The Natural History of Clay

by Alfred B. Searle

Re-Deposited Clays.

Although many clays and other materials have been transported and accumulated in the manner described, the majority of those now available have been subjected to repeated transportation and deposition, owing to the frequent and enormous changes in the relative levels of land and water during the various geological epochs. So far as can be ascertained, it is during these changes of position and the recurrent exposure to air and to water containing various substances in solution, together with the almost incessant grinding which took place during the transportation and deposition, that most secondary clays became plastic. If this is the case, it explains the impossibility of increasing the plasticity of clay by artificial means, at any rate on a large scale.

The simplest of the agents of re-deposition are rain-storms and floods which, forming suddenly, may cause the water of a stream or river to flow with unwonted velocity and so carry away previously formed deposits of various kinds. Clays transported in this way are termed by Ries ([6]) colluvial clays, the term 'diluvial' is generally employed in this country. If these are derived from a primary clay which has not travelled far since it left the original granite from which it was formed, they will usually be white-burning and of only slight plasticity, but if the flood affects materials which have already been re-deposited several times, the colluvial clays may be of almost any imaginable composition. Floods of a different character—due to the subsidence of the land so that it is partially covered with lake- or sea-water, which beats on its shores and erodes it in the manner already described—are also important factors in the transportation of clays.

So far as clays are concerned, the action of the sea is both erosive and depository, though the sedimentation in it being that of the pelagic ooze at great depths the clayey material is quite inaccessible. Under certain conditions, however, the sea may erode land in one area and may return the transported material to the land in another area. The diluvial clay-silt known as warp in the valley of the Humber is of this character.

Quite apart from the action of water, however, much denudation, transportation and re-deposition of clays and associated materials has been due to the action of ice in the form of glaciers, though these do not appear to have had much effect in increasing the plasticity of the clays concerned.

Glacially deposited clays are characterised by their heterogeneous composition, some of them containing far more sand than true clay, whilst yet retaining a sufficient amount of plasticity to enable them to be used for rendering embankments impervious and for the manufacture of common bricks, and, occasionally, of coarse pottery; others contain so much sand as to be useless for these purposes. Most glacial deposits contain a considerable proportion of stones and gravel which must be removed before the clay can be used.

The large proportion of adventitious matter is due in great part to the much greater erosive force and carrying power of ice as compared with water, resulting in much larger pieces of material being carried, and as the whole of the ice-borne material is deposited almost simultaneously when the glacier melts, only a very small amount of separation of the material into different grades takes place. The comparative freedom from coarse sand of some glacial clays shows that some sorting does occur, but it is very limited in extent as compared with that wrought in materials which have been exclusively transported by water.

For the manufacture of bricks, tiles and coarse pottery in Yorkshire, Lancashire and some of the more northern counties of Great Britain, glacially deposited clays are of great importance in spite of their irregular composition. They are frequently termed 'boulder clays' or 'drift clays' ([p. 65]), but in using these or any other terms for clays transported by glacial action it is important that they should not be understood to refer to the whole of the deposited matter. Large 'pockets' of coarse sand and gravel frequently occur in deposits of this character and veins of the same materials are by no means uncommon. The custom of some geologists of referring to the whole of a glacial deposit as 'boulder clay' has, in a number of cases, led to serious financial loss to clayworkers who have erroneously assumed that, because some 'boulder clays' are used for brick and tile manufacture, all deposits bearing a similar title would be equally suitable. This difficulty would largely be avoided if, as is now increasingly the case, the term 'drift' or 'glacial deposit' were used for the deposits as a whole, the term 'boulder clay' being restricted to the plastic portions and not including pockets of sand, gravel and other non-plastic materials.

Boulder clays—using this term in the limited sense just mentioned—consist of variable quantities of sand and clay, stones and gravel being generally associated with them. The stones may usually be removed by careful picking, and the gravel by means of a 'clay cleaner' which forces the plastic material through apertures too small to allow the gravel to pass. The plastic material so separated is far from being a pure clay and may contain almost half its weight of sand, the greater part of which is readily separated by washing the material.

Boulder clays, when freed from stones and gravel, are sufficiently plastic to meet the needs of most users, without being so highly plastic and contractile as to necessitate admixture with sand or similar material.