The boulder clays--considered apart from the stones, gravel and sandy materials occurring with them--are usually red-burning, stiff and very plastic, but the gravel, sand and crushed stones mixed with them in the formation of the material usually render them of medium plasticity. By careful washing, most boulder clays may be purified sufficiently to enable coarse brown pottery to be made from them. Clean deposits of sufficient size to be worked without any purification are occasionally found. Usually, however, the boulder clay formation is somewhat treacherous as it is difficult to ascertain its nature; boreholes are apt to be quite misleading as the formation is so irregular in character. Agriculturally, drift or boulder clays are poor soils, but by judicious management and careful mixing they may be made more fertile. Where it contains chalk--as in Norfolk and Suffolk--boulder drift forms an excellent arable soil. =Pleistocene or Recent clays= are amongst the most important brickmaking materials in the South of England. They are of remarkably varied character, having been derived from a number of other formations. Usually the deposits are somewhat shallow and irregular in form, but beds of considerable thickness occur in some localities. Agriculturally, they are of considerable importance. Most of the =brick earths= used in the south-east of England are of Recent formation, those of the Thames Valley being of special importance in this connection, particularly where they are associated with chalk; thus forming natural _marls_ or enabling artificial marls to be produced.