SEDIMENTARY ROCKS. In petrology, this major division of rocks includes those which have been deposited as beds, with more or less definite upper and lower limits, either on the floors of rivers, lakes and seas, or through the agency of the wind, on the surface of the land. The materials which constitute these deposits result from the disintegration of existent rocks under the action of geological agents such as changes of tempera ture, frost, wind, rain, running water, ice and the waves of the sea. The work of these agents is assisted by force of gravity, which tends to bring the detritus to lower levels, so that, while temporary halts on the downward journey may be made on land and on the beds of rivers and lakes, the ultimate resting-place is the sea-bottom. The geological agents operate both mechanically and chemically; in the former case, fragments of rocks and minerals are isolated from the parent-mass and are transported as such, but in the latter case minerals are decomposed by weathering, in part reduced to a colloidal condition and in part dissolved. Many soluble rock-constituents are in this way carried down to increase the quantity of salts in solution in lakes or in the ocean.
A complex mixture of materials is thus gathered from all the rocks available in a drainage-area, but as soon as transport be gins, a differentiation or sorting out of constituents is inaugurated. This sorting, which is definite but at a minimum on the land surface under the action of the weather and of gravity, increases in torrents and becomes more evident in large rivers and lakes. It is most effective in the sea. As the velocity of upland streams diminishes, boulders can no longer be rolled and pebbles are dropped. Thus boulder-beds, gravels, breccias and conglomerates (q.v.) are formed. Examples of these are seen in elevated river terraces and in the alluvial cones or "dry deltas" built up where mountain-streams debouch on plains or wide, open valleys. Similarly, where harder types of rock are subjected to the action of the sea, large rock-fragments are broken off, rounded by the waves, and accumulate near high-tide level as shingle-banks, often accompanied by sand-bars and sandy beaches. With in creasing distance from the source of supply, the boulders and pebbles are replaced by sand, silt and clay.
Where variations in the velocities of rivers occur, a certain degree of sorting of the detritus according to size is effected, coarser sand and gravel being laid down in the form of banks. While some of the finer clayey and silty material may be deposited en route as a result of flocculation (see below), much of it reaches the sea. If lakes intervene in the drainage-system, even the finest material may be trapped, and the rivers effectively freed from suspended detritus (e.g., the Rhone at Lake Geneva). If abund ant decomposing organic matter is present, the clay and silt may be rapidly deposited, carrying down in the process even sand; heterogeneous or "poorly-graded" deposits thus result. In the great lakes which were formed by the melting of the ice-sheets at the close of the Glacial Period, and in those lakes like Lake Louise and Lake Geneva which to-day are fed by glacial streams, a seasonal differentiation has been observed in the accumulations of detritus on the lake-bottom. The sediment is found to consist of alternating bands of finer and coarser material, called varves. The more sandy layers or portions of a layer represent the ma terial which settles out almost at once after summer floods.
With the cessation of the stream-flow during the winter freez ing, opportunity is afforded for the finest mud in the lake-water to settle down slowly, first as silt and later as clay. Thus is effected (if we exclude the action of the wind, which on land occasionally sorts material into sand, silt and dust very efficiently) an example of the most perfect grading of sedimentary rocks. An individual band or varve may show a gradual passage upwards from sand or silt at its base to an impalpably fine, indeed colloidal, clay at its top. This clay is then succeeded abruptly by the coarser sand or silt resulting from the next flood. Rocks of various geological ages display similar banding and periodicity of supply of coarse and fine sediment, but none so perfectly as glacial deposits. The extent to which this periodicity is seasonal has not yet been established with certainty.