The Cave

Cave Morphology

Posted by steven on 14/02/2005 at 03:44 PM

The Uamh an Ard Achadh system is quite different, in many ways, from other caves in the area.  The cave displays at least two separate stages of development and has the appearance of being of greater age than most of the systems found on Beinn an Dubhaich.  The long, and generally ‘roomy’ stream passage, is much less steeply graded in longitudinal section than most local caves, and is more reminiscent of a typical Yorkshire cave (Ryder, 1974:121-23). 

The sizeable stream that feeds the cave is in an obvious valley, running in a north-easterly direction, and sinks besides a 3 metre deep hole filled with old domestic rubbish.  The actual entrance to the cave, excavated through a boulder-choked shaft, is situated some 25 metres to the northeast.  A descent through the boulders leads into the streamway, which soon enlarges to a stooping size passage showing an attractive cross-section and some pleasant formations.  Forty metres down the streamway, a boulder slope on the right leads up into the dry passage containing the archaeological deposits, while continuing downstream leads to a junction.  Left leads into a dry high-level oxbow ending in a boulder choke, while straight on leads to a circular chamber containing a 0.6 metre deep pool.  An easy duck under the left wall (0.2m airspace) followed by a short crawl, continues to larger passage.

A second high-level oxbow passage splits off from the main streamway here to the left, while the continuation downstream progresses to a 3 metre high waterfall over an igneous dyke.  The stream passage now continues as a high rift, which in places divides into an upper and lower level, and eventually ends in the terminal chamber. This lies directly beneath loosely packed boulders at the head of a shallow valley. This at some time in the past was the main resurgence for the cave and probably still acts as such in extreme flood conditions. Between Terminal Chamber and the present day resurgence the passage is completely flooded, is much smaller in cross-section, and has not been penetrated by cavers for more than a few metres.

Figure 4.  High Pasture Cave: Moldywarps Speleo Group BCRA Grade 5c survey.  (Courtesy of P. Ryder & GSG). The red boxes indicate where bone deposits have been recovered during the Project

Throughout the cave igneous intrusions in the form of dykes have guided the formation and development of the cave, and three tentative phases of passage development have been suggested by Ryder within an article submitted to the BCRA Transactions (Ryder, 1974:123 and see Fig.4).  The following sections are taken from this report: 

Possibly Pre-Glacial.  A streamway existed, now represented by the dry side passages and high-level oxbows in the section of the cave above the 3 metre waterfall, fed by the surface stream sinking at a point lower down the present dry valley than the now active sink.  The furthest upstream point of this old system would seem to be the dry side passage containing the archaeological deposits, which still retains a considerable amount of fill that may conceivably be of Glacial origin.  Further downstream, the high level passages and oxbows are relicts of this older streamway, gradually descending to merge with the present active stream passage just before the terminal chamber.

Possibly Pre-Glacial, or Inter-Glacial.  The present entrance series developed, the sink either being in the present position, or perhaps where the entrance is situated now.  Either during this stage or the next, the lower levels of the Lower Streamway developed below the 3 metre waterfall, possibly during a period when the stream was reactivated, possibly by melting ice following a period of quiescence.

Possibly Post-Glacial.  There is little direct evidence for separating the postulated stages of (ii) and (iii) beyond the fact that the entrance series does appear to show more development than the streamway between the second and third dry passages above the waterfall.  Therefore, it seems that the stream abandoned the section of its former route now represented by these high level passages, developing a new route in the lower section containing the duck with limited airspace.  The older stream course was possibly blocked by a major collapse, represented by the calcited chokes in the oxbow passages. 

Therefore, the passage containing the archaeological deposits may have served as the original inlet for the cave, some form of sink or entrance being situated away to the south.  Unfortunately, no evidence for such an entrance can now be seen on the surface within the dry valley, this possibly having been removed by glacial or post-glacial processes or later agricultural developments within the valley.  The latter seems likely, although additional acceptable scenarios would be the infilling of this original cave entrance or sink by glacial deposits, or failure of the steeply inclined northwest face of Beinn an Dubhaich (Lawson, pers comm.).

The preliminary report for the geomorphology survey carried out during 2004 by Tim Lawson and Ivan Young (Lawson & Young, 2005) can be found in Appendix 1 at the end of this section. 

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