GeologyRocks

About the author

Jon
University of Edinburgh

Introduction

The Pentlands are a series of hills just south of Edinburgh. They consist of volcanic and sedimentary rocks of Devonian and Silurian age. The sediments are both terrestrial and marine, with the oldest being the marine Silurian.  These Silurian rocks are the most interesting feature, forming three inliers or "windows". One of these inliers, the North Esk Inlier, is very interesting both geologically and palaeontologically. 

Note: This area is an SSSI - do not hammer the exposed bedrock. There are plenty of fossils in the loose scree.

Geological History

The North Esk inlier is the southernmost of the three inliers. There are four formations that make up the inlier, which young in the north-west direction (Figure).The formations are:

• The Deerhope formation
• The Cock Rig formation
• The Weather Linn Law formation
• The Henshaw formation

The Silurian rocks that make up the inlier were deposited in a marine/coastal setting and have since been tilted to near-vertical. At the time, sea-level was falling and, as a consequence, the formation prograded forward (Figure). The Deerhope formation  consists of shales, deposited on an offshore slope, and grades upwards into the Cock Rig formation. The Cock Rig formation was an offshore island barrier, rather like Spurn Head which is currently forming in the Mouth of the River Humber. The Cock Rig formation consists of cross-bedded sandstones and conglomerate horizons, often with herringbone cross stratification - a clear indication of a tidal environment. The upper part of the Cock Rig Formation consists of flat sand sheets, typical of beaches. This might be the coastline, but above this is the Weather Law Linn formation which was a quiet marine lagoon. Like the Deerhope formation this  formation is mainly shales, but there is a white clay layer - bentonite from a volcanic ash fall. This is the most fossiliferous of the formations in the Pentlands. Finally, the Henshaw formation is essentially the coastline and is a red sandstone, indicating semi-arid environment. Indeed, there are horizons of conglomerates that are indicative of flash floods. These beds are typical of the Devonian Era, but here they are found in the Mid-Silurian.

These formations form an ideal offshore barrier-lagoon system, which prograded forward in time as sea-levels fell.  The system retained its form for a prolonged period of time, though it is impossible to say how long a time. As these rocks are now near vertical it is possible to walk from the deep water shales, up through the barrier island, into the lagoon and finally onto the shoreline as you walk from SE to NW.

Figure 1 show the palaeoenvironment of the North Esk Inlier. Relative sea-level fell in the subsiding basin causing progradation of the whole sequence. The most fossiliferous formation is the Weather Law Linn, which is behind the barrier islands that make up the Cock Rig formation.

Figure 1: Cartoon sketch showing the relationships between the Silurian strata in the North Esk inlier in the palaeoenvironment.

Localities

There are a few good localities in this area (Figure 2). Two are in the Weather Law Linn formation, one in the Deerhope formation. There is limited exposure of the Cock Rig formation.The formations can be seen in a single day trip, starting at Carlops, on the A702 (Edinburgh to Carlisle road). Parking is available in a small car park at the south of Carlops on the left-hand sside of the road going south

Figure 2: Top map shows topography in the Pentland Hills, just south of Edinburgh (from Google Maps). The lower map shows Carlops, with the parking (and pub!) marked. Produced under Crown Copyright from from Ordnance Survey's Get-a-map service.Image reproduced with permission of Ordnance Survey.

Carlops (marked with "A" on upper map) is about 30 minutes drive south of Edinburgh on the A702 (follow signs to Carlise). Parking (free) is on the south side of Carlops in a disused quarry. It is possible to drive up the rough road just south and opposite the car park) to Fairliehope, past the treatment plant. This is also a pleasant walk.

The three localities are just to the North-East of the the North Esk Reservoir (Figure 3).

Figure 3: Localities on the North Esk inlier, with the approximate location of the four formations.

OS map of the area with fossil localities are marked by black circles. The colours match the palaeoenvironment diagram above (Grey: Deerhope; Yellow: Cock Rig; Green: Weather Law Linn; Brown: Henshaw). The southermost locality is in the Deerhope Formation. Unfortunately, we didn't have much time to explore this as we were moved on by a shooting party!

Locality 2 is just off the main path (take the left-hand path when you get to the gate), when you reached the disused quarry, keep going over the next burn and the locality is to the left. This locality is the Weather Law Linn Formation. The disused quarry just south of this contains an igneous intrusion and the Cock Rig Formation, which can be seen in the hillside opposite too.

Locality three is interesting and deserves a more thorough description. Again, this locality is in the Weather Law Linn Formation.

An interesting feature here is a thin band of bentonite - volcanic ash. This would have smothered the lagoon at the time of eruption. Although sometimes difficult to find, once found it gives a clue to where to find the better fossils. The bentonite is exposed as a white clay layer.

The layer passes from underneath my foot, to the top-right corner and the large lump of white is the clearest indication of its presence! The volcanic eruption produced a massive change in the fauna of the lagoon.

Example Finds

The above images are from the Weather Law Linn formation containing Eoplectodonta and Leptaena. This is from locality 2.

Trilobites are probably the most sought after finds. We found several parts, but no whole ones:

There are lot's of other types of fossils to be found, such as sponges, corals and crinoids. More examples will be posted to the images section as we identify them.

Note: This area is an SSSI - do not hammer the exposed bedrock. There are plenty of fossils in the loose scree.