Note On The Geological Features Of Rift Pot

By Harold Brodrick.

Rift Pot differs from the other pot-holes of the Craven district, so far as my observations there extend, in that its formation throughout its greater part is not the result of water action.  It would be more accurately described as a fault-fissure than as a pot-hole.  Its chief portions consist of a fissure ranging from five to fifteen feet wide, with platforms of jammed stones at intervals of its depth.

At the moor level the fissure is sixty feet long and seven feet six inches wide in the middle; at the northern end it thins to a crack a few inches wide; while towards the south it widens to about twenty feet.  The slopes at each end of this fissure seem to be composed entirely of jammed stones and are considerably undercut, the south platform being so for a horizontal distance of at least fifteen feet, for all of which distance the lower edges of the jammed stones can be seen from below.

At twenty-five feet below the moor the fissure narrows to about three feet, and only widens to about twelve feet at a depth of sixty feet from the moor level, at which point the roof of the main chamber begins and runs more or less level to its east wall. With one slight exception, which will be referred to later, there are no signs of any water action in or above the main chamber; the walls are flat and contain none of the groovings (formed by running water) which are usually found in pot-holes.  The floor of the main chamber is composed of loose rocks, through which in one place there is a hole at least twenty-five feet deep and possibly considerably deeper; the sides of this were, however, in too unstable a condition to permit of full examination with safety.

At the northern end of the main chamber the walls come gradually together and finally form a crack only a few inches wide.  Towards this end, and partially filling the crack, is a bank of heavy grey clay twenty feet high which is formed of the insoluble portions of limestone, and which also contains a few quartz grains.

The only evidence of water action to be found in the main chamber is near the northern end.  The deep hole previously referred to, in the floor, passes through large boulders thinly coated with stalagmite, and as these boulders are directly beneath a small sink on the moor it is almost certain that the deposit on them has been formed by water filtering through from above, and in quantities too small to have had anything to do with the formation of the Pot.

Running water first makes its appearance at a fissure above the Pinnacle Ledge. The second shaft shews water-grooving of the usual kind, and is more or less circular in shape.  It is not known how high the top of it reaches; but as from the rock slope near the south end of the main chamber it can be seen still reaching upwards it is probable that its upper end is not far from the surface.  At some unknown, level in this shaft considerable percolation of water occurs.

At the northern end, near the moor level, the east wall of the fissure is slickensided, though the slickensides do not cover much of the rock, the greater portion probably having been removed by atmospheric influences; in the first chamber, however, the east wall is slickensided over an area fifty feet in length and at least twenty feet in height.  Near the surface the slickensides occur along successive master joints, while those in the chamber occur along another master joint horizontally distant about fifteen feet from those at the surface, thus proving that the faulting occurred along several parallel lines. These slickensides are horizontal, and as the beds of limestone on either side of the upper part of the Pot correspond in level it is probable that no vertical movement accompanied the faulting.

No slickensides could be found on any of the west walls of the Pot.  Near the surface the slickensides are coated with clear crystals of calcite which can be pulled off in slabs several inches in area, exposing the slickensides, which here, not having been acted upon by the atmosphere, are very clearly marked.  The crystals have obviously been formed by infiltration of water along the line of the open joint.

Where horizontal faulting is met with in such rocks as the Triassic sandstones small open joints or fissures are occasionally found between the two faulted sides, but I do not know of any fissure of the size of Rift Pot which so obviously owes its origin to a fault, although I think it is likely that the deeper pot-holes, such as Gaping Ghyll and Alum Pot, owe their origin to the presence of faults the existence of which has not, as yet, been proved.

The narrowest end of the fissure, down to a level of about twenty feet from the surface, is choked with angular fragments, of limestone ranging up to a foot or more in diameter, and these have been almost entirely coated with clear crystals of calcite which are now etched and polished as if water containing fine powder had run over them. In colour, they are a darker shade of grey than most of the limestone of the district, being probably formed from the so-called ‘marble’ of the district, several beds of which occur in various pot-holes near.  On examining a thin section of one of these fragments under the microscope it is seen to be made up of smaller fragments of limestone cemented together by veins of calcite, the limestone itself being exceedingly fossiliferous and containing polyzoa, brachiopods, encrinital stems, corals, and foraminifera in abundance.  In the limestone there also occur small pieces of quartz, angular and splintery, which have been formed as ‘fault-stuff’ between the walls of the fault, and have subsequently been cemented together and coated with calcite deposited by percolating water.

The deposit of clay in the first chamber, mentioned earlier, is of a. very dark blue-grey when wet, but is much lighter in shade when dry; it consists almost entirely of an exceedingly fine powder, and is almost certainly the residue after the solution of the limestone.  If this clay is washed for a long time a small residue is left of quartz grains similar to those found in the limestone and probably derived from it.  It has been suggested that this clay is glacial clay washed in from the moor, but of this the evidence is doubtful.

The rocky slope to the south of the main chamber is composed of large boulders, which are quite free from incrustation or stalagmite.  Below this point are a series of slopes and platforms entirely composed of jammed stones, the platforms in many places being quite thin, and containing holes, through which, if a stone drops, it eventually reaches the bottom of the pot.  These slopes and platforms lie one below another the vertical walls on either side varying from two to six feet apart, and following the form of the fissure at the surface in being wider towards the south.  The walls, both above and below ‘the Eye,’ are in a very unstable condition, being composed of loose blocks of limestone which come away at the least touch and do not exhibit any evidences of having been acted upon by water.