2 References
The object of this excursion is the examination of Silurian greywackes in some excellent exposures on the Berwickshire coast, at the north-eastern end of the extensive outcrop of Lower Palaeozoic rocks which forms the Southern Uplands of Scotland. Attention is drawn particularly to tectonic and sedimentological features of the rocks. Certain glacial deposits and landforms will also be noticed (Geikie 1864, Greig 1988).
The area lies between Siccar Point and St Abb's Head, north of the A1107 between Coldingham and the A1 road south of Cockburnspath. The road to Dowlaw and Fast Castle turns off the A1107 road at Old Cambus Wood (NT 828 688), 8 km from Coldingham and 5 km from the A1. The excursion lies within the farms of Dowlaw and Lumsdaine to the east. It involves between 5 and 6 km of walking, mainly over hilly grassland, but some 400 m are across a rocky shore, which is reached via steep slopes of vegetated scree between 50 and 100 m high. Additional descent, and ascent, of about 100 m of scree may be considered worthwhile at one locality. Stout footwear is essential, with a good grip in the soles.
The greywackes are of Silurian (Llandovery) age, part of the Gala Group, and were deposited by turbidity currents which swept down submarine slopes and deposited their load of sand and mud in the flatter areas of the sea-floor. Each bed corresponds to one episode of flow and in many cases the lower, coarser part of the bed is graded, the coarsest material having settled first. Bed thickness is generally up to 2 m, but in exceptional examples may range up to 6 m. Near the bases of a few of the thicker beds tiny angular pebbles or shell fragments can be seen with the naked eye together with larger, detached pellets of the very fine shaly beds. Structures related to the slumping of unconsolidated sediment can be identified in places and solemarks on the bases of greywacke beds are very common. The upper surfaces of greywacke beds may preserve ripple-marks, in most cases transverse to the linear bottom structures (sole marks) and hence to the current which caused them. The spectacular folding of the strata is the result of sequential deformation during the late Silurian caused by the closure of the ocean in which the rocks were deposited. This has left bedding steeply inclined, commonly vertical or inverted, and the recognition of grading and of top- and bottom-structures is an essential technique if the order of deposition of the beds is to be established. Fossils are not abundant. Traces of graptolites may be visible in the field occurring in apparently random pockets among the fine-grained shaly rocks. They are all of upper Llandovery age (Strachan 1982).
1. Harly Darlies: Greywacke, Glacial Deposits
In normal weather this in an excellent point for viewing the East Lothian coastal plain (Lower Carboniferous), the outer Firth of Forth, and Fife. Much nearer is the historic locality of Siccar Point.
2. Dowlaw Dean: Post-Glacial Gorge
The farmer at Dowlaw (NT 856 702) should be consulted about the parking of vehicles. From the farm follow a track eastwards down to a bridge across the Dowlaw Burn on to the lands of Lumsdaine, and then cross the fields along the right bank of the deeply cut Dowlaw Dean, largely Post-Glacial, to the cliff-top near Brander. On the way notice high on the opposite side of the Dean one or two short sloping benches, developed by erosion at an early stage in the cutting of the gorge.
3. Brander: Anticline
This narrow headland is formed by greywackes dipping steeply to north-west, close to a spectacular anticline which can be viewed from several points on the cliff-top and the screes south-west of the headland. A good head for heights is essential in places! Where first seen, looking seaward across the screes, the fold has a well rounded unbroken form, approximately symmetrical about a very steep axial plane, each bed maintaining its thickness across the nearly horizontal crest. Within somewhat steeper beds immediately to the left (north-west) the crest of a subsidiary anticline can be seen, plunging towards the viewer at 30°. If these subsidiary folds are congruous with the main anticline the thin-bedded strata to the right must include a complementary syncline, perhaps represented by a plane of bed-parallel faulting. These subsidiary folds are isoclinal, their axial planes parallel to the bedding and dipping steeply to NNW. They are seen again to south-west, on the landward side of the scree, but here they are more open and more widely separated, though still with a marked plunge to south-west, and are perceptibly diverging from the main fold. (The main fold also changes inform. From the sea it appears to have an angular crest and to be somewhat disturbed by steep faulting). An analogous development of subsidiary folding will be seen again at Locality 5.
4. Muscle Craigs: Syncline
Descend to the shore by the screes west of Brander. About 40 m north-west ofthe angle in the tide-line a syncline is well exposed in a small upstanding rock, from which it can be followed seaward across the shore, clearly indicated by the gradual convergence of the beds. This difference in strike across the fold results from an axial plunge of 10° to WSW, seen on the small stack, where the axial plane is seen to dip very steeply to NNW. Greywacke beds approaching 6 m in thickness occur on Brander and are seen again to north-west, beyond a run of thinner greywackes and siltstones in the core of the syncline. It has not been established, however, that they are the same strata. Low-angle accommodation faults, variously oriented, are seen at several points among the near-vertical beds on the north-western limb of the syncline.
5. Dowlaw Burnfoot: Sedimentary and Structural Features
6. Souter: Stack, Glacial DepositsClimb up the screes and bracken-covered slopes, 50 m high, behind the beach, and continue northward along the cliff-top to Souter Brae.
7. Big Byrips: Multiple Anticline
The second erosion level forms a small patch of brown clay at the cliff-top 250 m west of Souter Brae. The next major fold can be seen only on the foreshore north-east of here. It is a multiple anticline, made up of at least nine closely spaced subsidiary folds, separated by 220 m of very steep strata from the syncline at Locality 6, and with a much less steep limb to north-west, as in the folds seen earlier. (Descent to the shore here is recommended only to the enthusiast with time available.
8. Hawks Heugh: Syncline
From the cliff-top some 150 m farther west a broad syncline can be clearly seen on the long east-facing cliff, Hawks Heugh, which runs southward from Fast Castle. Although apparently a simple symmetric fold with both limbs dipping at 60°, closer inspection shows it to comprise at least three elements, and in addition it is displaced by a low-angle fault clearly visible on the cliff-face. The cross-strike width of the south-eastern limb of the syncline is just over 300 m, and of the north-western 200 m.
9. Fast Castle: Complex Anticline. Nothing was found. The castle is also believed by some to have been the setting of Scott's novel The Bride of Lammermoor. At the site the geologist can contemplate the dip-slopes of greywacke which drop steeply towards the sea to south-east, but the principal interest here lies in the complex antiformal structure which traverses the shore on the north-western side. This feature may be studied from the cliff-top to south-west but the spectacular changes in fold profile can be appreciated only by descending the 100 m to the shore. The least difficult approach is probably by a scree-slope 250 m south-west of the castle. The ‘antiform’ is seen to consist of a complex hinge involving at least five subsidiary folds, but towards the south-west the central of the three anticlines degenerates into a minor irregularity in the axial region of a syncline.
10. The Little Rooks: Multiple Fold, Fault Breccia
From the top of the screes a footpath ascends gradually across the heather to south-west and then southwards, through a gateway, to Dowlaw, but if time allows the cliff-top may be followed westward for 700 m, as far as the burn which descends north-westwards from the farm. This part of the coast runs sub-parallel to the strike of the greywackes, which dip north-westward at 60°. Some 800 m west of Fast Castle, and 300 m across the strike of the beds from the anticline there, a syncline is exposed at the cliff-top and may correlate with the hinge better exposed near L.W.M. 150 m east of The Little Rooks. The multiple nature of this structure is seen in the burn within 50 m or so upstream from the cliff-top. This last example of folding is typical of the style seen across the extensive inland outcrop. Bands of fault-breccia, not seen elsewhere along the coast, follow the shoreline from The Little Rooks to Green Stane, and are taken to mark a significant fracture, parallel to the strike, separating the rocks of these seaward stacks from those of the cliff sand the area to south-east. No change of lithology is apparent. Return to Dowlaw by following the burn upstream.
- Geikie, A. 1864. The Geology of Eastern Berwickshire. Mem. Geol. Surv. U.K.
- Greig, D.C. 1988. Geology of the Eyemouth district. Mem. Geol. Surv. U.K.
- Strachan, I. 1982. The distribution of graptolites in south-east Scotland, with notes on the fauna of Old Cambus Quarry. Scottish Journal of Geology, Vol.18, 157-165.
- Douglas, F. 1971. Gold at Wolf's Crag? Oliver and Boyd. Edinburgh.
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