What’s on : Walks-and-field-studies

In the Footsteps of John Phillips: Askrigg Field Day

19 May 2022
Start time
12:00 AM
In the Footsteps of John Phillips:  Askrigg Field Day

Event Information

In The Footsteps of John Phillips Askrigg,
19th May 2022

Bill Fraser of Leeds Geological Association will lead a small group on an energetic tour of sites identified as Yoredale Cyclotherms by John Phillips at Mill Gill, Askrigg. Paths are steep and uneven. Rock surfaces close to the stream can be slippery John Phillips (1800-1874) was the first Keeper at the Yorkshire Museum and was Secretary to the YPS and the British Association for the Advancement of Science. Yoredale was the original name for Wensleydale carrying the river Ure. Phillips named the distinctive set of Carboniferous rocks which underlies the series of terraces the Yoredale Group. These sit on the Askrigg Block and demonstrate up to eleven cycles of sedimentation called cyclotherms.

Transport will be by own vehicles and the party will be a maximum of twelve. Good boots and protection from the weather is advised.

If interested, please email Paul Thornley on ypsgeology@gmail.com

Image Credit: Chris Cox / Mill Gill Force / CC BY-SA 2.0

Member’s report

The small group met early at Askrigg opposite ‘Skeldale House’ used in the BBC filming of James Herriot. Bill outlined the story of John Phillips touring Yorkshire with his Uncle William Smith and then becoming lecturer to YPS and keeper of the Yorkshire Museum. Phillips’s journeys round the Yorkshire Dales, many on foot, were numerous and lasted many months. The result was the publication of ‘The Mountain Limestone’, part two of ‘Illustrations of Yorkshire Geology’, in 1839.

In 1825 Phillips visited Mill Gill and chose Upper Wensleydale as the type area for the group of rocks he named the Yoredale Series. The cyclical nature of strata deposits form terraces on the high ground around. Each cycle is known as a cyclotherm. Not all cyclyothems are complete, but generally shallow seas formed limestone, sediment washing in from high land created shales. As the deltas advanced, silts and fine sand were deposited on slopes, then courser sands on the delta front became sandstones. As the land built above sea level, vegetation grew tall which was flooded rapidly to lay down coal seams and the shallow seas returned. Bill explained the Block and Basin framework of Northern sediment formation. That Mill Gill lies on the Askrigg block determines both the nature of the deposits and their relative lack of depth and periods of erosion.

There are 11 of these cyclotherms in the Yoredale or Wensleydale Group, each named after the limestone at their base which can be traced across a wide area. On the high ground, the hard limestones and sandstones form the steps and the softer mudstones the gentle slopes.

Not only did Phillips catalogue and describe the rocks but he also understood how to use them for correlation purposes and to interpret the nature of the environmental changes that must have occurred to form them, ‘variable inundations of the land and inconstant movements near the shores of the sea’.

Bill led us up the Mill Gill through gorges, waterfalls and across boulder fields and limestone pavements on an energetic trip to see three of the cycles, some with minor cycles within them: Gayle, Hardraw Scar and Simonstone. Three waterfalls offered opportunities to examine the variety of fossils in the limestones and marine mudstone; in the latter, ironstone concretions, root marks, club moss imprints showing that emergence and swamp conditions prevailed. Tufa was seen on rock faces below falls in lime rich waters.

At Whitfield Gill Force, it was possible to see the contact of the thick sandstone blocks with the 4-5 m of thickly bedded grey Simonstone Limestone with hour-glass shaped solution hollows all along its base from the line of springs formed at the line of impervious sandstone.

The day was fine and clear. We had the area to ourselves and on the return journey had good views across Wensleydale to Addleborough with its steps of 7 cyclotherms and Millstone Grit cap. The floor of Wensleydale is covered in glacial features, with drumlins and lateral moraines.

The group thanked Bill warmly for his expert guidance, preparation for the trip and enthusiasm.

Paul Thornley

Photo of Bill Fraser at Whitfield Gill Force by Paul Thornley