Event Reminder: 9th June, Slump Folding in Kirkstile Formation, Wythop Woods

WEDNESDAY 9 JUNE

SLUMP FOLDING IN KIRKSTILE FORMATION, WYTHOP WOODS

Leader Phil Davies

Meet at 6.00 pm at Beck Wythop Car Park, alongside Bassenthwaite Lake NY 214284. NB This is the one at the Keswick end of the dual carriageway on the A66, not the Woodend carpark at the Thornthwaite junction. It’s a ‘there and back’ route. The outbound leg is about a mile gently uphill. The tracks aren’t too rough – stout footwear is always a good idea, but not necessarily heavy boots for this one.

The purpose of the trip is to examine Ordovician Kirk Stile Formation (part of the Skiddaw Group) which is well exposed in about ten forestry track cuttings. Everything can be seen conveniently from track level.
The mudstones, siltstones and sandstones are strongly affected by slump folding, probably influenced by later tectonic folding as well. Some of the slumps are fairly dramatic ‘crumples’. At least one fold has a neat ‘v’ shape, somewhat out of character with the crumpled / curved slump folds. That doesn’t mean it isn’t also a slump feature! Some very tight folding is also present, but not so easy to spot.

Despite the rather chaotic first impression, a general structural pattern can be revealed. Most bedding dips are towards the SE or NW – the NE to SW strike is reflected in the trends of the axes of small folds that are visible in the cuttings. Fold axes generally plunge at shallow angles. The limb of one fold can be examined, and the axis can be seen to be curved.

There must be faults of varying magnitude that juxtapose rock units of different bedding orientation. Some faults can be seen in the cuttings.

There is at least one suspected intrusion – something to look out for and discuss. We should also consider the range of sedimentary strata and if there are any believable way-up indicators.

This is a pleasant place for a walk, and this trip could be regarded as an introduction for further explorations across to Lothwaite and Peil Wyke. The structural configuration around Peil Wyke forestry cuttings (to the north) is superficially similar to that in Wythop Woods. However there is an area in between where the bedding dip is predominantly to the south, which is a bit of a head-scratcher. BGS publications refer to a Wythop Syncline – perhaps this ‘big picture’ interpretation has been deduced from consideration of interfaces of the Kirk Stile Formation with underlying Loweswater Formation but it’s hard to reconcile this with evidence in the forestry cuttings. Discuss!