Fig. 15. Lacustrine clay at Skipsea. (By courtesy of T. Sheppard Esq.)
The greater purity of lacustrine clays, as compared with fluviatile ones, is attributed to the much larger area over which the deposit is spread,—enabling variations in the deposits to be much less noticeable than when a smaller area is covered—and to the very small velocity of the water in lakes, whereby all the coarser particles are deposited a considerable distance away from the clays and silts.
Ries ([6]) has pointed out that many (American) lake-clays are of glacial origin, having been laid in basins or hollows along the margin of the ice-sheet or in valleys which have been dammed by an accumulation of drift across them. Such clay beds are usually surface deposits of variable thickness and frequently impure. Like all lacustrine deposits they show (though in a more marked degree than in the older and larger lakes) alternate layers of sand and clay, though the former are usually too thin to be noticeable except for their action in enabling the deposited material to be easily split along the lines of bedding.
Estuarine deposits partake of the nature of both fluviatile and marine beds, according to their position relative to the river from which they originate. They are usually uncertain in character and are often irregular in composition owing to the variations in the flow of the water. The Estuarine clays of Great Britain—with the possible exception of the Jurassic deposits in Northamptonshire and Lincolnshire—are of minor importance, but in some countries they form a valuable source of clay.
Fig. 16. Clay at Nostel, showing Marine Band. (By courtesy of T. Sheppard Esq.)
Marine clays are, as their name implies, those deposited from sea water. They are frequently found at a considerable distance from the shores of the ocean in which they were laid down, and subsequent risings and fallings of the surface of the earth have so altered the areas occupied by sea water, that a large number of marine deposits now form dry land. Though usually of enormous size and of generally persistent character, marine clay deposits vary considerably in the composition of the material at different depths, as well as in different areas. This is only to be expected from the manner of their deposition, from the varied sources of the material and from the numerous river- and ocean-currents taking part in their formation. For this reason it is generally necessary to mix together portions of the deposit drawn from various depths in order to secure a greater uniformity than would be obtained if a larger area were to be worked to a smaller depth.