The Geology of Casterton and Leck Fells

Easegill Caverns, Link Pot and Pippikin Pot are all found in nearly horizontal beds of Lower Carboniferous Limestone of the S2/D1 periods. The D1 limestone zone is where most of the Dales potholes are formed and is responsible for the majority of visible karstic features. These beds lie unconformably on Silurian grits. The area is affected by two major fault systems: on the west by the Dent Fault, and to the south by the Craven Faults. The existence of the intersection of two faults close to Leck Beck Head makes the hydrology rather complex and because of this very little progress has been made by divers in Leck Beck Head and Witch's Cave. However, the lack of dip in the limestone beds has meant that phreatic horizons exist at the same level throughout the Easegill-Pippikin system.

The Development of Link Pot

There is a horizon of phreatic development throughout the Easegill System at 825 feet O.D. as noted by Ashmead (1); it can be seen in the roof of Fall Pot and in numerous phreatic tubes around the Graveyard.

The phreatic development is equally visible in Link Pot, especially in the high levels of Serendipity and the upper levels of the 'Night Shift' series. Further down the valley the same phenomenon is visible in Pegleg Pot.

The seven metres diameter phreatic passage seen in Hylton Hall suggests that at the same time there was a large phreatic drainage system running beneath, and parallel to, the gill. This can be seen further up the gill at Howgill Sink and, presumably, exists as far up as Cow Dub. Downstream, this tube is blocked by loose blocks and gritstone boulders, but it reappears at the Hobbit in Pippikin where it turns under the fell via the Hall of the Ten, and terminates in a choke at Gour Hall. A similar sized phreatic tube can be seen in Hand Pump Hall but, as yet, it has no outlets.

All the major phreatic passages in Link show extensive corrosion by vadose entrenchment. This can be seen at the end of Hylton Hall which has a characteristic trench where the invading stream has cut down. Other examples are found in the Canyon and in Seredipity main streamway (Easy Street). The last mentioned also exhibits considerable block breakdown due to undercutting.

Presumably, the stream which descends Serendipity used to flow along the upper series and descend into the canyon by various routes such as Tiger's inlet. At the base of the Ramp is a phreatic tunnel associated with Cobble Crawl.

The dry Link passage which enters Pippikin at Dusty Junction is part of the same phreatic development as the rest of Link Pot. The inlet to Dusty Junction has been attributed by Waltham (2) to be the old passage from Pippikin Pot but it appears as though this has yet to be found.

Potential extensions of the system

The present day hydrology of Easegill Beck is complex, as the presence of the gill acts as a watershed between Casterton and Leck Fells. The water in the beck first sinks at Top Sink and, with increased flow, the stream sinks further down the valley at County Pot and, eventually, Cow Dub. All these sinks take the water to Lancaster Main Drain and thence to Leck Beck Head. Below Cow Dub the drainage becomes more complicated. The water percolates through the beck to Maracaibo and to Lancaster Main Drain; recent tests by the RRCPC have shown that the sink in Howgill Beck also drains to Lancaster Hole.

A further one hundred feet down the valley Howgill Sink is presumed to feed Serendipity inlet in Link Pot and drain towards the phreas below Pippikin Pot. Still further down the valley, this trend continues with Mistral, Crumbles, Pegleg and Kirk Pot all entering the phreas via passages under Leck Fell.

However, at Link Pot, the water from the two sinks in Hylton Hall drains to Leck Beck Head via the Earby Series.

An interesting problem is the extensive cross drainage beneath Leck Fell. The main inlets on the north-east of Pippikin are the Dickeries and Rat Bag Inlet. On casual inspection these would appear to come from sinks in Easegill or be from Link Pot. However, the source of these inlets is yet to be discovered.

All the active passages of Link pass about twenty to thirty metres below Pippikin. The water in the Ramp flows directly into the phreas below Pippikin, as does the water in Serendipity; these two streamways are interconnected via a partially choked phreatic tube, Cobble Crawl.

The other inlet to Pippikin is Cigalère, which sinks on Leck Fell and flows down to disappear beneath Hall of Ten in a mass of boulders. This stream has been pirated by the Serendipity streamway via Waterall Chamber. Presumably, this stream used to continue under Hall of Ten into an, as yet, undiscovered passage.

Further down Easegill valley are Pegleg Pot and Kirk Pot which both resurge into Leck Beck Head. It is assumed that, by comparing levels of sumps, these two pots are interconnected via a 'Deep Rift' and that this water is directly connected with Leck Beck Head and Witch's Cave. This is because Witch's cave begins to flow as soon as the flood pulse in Easegill hits Kirk Pot.

The phreatic passages associated with Leck Beck Head are very extensive, stretching far under Leck and Casterton Fells. In 1970, as a joint project with RRCPC, the NPC did a flood pulse test in Lancaster Main Drain. The pulse too only seventeen minutes to reappear at Leck Beck Head. A similar experiment was conducted in the main drain of Lost Johns' and this pulse took thirty minues to reach Leck Beck Head. These results show that there is little or no airspace in the passages beyond the sumps and so there is little hope of extending the system downstream.

The old phreatic level at 825' O.D. also appears in Lost Johns' and Gavel Pots. Waltham (2) suggests that the upper levels of Gavel Pot are an abandoned flooded phreas which used to rise at some long since eroded resurgence on Leck Fell. This theory fell into some disfavour when the sump in Gavel was found to connect with Lost Johns' sump and not to be perched 100 feet above it. The distance between Pippikin and Gavel is about 2500 feet with a vertical difference of 200 feet; but such a gentle gradient appears in the upper passages of most Leck Fell potholes, so the possibility of a connection is not that remote.

The discovery of Link Pot gives the theory more justification as such a passage woulkd have flowed directly into Link via the high-level series above Serendipity.

Similarly, the system of phreatic tunnels in Link must also extend to the other side of the beck. Some excavation of the choked crawls of Link Pot could produce another system comparable with the Earby '71' series.

References

1) Trans C.R.G. vol. 9 no. 2: "Lancaster Hole and the Easegill Caverns, Casterton Fell, Westmorland".

2) "The Limestone and Caves of Northwest England", ch. 4: edited by A.C.Waltham; David and Charles, 1974

Further Reading

"The Geology and Mineral Resources of Yorkshire"; the Yorkshire Geological Society

Red Rose C.P.C. Journals nos. 5, 6 and 7

I.Crossley