News

15 members spent an excellent day under the very able leadership of Noel Pearson to the two current Carboniferous Stage Stratotype sites in Cumbria. Both have recently been the subject of a great deal of new research by specialist palaeontologists, focussed not only on the macro fauna, but on advances in the field of micro fossils.
The leader gave members an excellent summary of the history of research on these two sites and an update on recent findings. Each stage has a basal boundary defined at a specific point in the marine rock section. Stages are chronostratigraphical (time
units). We are fortunate here in east Cumbria to have these two classic sites a short distance apart. The Asbian basal boundary at Little Asby Scar (NY 699083) and the Brigantian at Janny Wood, south of Kirkby Steven (NY 783038).

The morning was spent walking up the Potts Beck valley to the Little Asby Scar location. A short climb up the Scar brought us to the key exposure – a clean rocky scar section where the Stage junction is currently defined. Yellow markings on the cliff face and the obvious signs of coring either side of the junction mark the spot. The finer points of the palaeontology are not obvious in the field, but the whole site is remarkably fossil rich. Some magnificent specimens of corals and sponges kept members occupied for some time. Some unusual dark bands of material also stretched member’s minds. The geomorphological features of the wider region were discussed from the scarp top, which affords excellent views over the karst terrain, the spectacular meandering Potts Beck and the Howgill Fells to the south. The rather complex recent drainage and glacial history of the area was discussed.

A short drive eastwards for the afternoon session brought us to the River Eden Valley and the type section of the Brigantian in the steeply dipping beds exposed along the river bed. A detailed log of the beds was provided to view the section as the group walked upstream from a notable series of waterfalls in the river below the wood. En route we were introduced to a new term at Catagill Scar – a ‘stratomorph’ – an exposed bedding plane surface which now forms a 45º slab of rock on the west bank of the Eden. Rather heavy rain somewhat shortened deliberations on this site, but the day as a whole was an insight to members on how very detailed palaeontology is enabling these sections to be logged and interpreted.

Leader Noel Pearson at the Little Asby Scar site – walking pole to his left lying horizontally at the base of the Asbian.

Janny Wood site. Carboniferous beds dipping upstream (to right in the view). The Brigantian boundary is at the extreme right of the picture.

Field Trip to Askham Fell 1st June 2016

The party met on the open fell at 6.30pm on a fine sunny evening. John Rodgers, our leader, began by describing the features of the Lowther Valley to the south. This valley is bordered on the east by Carboniferous limestone escarpments and on the west by the rugged terrain of the Borrowdale Volcanics. Between the two the relatively easily eroded Skiddaw Group rocks occur and underlie the lower part of the valley.

We then walked northwards over the fell, noticing sink holes and one small exposure of the limestone. We crossed the narrow sandstone band of the Ravonstonedale Group, which is very poorly exposed but members located a few specimens of the well-sorted, gingery coloured sandstone. A pause was made to consider a line of enigmatic depressions which were probably man made rather than sink holes. The party continued to Heughscar Hill to enjoy the magnificent view along Ullswater. Again the underlying geology was reflected in the topography. Here the BVG group form the high rugged fells to southern and southeastern boundaries of the Lake. To the north west Great Mell Fell and Little Mell Fell and the smaller fells of Soulby Fell and Bowerbank are smooth, rounded hills of the Mell Fell Conglomerate. Closer to the viewpoint the low land is again a reflection of the erosion of the Skiddaw Group rocks which underlie this area.

From this panorama the group moved eastwards to a small exposure of limestone pavement and a tumbledown limestone wall where a range of fossils – colonial and solitary corals, and a variety of brachiopods – were identified.

This was the last stop after which the party returned to the cars and thanked John for a very enjoyable evening.

Excursion Report

A small group of members examined the Carrock Fell Complex spending most of their time in the southern mafic plutonic component considered to be associated with the Eycott Volcanic Group.  Discussion covered the range of published interpretations for this body of layered cumulate gabbros pooling our collective knowledge of magma-chamber processes and guided by Prof. Joe Cann.  The gabbroic body is bounded by sub-vertical faults and the internal igneous layering is steeply inclined.  Published explanations cover a spectrum from layered gabbro accumulation in a sub-horizontal sheet followed by high degrees of rotation to crystallisation on an inclined floor of a magma chamber with no subsequent rotation.  If the former proposal is correct, the challenge is to understand how and when the very significant rotation happened; could it have been in the murky events associated with the initiation of subduction at the leading edge of Avalonia or a volcano-tectonic fault, or a younger event?  Lighter and darker banding in the gabbro was first seen in the quarry along the road from Mosedale Bridge.  Within the mafic body we observed the layering mainly dipping at around 50 degrees though steeper dips are found on the margin.  The party climbed up the steep slopes of Carrock Fell, approximately along the steep contact of the gabbro with the metamorphosed and deformed Skiddaw Group.  Some large xenoliths of intensely folded hornfelsed Skiddaw rock were seen within the intrusion in amongst moderate-sized exposures of layered gabbro.

On the summit of Carrock Fell, the group encountered the second, and younger, component of the complex which is a gabbro to microgranite intrusive suite unrelated to the gabbro cumulates.  Here exposed granophyre weathers to a distinctive pinkish colour.  Boulders of gabbro on the summit occur as sparse erratics, carried upwards and northwards by ice.

All agreed that the Carrock Fell complex is indeed complex, and would merit further study.

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Non-members are welcome to join excursions and lectures on a try out basis free of charge. Further information can be found on the events and membership pages of the website.