North Esk Inlier, Gutterford Burn - an excursion

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By G. Robertson. From: Lothian geology: an excursion guide. Edited by A.D. McAdam and E.N.K. Clarkson. 1996

O.S. 1:50000 Sheet 65 Falkirk and West Lothian

B.G.S. 1:50000 Sheet 32W Livingston

Introduction[edit]

Silurian rocks outcrop in three inliers in the Pentland Hills. The largest of these, the North Esk Inlier, forms the subject of this excursion. The others are the Bavelaw Castle and Loganlea-Craigenterrie inliers (p. 162). Silurian strata in the inliers are generally vertical though rarely gently inclined. They strike SW-NE, young NW and are overlain with angular unconformity by gently dipping Lower Old Red Sandstone greywacke conglomerates. Silurian fossils were first recorded from the Pentlands by McLaren in 1838. Since then local geologists have collected extensively from the inliers. the most prominent being Hardie and Henderson, whose large collections made last century are in the Royal Museum of Scotland. Originally the sediments were thought to be of Wenlock to Downtonian age (Peach & Horne, 1899). However, in an extensive reinvestigation of the fauna, Lamont (1947) determined ages ranging from Upper Llandovery, probably crellulata biozone, to Wenlock for the North Esk Group as exposed in the North Esk Inlier. These ages were incorporated in the third survey by the Geological Survey (Mykura & Smith in Mitchell & Mykura 1962) and are generally accepted by later workers.

The succession as given by Tipper (1976) and Robertson (in prep.) is as follows, with the localities to be visited numbered:

metres
North Esk Group
Henshaw Formation (?Wenlock) (16-17, 26-29)
Red conglomerates, cross-bedded micaceous sandstones, olive green shales with one or two bands of fish fragments 730
Wether Law Linn Formation (Llandovery) (11-15, 25, 30)
Well-laminated brown siltstones with shelly fauna 125
Poorly stratified brown sandy siltstones 45
Highly fossiliferous laminated and bioturbated yellow-brown silty mudstones 40
Cock Rig Formation (Llandovery) (10)
Cross stratified red-brown conglomerates and sandstones 110
Deerhope Formation (Llandovery) (6-9)
Fossiliferous blue-grey micaceous siltstones and mudstones 250
Reservoir Formation (Llandovery) (1-5, 18-24)
Interbedded greenish sandy siltstones and mudstones 1400

The sequence represents a rapid regression from an offshore submarine fan environment (Reservoir Formation, Deerhope Formation and Cock Rig Formation) through a shallow shelf environment in which an abundant and diverse fauna existed, though changing through time (Wether Law Linn Formation) to a terrestrial environment (Henshaw Formation). The most recent bibliography of works on the fossils of the Pentland Hills is given by Clarkson & Howells (1981).

For access to the eastern part of the inlier (River North Esk and Gutterford Burn excursions) parking can be found in Carlops and the 3 km walk taken either by the east bank of the River North Esk or by the farm track to Fairliehope and to the cottage at the reservoir. Private cars may be parked near the cottage. The western end of the inlier (Lyne Water excursion) can be reached from West Linton by following the road to Baddinsgill Reservoir where car parking is provided 1 km from the reservoir. The itinerary forms the basis for one day and two half-day excursions. They are:

Gutterford Burn[edit]

The object of this excursion is to study several famous localities in the Gutterford Burn within the Reservoir Formation. The starting point is also the cottage at the south-west end of the North Esk Reservoir.

North Esk Inlier - excursion map

18. Reservoir Formation-Lowest Beds[edit]

Good exposures of the Reservoir Formation, noted in Excursion A along the east bank (3) and north bank (4) of the North Esk Reservoir, continue up the Gutterford Burn which flows into the north-east corner of the reservoir. Purple and green finely laminated silty shales are interbedded with discrete beds 1 cm to 1 m thick, of siltstone and fine sandstone, sparsely fossiliferous. The stream runs roughly parallel to the strike allowing individual beds to be followed for long distances.

19. Reservoir Formation-Graptolite Beds[edit]

Purple and grey mudstones further upstream have yielded rare specimens of the smooth-shelled brachiopod Lissatrypa atheroidea and of several species of graptolites. The fauna is dominated by Monoclimacis vomerinus (s.l.) possible M. crenuiala (sensu Elles & Wood) with occasional Monograptus priodon and M. spiralis and up to three species of dendroid graptolites. Precise identification is difficult but the fauna indicates a pre-Wenlock age for the Reservoir Formation.

20. Reservoir Formation-Gutterford Burn Limestone[edit]

Some siltstone to fine sandstone beds higher in the formation have thin basal lags of crinoid ossicles. One such bed which has a rich fossil content making it a highly calcareous siltstone, is known as the Gutterford Burn Limestone. Its outcrop can only be found with difficulty high on the eastern bank of the valley. It is a 25 cm thick lag deposit containing abundant broken corals and bryozoans, disarticulated brachiopods and bivalves, together with gastropods, trilobites, ostracods and crinoid ossicles draped over internal sedimentary structures.

21. Reservoir Formation-Eurypterid Bed[edit]

The next large outcrop upstream is the site of the Eurypterid Bed. These arthropod bearing laminated siltstones are dominated by stylonurids (Waterston. 1979) and contain one of the most extensive eurypterid faunas in the world. As a result of extensive excavations by Laurie (1892. 1899) the bed is now buried beneath a large pile of debris, but occasional fragments of the siltstones, along with abundant Dictyocaris, are to be found in the scree.

22. Fairliehope-Gutterford Intrusion[edit]

In the next large outcrop the sediments are cut by the Fairliehope-Gutterford Intrusion. It is necessary to examine the outcrop closely to differentiate igneous rock from baked siltstone. The intrusion is folded by tight F1 folds three separate outcrops in the Gutterford Burn, formerly taken to be three separate intrusions. The intrusion is pre-Old Red Sandstone in age, since the body is affected by Silurian folding. From a position high on the north bank of the stream these tight F1 folds can be seen to be themselves folded by younger more open F2 kink folds of the kind exposed on the east bank of the reservoir (locality 3).

23 and 24. Reservoir Formation-Starfish Beds[edit]

Two starfish localities have been recorded high in the Gutterford Burn. The first (23) is on the north bank 100 m downstream from the wall across the burn and the second

(24) on the south bank 50 m upstream from the wall, is the classic starfish bed of Peach and Horne (1899). The common starfish Crepidosoma wenlocki is found in fine micaceous silty shales with relatively rare brachiopods Lissatrypa atheroidea. Clorinda, hyolithids, cephalopods and the echinoid Aptilechinus caledonensis.

To return, it is easiest to follow the path high on the cast bank of the burn back to the reservoir.

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