 |
| The Bradgate Formation at Billa Barra Hill |
In the first week in March 2020, having already visited Edensor and Conisbrough, I quite unexpectedly finished it with a trip to Charnwood Forest with Paul May of the Sheffield U3A Geology Group, to undertake a recce for our next day out.
On the Friday evening, he had phoned me to ask if I wanted to accompany him on the recce for the field trip that had been scheduled for 19th March - to explore the Precambrian rocks around Billa Barra Hill and Markfield.
 |
| Billa Barra Nature Reserve |
Arriving at 11.15 at the car park for Billa Barra Hill Nature Reserve, our plan was to follow the guide produced by Dr. Annette McGrath - A geological walk around Cliffe Hill Quarry – and, after briefly stopping to examine the dry stone wall at the Charnwood Noon Column, we headed off to explore Billa Barra Hill.
 |
| The Charnwood Noon Column |
It comprises a small knoll of volcaniclastic siltstone of the Bradgate Formation, part of the Charnian Supergroup laid down during the Ediacaran Period, in a local environment dominated by explosive volcanic eruptions of gaseous and silica rich magma, with frequent pyroclastic flows. The island of Montserrat in the Caribbean is considered to be a good modern analogy of its geological setting.
 |
| Joints on a bedding plane |
Hidden amongst the trees on top of the hill, there are various small jagged outcrops of tilted rock with a well defined rhomboidal pattern of jointing and weak cleavage that generally pervades the body of the rock.
 |
| Weakly developed cleavage |
The outcrops are all very pale pink/grey in colour and, on weathered surfaces, extremely fine laminations can be seen, with graded bedding and differential weathering highlighting its sedimentary nature. No other larger sedimentary structures were observed here and, like other rocks in Charnwood Forest that are described elsewhere, these are thought to have been deposited as a distal turbidite fan.
 |
| A sample from Billa Barra Hill |
Breaking open a piece of similarly weathered rock that I picked up from the ground, the fresh surface is seen to be pale grey/green, with pinkish and darker bands, which intensify in colour when wet. The surface of the stone has weathered to a form a dark brown skin, which appears to be a concentration of iron oxides/hydroxides, and there is no reaction with hydrochloric acid.
 |
| A fresh surface |
An inspection with a hand lens reveals that the rock is dense and siliceous and without obvious grains seen on a fresh surface, except a scattering of tiny dark green/black grains of an undetermined mineral, and there is a general green/ grey or pink colouration to the banding.
 |
| Slickensides at Billa Barra Hill |
A quarry is cut into the south-west side of the hill at a lower level, but is now water filled and the rock faces are inaccessible. Although largely covered in moss, after an area had been roughly cleaned, the old quarry floor exposes well defined slickensides.
 |
| An examination of slickensides |
Bearing in mind the age and mobility of the group as a whole, we agreed that the slope down to the old quarry and back to the path was quite safe and manageable and, having finished this part of the walk, we went in search of New Cliffe Hill Quarry, where the South Charnwood diorite is exposed.
 |
| A notice |
No comments:
Post a Comment