The action of the sea in the transport of rock-materials is more intense than that of rivers, the coasts being worn away by repeated blows from the waves and the pebbles and sand grains the latter contain. The ocean currents carry the materials dislodged by the waves and transport them, sometimes to enormous distances, usually allowing a considerable amount of separation to take place during the transit. In this way they act in a similar manner to rivers and streams.
Glaciers may be regarded as rivers of ice which erode their banks and bed in a manner similar to, but more rapidly than, streams of water. Owing to their much greater viscosity, glaciers are able to carry large boulders as well as gravel, sand and clay, so that the materials transported by them are far more complex in composition and size than are those carried by flowing water.
Fig. 14. Illustrating the successive deposition of different strata.
Separation and Sedimentation.
The clay and other particles having been placed in suspension in water by one or more of the natural forces already mentioned, they soon undergo a process of sorting or separation, previous to their deposition. The power of water for carrying matter in suspension depends largely on its velocity, and when this is reduced, as when a river discharges into a lake or sea, the larger and denser particles at once commence to settle, the smaller ones remaining longer in suspension, though if the velocity of the water is reduced sufficiently all the particles will be deposited. Hence, the deposits in lakes (lacustrine) and at the mouths of rivers (estuarine) increase more or less regularly in fineness according to their distance from the point at which the water enters, the gravel and stones being deposited first, then the coarse sand, next the finer sand and finally the silt and clay ([fig. 14]). If cross-currents are present, the deposits will, naturally, be made more irregular, and in some cases variations in the flow of the transporting water may cause the coarser particles to be carried further than usual so that they may cover some of the finer deposits previously formed; but as the clay and silt particles are so much finer than sand and gravel they usually travel so far before settling that their deposits are very uniform if the area over which they are spread is sufficiently large. Lake-deposited clays are for this reason more uniform than estuarine beds, whilst beds deposited at considerable depths in the sea and at a great distance from land are still more uniform.
A lacustrine clay is usually more persistent and uniform than fluviatile beds though sometimes difficult to distinguish from the latter. Some of the most valuable clay deposits are of lacustrine formation; their comparative purity and great uniformity enabling ware of excellent colour and texture to be produced without much difficulty. Thus the Reading mottled clays of the Hampshire basin, on the outskirts of the London basin and in Northern France are well known for the admirable red bricks, tiles and terra-cotta produced from them. Still purer clays deposited at Bovey Heathfield in Devonshire are also of lacustrine origin, though they differ in many respects from the ordinary lake-deposited clays and are of unusual thickness for deposits formed in this manner.
