▁next ▁to ▁ ▁a ▁fresh water ▁lake ▁and ▁killed ▁large ▁m amm als ▁such ▁as ▁hi pp op ot am ids . ▁ ▁Upper ▁Her to ▁The ▁Upper ▁Her to ▁member ▁changes ▁ ▁to ▁yellow ▁sand stone ▁( from ▁the ▁ ▁fl uv ial ▁and ▁lake - margin ▁depos its ▁of ▁the ▁lower ▁layer ), ▁and ▁dates ▁to ▁between ▁ 1 6 0 ▁and ▁ 1 5 4 ▁k ya . ▁Im medi ately ▁above ▁the ▁er os ion ▁surface ▁separ ating ▁the ▁two ▁layers ▁is ▁a ▁sequence ▁of ▁vol can ic ▁sand stone ▁and ▁gra vel ▁depos its ▁of ▁variable ▁thick ness es . ▁It ▁shows ▁cross - bed ded ▁sed iment ation ▁containing ▁p um ice ▁rocks ▁and ▁has ▁produced ▁all ▁the ▁human ▁foss ils ▁and ▁tools ▁found ▁in ▁the ▁ ▁Upper ▁layer . ▁The ▁Upper ▁layer ▁is ▁to pped ▁by ▁a ▁vol can ic ▁t uff . ▁Two ▁vol can ic ▁layers ▁of ▁very ▁fine ▁ash ▁occur , ▁one ▁just ▁below ▁the ▁hom in in ▁foss ils ▁and ▁one ▁just ▁above ; ▁this ▁important ▁feature ▁allows ▁accurate ▁arg on – arg on ▁d ating ▁of ▁adjacent ▁sed iment ary ▁layers ▁and ▁their ▁foss ils , ▁as ▁reported ▁above . ▁This ▁is ▁valuable ▁" .. because ▁the ▁accurate ▁d ating ▁of ▁fa unas ▁and ▁arte fact s ▁of ▁many ▁sites ▁of ▁this ▁general ▁anti qu ity ▁in ▁Ple ist oc ene ▁Africa ▁has ▁proved ▁not or iously ▁difficult ." ▁ ▁In ▁this ▁layer ▁have ▁been ▁found ▁early ▁Middle ▁Stone ▁Age ▁(