Author: cumberlandgeolsoc

Sunday 13th August

Starts: 10:30

Location: A5086 road at Parkside
Description:

An Examination of the Ravenstonedale Group.

16 members assembled at Wath on A685 for a briefing by leader Noel Pearson who has done considerable research into the Lower Carboniferous of this area. Three separate localities had been identified to represent the stratigraphy, involving various levels of difficulty, but thankfully in dry and bright weather conditions.

1  A short visit to an exposure of bedrock in the track just above Flakebridge Farm allowed examination of an unconformity between steeply dipping Silurian mudstones and overlying low-angle Carboniferous sediments. This is a rare example of being able to stand with the gap between your feet representing a time gap of at least 50 million years.

2  A transfer by car took us to Pinksey Gill. After a brief explanation of the surrounding landscape from an elevated viewpoint, the group followed the stream bed upwards through the shallow water marine beds of the Pinksey Gill Formation and into the Marsett Formation. A change from the varied clastic and carbonate beds was noted with the outcrop of a conglomerate band which has been equated with that of the well-known Shap Wells unconformity, and marking a temporary regression.

3  After a stream-side lunch stop the party moved on to Ravenstonedale village and embarked on a section of the youngest sub-division of the Ravenstonedale Group, the Stone Gill Formation. This involved following the bedrock downstream, eventually into Stone Gill itself, and searching on the way for a variety of fossils, including bands of algal limestone, brachiopods, corals, gastropods and traces of burrows. The disappearance of the stream into underground drainage channels, extensive sections of dry stream bed and the reappearance of flowing water further down the gill was noted, and Noel outlined attempts to trace the sub-surface drainage using introduction of dyes (which were not then recorded at the expected point, clearly indicating a complex system below).

 

After a team effort negotiating stream crossings and the challenge of a watergate, Noel was thanked for the considerable preparation he had taken and the detailed explanation of the earliest Carboniferous beds in this part of Cumbria.

 

JR

Railway Footpaths and Old Mining Areas in West Cumbria

This event is now taking place on Sunday 13th August, one week earlier than originally advertised.

Starts: 10:30

Location: A5086 road at Parkside
Description:

Fold Analysis Gawthwaite Moor Saturday 10^th June

Sixteen Members gathered at the Greyhound Inn, Grizebeck where our leader, Clive Boulter described procedures for measuring dip and strike.   In appalling weather the party then set off for Gawthwaite Moor and after traversing some very boggy ground successfully took dip and strike and location measurements in three different areas.   The wind and rain continued relentlessly so the party returned to the Greyhound Inn where Clive described how to plot the data onto stereographic projections, and carry out an interpretation, which every member succeeded in doing.   The original intention had been to take very many more measurements to make a statistically viable recording that could be published in the Proceedings.  The weather prevented this but everyone present had thoroughly enjoyed the day and a request was made that another day be arranged to complete the work.  SB

Carrock Fell Igneous Complex- Revisited 14 May 2017

Led by Alan Wise

On a bright clear morning 8 members set off from Mosedale to consider the Carrock Fell Complex by traversing the eastern side.

The leader had first mapped Carrock Fell as part of his thesis in the mid ‘70s. As well as distributing some written notes and diagrams, he set the scene by showing us his hand drawn map of the area and explaining that the Carrock Fell Igneous complex is made up of 12 different rock types between the Skiddaw slates to the South and the Eycott volcanics to the North, this includes 5 different bands of gabbro. He noted that there were xenoliths of Skiddaw slates and of Eycott volcanics in the gabbros.

The area was last surveyed by the BGS in 1928/32 and published in 1968, when it was interpreted as differentiation in situ. However, understanding and interpretation of gabbros had advanced over the last 80 years.

The party set off up Carrock Fell broadly along the contact of the Skiddaw slates and the gabbro and paused to examine a contact. It was noted that the slates were hard but no metamorphic aureole associated with Carrock Fell was observed. The leader suggested that the magma had been injected in a series of pulses as a composite sill, with cooling between each pulse, then turned so it was now almost vertical. He noted that there was evidence of chilled margins in the gabbros so that ruled out magma differentiation.

He demonstrated that the magnetite and ilmenite in the gabbros meant that compass readings were unreliable, a real problem in the 1970’s.

At the next exposed junctions of the slates and gabbros we observed some folding in the slates and looked in vain for definitive evidence that the Skiddaw slates had been overturned.

Noticing the change of slope we moved up and away from the slate and onto the gabbro. The Marginal gabbro was initially fine grained, then as we moved north, courser grained with bigger crystals and some pods of crystals.

Then we came to a fine grained andesite outcrop of some 100 meters and examined the contact at the north end with the gabbro. No evidence was known of mixing between the andesite and the gabbros. The wide andesite exposure was interpreted as the roof of the Eycott lavas which had collapsed into the gabbros.

Continuing the traverse of the slope in a northerly direction we examined an outcrop of Leucogabbro which had a fabric and crystal sedimentation then an outcrop of the Buck Kirk gabbro which had no fabric and was darker and finer grained, so was postulated as being intruded at a lower temperature as a crystal mush. An andesite dyke some 4 meters wide had been intruded later. Further exposures of light and dark bands and pegmatites were considered before crossing Further Gill Syke.

North of Further Gill Syke we moved from the gabbro through 6 types of granophyre. These were interpreted as later pulses from the same magma source as the gabbros, which had become more granitic.

We then descended to the road, avoiding the steeper parts of Further Gill Syke. As we walked to the cars the leader pointed out white cumulates with a black base dipping 60 degrees to the North in the cliff.

The leader summarised his interpretation as Marginal gabbro was intruded between the Skiddaw Slates and the Eycott volcanics. The junction was dipping at about 35 degrees to the north. Four further pulses of gabbro were intruded along contraction joints. Three of these magmas were at a higher temperature so allowed for crystal sedimentation under gravity forming layers of cumulate with a preferred orientation. The Buck Kirk gabbro was intruded as a crustal mush at a lower temperature. Granophyre and Diabase were intruded along the northern margin of the complex. Skiddaw granite was intruded to the South of the Complex and rotates the complex to the North by some 60 degrees.

Although the ground was rough and steep the party benefitted that the day was dry and bright, the bracken had not yet shot up, the leader had brought his brushes to clean the various exposures and provided fresh specimens to examine.

When the party reached the car park they thanked Alan Wise for sharing his research and knowledge and for a very interesting and challenging day and encouraged him to write up his interpretation in the proceedings.