Clast-supported and matrix-supported raised beach deposits

Clast-supported storm beach at Abermawr, Pembs.  Here some of the cobbles are made of Irish Sea till!  There are no sediments in the spaces between the cobbles -- they are washed away to lower levels during every high tide. In the lower part of the storm beach ridge, beach sand fills all the spaces.

Matrix-supported raised beach cobbles in layers, Poppit, Teifi Estuary, West Wales

Matrix-supported raised beach at Porth Killier, St Agnes, Isles of Scilly.  The spaces between the cobbles are filled with a reddish colluvium

Cemented clast-supported raised beach from the Gower.   There is some fine-grained sediment between the clasts, but the clasts are very tightly packed together.

There is some discussion in the literature about the significance of clast-supported raised beach deposits and those that are matrix-supported.

We are talking, of course, about conglomerates, or unconsolidated conglomerates in which the clasts are for the most part well-rounded, and normally of a consistent size -- that size being dependent, to some degree, on the precise dynamics relating to wave energy and the type and availability of materials.

Clast-supported raised beaches are those in which the cobbles, boulders or pebbles are in contact with each other, with little no finer material in the intervening spaces.    In other words, the clasts "support" each other.

In contrast, in matrix-supported raised beaches the cobbles, boulders or pebbles may not actually be in contact, and the bulk of the material may be made up of a fine-grained matrix.  In other words, the matrix supports the larger particles.

There is a suggestion that clast-supported raised beaches are somehow more "authentic" and maybe younger than raised beaches that are matrix-supported;  and there are some -- in the debates about the Scilly Isles in particular -- who have argued that the matrix-supported deposits are very old indeed, and are in secondary positions, having been moved downslope during cold-climate or even periglacial conditions.

I do not think this is a sustainable argument -- and I think it would be a grave mistake to label one type of beach as old and redeposited, and the other as young and in situ.  To some degree all open-work storm beaches are kept clear of finer sediments as long as they are being washed and constantly rearranged by storm waves -- but as soon as they are isolated from marine coastal processes, modification kicks in.  If a beach on a platform is beneath a steep slope, it is inevitable that fine-grained sediments (generally labelled as "colluvium" will be washed down over and into the beach, gradually filling all available spaces.  In more open environments, alluvial or river processes will fulfil the same function.  And in yet other situations blown sand will be carried onto and into the beach, with sand grains filling the spaces.  Then we might have combinations of all these processes.

The main message is this:  if a raised beach is in a particular position, resting on a rock platform, it is in all probability in its "right" stratigraphic relationship with the deposits resting on it.  If you want to argue for something different, then you have to come up with some pretty smart additional evidence to support your case..........