4 Replies to “Irish Pebbles 2”

  1. I know what you are saying, Lucy.

    It is true that in a number of locations man-made construction materials end up on the beach, rolled by the surf to the shape of pebbles or beach stones. However, the pebbles in this picture come from an area where explosive volcanic activity during the Silurian resulted in thin layers of tiny fragments of lava accumulating as layers of ‘ash’. These layers were deposited in a shallow sea where it became interbedded with smooth siltstones. Some of the volcanic ash also fell on the land at the edge of the shallow sea where it became reworked with other loose material from sub-aerial deposits before being carried down to the sea by either rivers or flood waters (thus accounting for the admixture of small rounded particles with the sharp fragments within the matrix).

    Further north of the beach where these stones were found, Devonian conglomerates also outcrop – examples again of rocks composed of reworked wind- and water-borne fragments from different terrestrial and freshwater sources that have been transported to the marine environment and eventually consolidated.

    So I guess that any decision made about the origin of pebbles on the beach might depend on the geological and present day environmental context in which they are found, and would also need to factor in such things as tides, currents, and longshore drift that can transport pebbles from some distance away to wash up on the shore.


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