Field Trip to the Lesmahagow Inlier Geological Society of Glasgow and Edinburgh 21^st September 2002

Programme · Arrive at Lesmahagow (10:30am) and drive towards Logan Farm (30mins). · Park at Logan Farm and walk towards Birk Knowes (30mins) · Continue up stream to tholeitic dyke · Cross to Kip Burn following line of dyke (30mins) · Look at disruption of sediments by intrusive force of dyke (30mins) · Have lunch at about 13:00hrs · Look at sediments and ?fossils as we follow stream towards farm (2hrs) · Return to Farm (30mins).

Introduction Many important collections of Silurian arthropods and vertebrates have been made near Lesmahagow since the mid to late 1800’s. The Geological Society of Glasgow set up a camp in the 1890’s, which was aptly named “Camp Siluria”, from which members of the society collected a number of rare and complete specimens of fossil fish and eurypterids. It is now very difficult to obtain permission to collect from these rocks as a result of inappropriate collecting. It is still possible to find fossils from these rocks, but all fossils are rare and some may be usefully donated to a museum for research. The Lesmahagow Inlier is a block of Silurian sediments surrounded by sediments of Carboniferous age. The inlier consists of shales and sandstones with occasional pebble conglomerates of a lagoon or lake. The lower parts of the succession contain occasional marine fossils, including trilobites and brachiopods, but the higher parts of the succession lack any evidence of marine incursions becoming influenced more by river and deltaic conditions. The sequence seen here is part of a general regression that can be traced from western Ireland to Scandinavia. The earlier (Cambro-Ordovician) terrane accretion of the Midland Valley Terrane to the Laurentian continent by sinistral strike-slip controlled basin development, sedimentary facies and deformation from Llandovery through until the early Devonian times. This field trip will concentrate on the fossil-bearing sediments of the Logan Water and Kip Burn, although a 6m wide tholeitic dyke will also be looked at.

Logan Water 500m 2-3hours walk along stream, fairly flat but can be muddy Kip Burn 1km 2-3hours walk over gently sloping hill through scrub and marsh Sketch map of the geology of the Lesmahagow Inlier close to Logan Farm

Locality 1 (NS 737346) Birk Knowes SSSI is also known as the Jamoytius Horizon named after the rare soft-bodied fish found from this locality. Sadly, due to overzealous collecting, this site has been made off-limits to all collecting for the time being. From the debris, we might be able to find a number of the rare fossil animals found at this locality such as Loganellia scotica (thelodont fish), Jamoytius kerwoodi (agnathan fish), Ainiktozoon loganense (concavicarid arthropod), Ceratiocaris papilio (pod-shrimp), Pterygotus bilobus and Slimonia acuminata (eurypterids) as well as a number of other rare animals. The silts and shales represented here are all from the Patrick Burn Formation and probably represent a shallow brackish-water lagoon with occasional marine incursions and high sediment input.
Loganellia scotica	Jamoytius kerwoodi
Ainiktozoon loganense	Pterygotus bilobus
Slimonia acuminata	Ceratiocaris papilio

Locality 2 The tholeitic dyke is exposed on the hillside 200m south of the Birk Knowes locality on the Logan Water. The dyke exhibits a characteristically sharp edged with spheroidal weathering surface. The crystalline texture can be clearly seen on a fresh sample with aligned feldspar laths. The dyke is about 15m across and can be traced with ease between the Logan Water and the Kip Burn. It is part of a Tertiary swarm emanating from Mull and is one of only a few that extend well into the Midland Valley of Scotland.	Sketch map showing extent of dyke exposed in the Lesmahagow Inlier
Locality 3 The dyke is exposed in the Kip Burn about 700m northwest of locality 2. The sediments here have been disrupted by the intrusion of the dyke which has caused them to fold upwards close to the dyke. The sediments exposed here are of siltstones and shales of the Blaeberry Formation. The sediments here consist of pale olive sandstones interbedded with pale olive mudstones. The mudstones are extensively bioturbated and can contain small gastropods (Platyschisma helicites) and ostracodes (Beyrichia sp.) as well as a few bivalves. The pod-shrimp Ceratiocaris papilio and eurypterid fragments are rarely seen.	The succession of sediments in the Lesmahagow inlier south of Logan House correlates broadly with the Pentland Hills although less marine in nature.
Locality 4 Following the Kip Burn downstream, the sediments turn a greyish green and blue colour with dark carbonaceous laminations in places. This formation has yielded a large eurypterid fauna below the waterfalls and the pod-shrimp Ceratiocaris papilio is relatively common here. These sediments were probably deposited in quiet conditions over a long period of time.	Locality 5 The Castle Formation is a sequence of thick sandstones and greywackes which is largely unfossiliferous and mostly devoid of sedimentary structures although some ripple surfaces have been recorded. These sediments may reflect a more rapid period of deposition at the top of the Patrick Burn Formation forming a boundary between the more marine nature of the Patrick Burn Formation and the non-marine nature of the succeeding deposits. There is a fish bed recorded from immediately above the Castle Formation that can be seen in a number of exposures in the Lesmahagow Inlier.