All Quaternary units except talus (Qt) and undivided surficial deposits (Qs) that are shown on the map were defined by Demsey (1990) and descriptions of these units given here are repeated verbatim or condensed from her report. Deposits were classified by Demsey based on the estimated time since the end of deposition, which was inferred based on degree of channel entrenchment, erosional degradation and rounding of channel margins, height of surfaces above channel bottoms, and degree of development of soils and desert varnish. Contacts between Quaternary map units and bedrock were determined by Demsey largely by aerial photograph analysis and were generalized so that they could be represented at 1: 100,000 scale. More recent detailed maps of bedrock, primarily at 1 :24,000 scale, show that many areas outlined as bedrock by Demsey are in fact Quaternary surficial deposits. It was thus necessary to assign Quaternary map units to areas of surficial deposits that were depicted as bedrock by Demsey (1990). These areas typically consist of canyon bottoms and flanking talus where various Quaternary units are complexly intermixed, and are commonly represented on this map as undivided surficial deposits (Qs). In other areas units identified by Demsey were extended to include adjacent areas previously depicted as bedrock, and in a few areas aerial photograph analysis was used to map previously unmapped surficial deposits.