Grid reference SJ 1950 / 6570
Altitude 470 feet
Length 7000 feet
Vertical range 320 feet
Hesp Alyn was discovered in June 1973 by the North Wales Caving Club. This system is a classic example of a sub water table or phreatic cave, and marks the original resurgence of water sinking from the river Alyn near Loggerheads. The cave has been drained of water by lead mining operations in recent historical times and therefore offers a unique opportunity for the study of phreatic cave features which are completely unmodifed by vadose streams.
The passages near the entrance are more or less circular in cross section, meander very sharply and have developed along bedding planes in the limestone. Further into the cave, beyond a large domed chamber the character changes and passaged are mostly narrow, and locally very high, and are formed along vertical joints. Towards the present limit of exploration, the cave becomes very sporting and descends rapidly in a series of vertical pitches, one of which is 80 feet deep. These were developed by upward flow of water under hydrostatic pressure from further within the cave.
Having been full of water so recently, the cave is practically devoid of calcite formations, but the remarkable rock solution features more than compensate for this deficiency. In particular there are numerous fine solution domes in the passage that runs north south from the entrance, which supports the local tradition that a series of north south faults control the movement of underground water.
A 25 foot entrance pitch leads through a shaky bedding plane over thick wet clay. Within 100 feet, an easy duck appears. The main passage then becomes walking height. There is a large side passage on the left followed by a sloping tunnel on the right. Follow the passage to the right, down to a low curtain over a pool. Just past this pool, fork left. About 1500 feet from the entrance there is Sandy crawl, 30 feet long, which is frequently blocked by flooding. Beyond is a series of quite high rifts up to 55 feet high but generally 3 feet wide. A canal 50 feet long and at a comparatively high level follows, which can be traversed with extreme care. The ladder pitches start next, the first being the 80 foot pitch. Then comes a 40 foot descent, and another 40 foot pitch. This leads to a T junction, and left is the way on. This leads to sump 1.
I have got no further than this, and so further details can be obtained from the Concise Caves of North Wales by Tony Oldham.
DON'T underestimate the difficulty of this system, carrying enough equipment to reach sump one requires at least 3 people, and takes at least 6 hours for the round trip.
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