“…[3] Gusev crater was selected as the landing site for the Spirit rover with the intention of sampling sedimentary deposits emplaced during discharge from Ma'adim Vallis, which breaches the southern rim of the crater [Cabrol et al, 1996[Cabrol et al, , 1998a[Cabrol et al, , 1998bKuzmin et al, 2000;Irwin et al, 2002;Golombek et al, 2003a;Grant et al, 2004]. The surface traversed by the Spirit rover after landing (14.5692°S, 175.4729°E), however, is characterized by impact-modified volcanic plains of late Hesperian age [Milam et al, 2003;Squyres et al, 2004;Grant et al, 2004;McSween et al, 2004;Crumpler et al, 2005;Golombek et al, 2006] and Husband Hill to the southeast that is older than the surrounding plains (at least early Hesperian [Kuzmin et al, 2000, Greeley et al, 2005, but perhaps Noachian) and of uncertain origin [Squyres et al, 2006;Arvidson et al, 2006]. We examined the rock population of the surfaces at regular intervals along the traverse of the Spirit rover ( Figure 1) to determine whether there was any significant variability among presumably different geomorphic surfaces that might reflect a fluvial/alluvial contribution to the impact-derived blocks and/or modification by weathering or alternate geomorphic processes.…”