SEDIMENTARY ROCKS/Rudaceous Rocks 135 Figure Interrelationships between factors controlling a stable channel bed in a river (after Lane 1955) Lakeshore gravel beaches are common, but the low power in the waves generated on small lakes ensures that the beaches rarely achieve a fully mature condition The large clasts are commonly essentially static, show limited size and shape sorting, and are often poorly rounded Rods and spheres are generally more common in the coarser particles than in the smaller clasts In shallow tropical or subtropical seas carbonate sediment dominates These include in situ reef rock, and carbonate sands and muds All of these may become quickly cemented, and subsequently eroded during heavy storms generating large amounts of rudaceous detritus An apron of limestone breccia may extend into deep waters at the foot of reefs and other abrupt carbonate shelf margins Such reef apron breccias have been recognized in the Permian Capitan Reef of Texas and New Mexico, around many Carboniferous structures in Britain and Belgium, and round Cretaceous reefs in the French Juras Clasts on Alluvial Fans Adjacent to newly emergent mountains, or uplifting fault blocks rapid changes of stream bed gradient ensure that deposition of sediment occurs adjacent to the source, creating alluvial fans of coarse detritus These sediments are particularly well preserved in arid or semi-arid areas providing important sites in which rudaceous materials accumulate In the geological past, before evolution of extensive land-living plants in the Devonian, fans were widely developed and thick accumulations of pebbly sandstones mark the margins of areas of active uplift The evolving alluvial fans, or the coalescent adjacent fans forming a bajada, produce wedges of sediment, commonly but against a fault Repeated uplift generates successions of wedges of coarse materials stacked above each other, with thicker beds of coarse sediments near the source, grading to thinner beds of finer materials in more distal areas The uppermost parts of the fans are characterized by the presence of debris- and mud-flow deposits, each of which contain large clasts, but the latter also contain much fine sediment In essence the water drains from the moving sediment-enriched flow to induce deposition The fan head is dominated by ribbons of the mass flow deposits, through which the streams erode as they flow towards the lower, outer parts of the fan, where slopes decrease from 5 to less than 1 and the waters become divided into many distributaries The sediments of the outer part of the fan are dominated by sheets of sand or gravel from the often ephemeral, braided streams Characteristically, the fan sediments