Stratification

Stratification, the anangement of rocks in layers, the result of deposit. Though volcanic ashes may be stratified, and even lavas interstratified, or poured out on or between beds, stratification is mainly confined to sedimentary or aqueous rocks. Deep or open-sea deposits, or others in which the nature of the sediment continues long unvaried, are likely to be in thick beds, such as some freestones; whilst estuarine or shore deposits, laid down under the influence of changing currents, frequently vary repeatedly in composition, forming a succession of thin-bedded layers, such as Penarth beds in the Trias (q.v.), the Purbeck series, or the Woolwich beds. There is generally some absence of cohesion being two successive strata, especially if of different composition, and thus planes of bedding may form lines of passage not only for underground waters, but also in some cases for intrusive igneous rocks. Individual strata may range in thickness from an inch to many feet, and a stratnm may be subdivided by planes of lamination. [SHALE.] Beds will generally be greatest in thickness and coarsest in texture near the source of the sediment, becoming finer and thinner as they recede from it; and the coarser the rock be in grain, the more local it will be, as a rule, in area. By the thinning out of beds of one kind the general lithological character of a series may change totally in a small horizontal distance.