138 SEDIMENTARY ROCKS/Rudaceous Rocks Figure Exhumed cliffline and beach sediments of Early Torridonian age, Enard Bay, north west Scotland bars The braided rivers beyond the fans typically develop in areas with greatly varying water discharge histories The wadi-floor streams of desert and semidesert areas, for example, receive little water for many months before floods sweep through the area carrying sediment-charged waters capable of transporting material up to the size of large boulders Thus more people drown in deserts than die of thirst As the waters subside the sediments are rapidly deposited to give a layer of coarse, matrix-supported gravel Another important site for the formation of braided systems is in the periglacial sandurs and plains associated with glacial retreat, such as those of Iceland The rivers carry little water during the winter months, but in summer may carry large quantities of glacial melt water, accompanied by the transport of high sediment loads, which become deposited as the bed gradient and flow velocities fall These streams drain areas in which sediments of all size are available and movement is minimally restricted by vegetation In rift valleys rudaceous marginal fault-bounded fanglomerates may pass out into braided river sands and gravels on the floor the central parts of basin (Figure 10) The deposits of braided rivers, explored by Miall and Bluck, typically show successions of fillings of stacked channels, some of the major stream, and others of second or third-order channels (Figure 11) Typically upward fining sheets of sand and gravel result from the migration of mid-channel and overbank bars that are the principal sites of deposition Miall showed that in the upper reaches of some Figure 10 Suggested distribution of Upper Old Red Sandstone alluvial fans in central Scotland (after Trewin NH and Thirlwall MF (2003) Old Red Sandstone In: Trewin NH (Ed.) The Geology of Scotland, 4th edn, pp 213 251 London: The Geological Society of London) braided systems debris flows occur within the more normal upward fining cycles of the flood deposits He identified three pebble-rich assemblage types, dominated by channel gravels with intervening debris flows, by superposed channel bars or by channel floor gravels passing upwards into current bedded sands Pebbles also occurred less frequently in other sand-rich braid deposits In the British geological column the braided systems have been recognized from the Precambrian Torridonian, in both the upper and lower Old Red Sandstone (Figure 12) of the Devonian and in the New Red Sandstones of the Permo-Trias