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INDICATORS FOR CHARACTERIZING ALLUVIAL FANS 59 Such boundaries often can be identified on the basis of changes in the shapes of contour lines on topographic maps. For example, at the toe of a fan contour lines may become straighter or less concave when viewed downslope, although in the case of deeply dissected fans, contour lines may become more irregular and crenulated because of channel incision. The toe of the Cedar Creek' alluvial fan (Figure 3-2a) is defined by Bear Creek along the fan's western margin, and by the much larger valley floor of the Madison River into which Bear Creek flows along the fan's northwestern margin. Streams draining the northern part of the fan are more deeply incised because the Madison River valley floor forms a lower base level for erosion than its tributary valley floor along Bear Creek. The toe of some alluvial fans in arid regions is indicated by a relative increase in the amount, size, and type of vegetation because ground water is closer to the surface there than on the upper parts of the fan. The toe of some alluvial fans in humid regions may be indicated by relatively less vegetation because the recent deposits are less fertile than older sediments. A general sense of vegetation types often is indicated on topographic maps. Lateral Boundaries Lateral boundaries of alluvial fans are the edges of deposited and reworked alluvial materials. The lateral boundary of a single alluvial fan typically is a trough, channel, or swale formed at the lateral limits of deposition. Crenulations in contour Jines at fan boundary troughs can be observed along the margins of the Cedar Creek alluvial fan (Figure 3-2a). Lateral boundaries of single alluvial fans commonly are distinct contacts between light- colored, freshly abraded, alluvial deposits and darker-colored, weathered deposits with well- developed soils on piedmont plains. Soils of active alluvial fans typically are less oxidized and lower in clay content than soils on older landforms. As a consequence, the younger soils generally are lighter colored and more friable. Color and texture changes often are pronounced on aerial photos or infrared remote sensing imagery. In areas with rock varnish formation,2 the lighter surfaces of recent alluvial fan deposits in contact with undisturbed varnished surfaces of older deposits form a distinct boundary or contact that readily is distinguished by the relative darkness of the ground on aerial photographs and by on-the-ground inspection. Dark, undisturbed surfaces of rock varnish are found on old piedmont and valley deposits throughout the Basin and Range province of the western United States. In the case of multiple fans that coalesce to form bajadas, where deposits and reworked material of adjacent alluvial fans merge, the boundaries between adjacent fans may be less distinct than those of individual fans adjacent to streams, rivers, or smooth piedmonts, but generally are I The committee has not visited the Cedar Creek fan and inspected its surface and deposits. It is used as an example here because it has been studied intensively by prominent geomorphologislS, and thus much information is available regarding it In addition, it is a classic allnvial fan in shapc and history. 2 Rock varnish is a dark coating (from 2 to 500 microns Ihick) that forms on rocks at and near the Earth's surface as a result of mineral precipitation and eolian influx. The chemical composition of rock varnish typically is dominated by clay minerals and iron and/or manganese oxides and hydroxides, forming red and black varnishes, respectively. With lime, the thickness of Ihe coating increases if abrasion and burial of the rock surface do not occur. As a result, clastic sediments on alluvial fan surfaces that have been abandoned for long periods of time have much darker and thicker coatings of varnish than do younger deposits.