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Alluvial Terraces: ~ecords of Past Climate and Hydrologic Fluctuations -1 I j ~ " J ~- . Pleistocene Terraces in the Grand Canyon ?lei stocene terraces are not well preserved in most areas of the Grand Canyon. Nonetheless, SpringorlJTT1 (1965) identified two distinct levels of ?leistocene gravel deposits in a number of tributary valleys. These occur at about 8 m and 20 rn (26 and 65 feet) above modern average stream 1 evel and are graded to former hi gher 1 evel s of the Colorado Ri ver. The I"!ost compl ete sequences of Pl p.i stocp.negravel terraces occur in Nankoweap and Lava Creek valleys and include four terraces at about 13, 23, 45, a!1d 76 m (43, -75, 148, and 249 feet) above - the average stream 1 evel . These terrace remnants consi st of well to poorly sorted alluvial and colluvial sediments of lo~al derivation. Springorum (1965) concludes that the gravels were deposited during Pleistocene glacial intervals when sediment loads and discharge rates were much higher than those of today, and were di s sected and 1 eft as terraces near the end of 91 aci al times as streams dO\'mcut below these 1 evel s. Although there has been noabsol ute dati ngof these terraces, Springorum (1965) notes the lack of a distinct paleosol on these surfaces and thereby c6nsiders them latest Pleistocene or Wisconsin in age (approximately 70,000 to 15,000 years B.P.; Frye, Willman, and Slock 1965). However, I have observed a moderately well developed soil on the upper terraces in Nankoweap valley and concl ude that more detail ed analyses of these soils should be r;'Iade in order to facilitate better age a s S~ s Sl'1e n t s . Pleistocene terrace remnants along the Colorado River itself have been observed and mapped, but have not been well described in the literature. Hereford (19841)) notes that the base of a rockfall deposit near Nankoweap Canyon, which temporarily dammed the Colorado River, rests on a dissected Pleistocene terrace some 5 to 10 r.1 (16 to 33 feet) ( above the1110dern all uvi urn. In addition, a piece of Douglas-fir drift\'/ood embedded in si lts in Stanton I s Cave some 44 m above average stt'eam level in Mar!)l/? Canyon dates to 43,700 B.P. (+1800 and -1500) (Hereford 1984b:l05). Euler (1984) postulates that this driftwood could have ::>een deposi teddue t.o temporary rlamming of the Colorado River by ':-, l ">J '-1 ~ '1 I I . "j 20