North Esk Inlier, Lyne Water - 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:

Lyne Water[edit]

The object of this excursion is to examine the Wether Law Linn Formation and to study the various alluvial facies in the terrestrial Henshaw Formation which are better developed than in the River North Esk. The starting point is the bridge at the north-east corner of the Baddinsgill Reservoir. The walking distance is approximately 4 km and should occupy at least 3 hours.

North Esk Inlier - excursion map


25. Baddinsgill Reservoir: Wether Law Formation-Upper Member[edit]

Strata of the Upper Member of the Wether Law Linn Formation crop out 150 m beyond the bridge, in the small side stream that flows into the reservoir. The yellow-brown muddy siltstones contain Pentlandella pentlandica and (?)Uospira simulans. On the south side of the stream a vertical fault throws the mudstones to the south, up against Lower Old Red Sandstone conglomerates. Contact with the Henshaw Formation near the bridge on the north bank of the reservoir is not here exposed.

26. Baddinsgill Reservoir: Henshaw Formation-Igneous Conglomerate[edit]

Along the north shore of the reservoir the Igneous Conglomerate is interbedded with thick beds of red medium-grained sandstones. The well-rounded hematite-stained pebbles of granite, porphyry, trachyte, sub-basic lava and fine-grained sediments form beds up to 50 cm thick. Moving along the shore of the reservoir the red sandstones become dominant. These contain isolated pebbles and thick pebble horizons, mainly composed of quartzite. Trough cross-bedding is poorly developed.

27. Lyne Water: Henshaw Formation-Lyne Water Fish Bed[edit]

Above the red sandstones grey-green muddy siltstones are developed, displaying dessication cracks. On the east bank of the Lyne Water, 25 m north of the sheepfold, is the Lyne Water Fish Bed. These yellow-brown and grey laminated siltstones contain rare small broken fish fragments, mainly Ateleaspis tessellata, with some Lasanius problematicus and Birkenia elegans, as well as the crinoid Pisocrinus campana and worm tubes, probably representing a minor marine incursion.

Overlying the fish-beds are red medium-grained sandstones with well-developed trough crossbeds and quartzite pebbles, well-exposed since the Lyne Water runs along strike.

28. Lyne Water: Henshaw Formation, Unconformity[edit]

Red trough crossbedded medium-grained sandstones form the highest Silurian sediments exposed in the inlier. Gently northward dipping Lower Old Red Sandstone conglomerate is exposed above the Silurian-Old Red Sandstone unconformity.

29. Lynslie Burn: Henshaw Formation-Lyne Water Fish Bed[edit]

Retrace one's steps and proceed up the Lynslie Burn. Red sandstones give way to interbedded red siltstones and shales. Low amplitude ripples indicate some palaeocurrent directions. Near the junction with the Lynslie Burn there is a further outcrop of the Lyne Water Fish-Bed, formerly regarded as a separate higher fish bed, the Lynslie Burn Fish Bed (Mykura & Smith, 1962).

30. Lynslie Burn: Wether Law Linn Formation-Lower Member[edit]

Nearer the head of the Lynslie Burn on the north bank is a good fossil-collecting locality in the Lower Member of the Wether Law Linn Formation (locality 11). To return it is easiest to descend the valley back to the reservoir.

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