Abstract

Pisoliths, rimstone pools, and laminated crystalline crusts are described from Dyfed, South Wales, which resemble cave pearls and stalagmitic deposits but are formed in streams on an open hillside. The crystalline deposits are entirely calcite and share a common petrographic character. Crystals may be divided into three groups: 1) acicular fibres, which form radial or spherular clusters defining a succession of botryoidal surfaces 2) prismatic crystals, with rhomboidal terminations which become rounded or flattened by solution at right angles to their axes and 3) prismatic crystals, with ragged boundaries formed by the lateral coalescence of fibrous or prismatic elements separated by low-angle tilt boundaries. The latter contain linear inclusions which are relicts of their fibrous precursors. Associated sediments include plant fragments (principally Polytrichum ) and gastropods ( Oxychilus ) crusted with radial crystalline fringes, but are of two main kinds: 1) poorly sorted aggregated and peloidal micrites which contain a variety of lithoclasts and 2) crystal cumulates formed by the mechanical breakdown and re-sedimentation of the crystals described above. The form of the crystals is related to the presence of foreign ions in solution. The external morphology of the deposits is controlled by the rate of flow and the depth of water films. Pisoliths do not require continuous agitation.

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