5 4 ▁k ya — f oss ils ▁of ▁H omo ▁s api ens ▁id alt u ▁were ▁found . ▁ ▁The ▁area ▁is ▁important ▁because ▁active ▁t ect on ics ▁in ▁the ▁Af ar ▁De pression ▁created ▁varying ▁habit ats ▁for ▁early ▁hom in ins ▁across ▁the ▁most ▁recent ▁few ▁million ▁years ▁known ▁inform ally ▁as ▁the ▁P lio - P le ist oc ene . ▁These ▁habit ats , ▁laid ▁down ▁in ▁sed iment ary ▁rocks , ▁have ▁since ▁been ▁u pl ift ed , ▁which ▁allowed ▁their ▁er os ion ▁over ▁time ▁and ▁their ▁access ibility ▁today ▁to ▁pale o anth rop olog ists . ▁Vol can ic ▁er u ptions ▁have ▁left ▁vol can ic ▁t uff ▁layers ▁that ▁enable ▁accurate ▁d ating ▁of ▁the ▁sed iment ary ▁depos its ▁via ▁Arg on – arg on ▁d ating . ▁ ▁Hat ay ae ▁The ▁Hat ay ae ▁layer ▁is ▁ 4 0 ▁m ▁thick ▁at ▁its ▁base ▁and ▁is ▁composed ▁of ▁var ieg ated ▁s ilt ▁cl ay ▁and ▁pale os ols , ▁ze olit ic ▁and ▁bent on it ic ▁t uff s , ▁carbon ates ▁that ▁are ▁ped ogen ic , ▁sand stone ▁with ▁b ival ve ▁and ▁g ast rop od ▁shell s , ▁and ▁mud stone . ▁It ▁was ▁depos ited ▁in ▁a ▁flo od plain ▁ ▁alongside ▁ ▁river ▁delta ▁channels ▁and ▁a ▁shall ow ▁fl uct u ating ▁lake ▁dated ▁to ▁around ▁ 2 . 5 ▁my a . ▁ ▁In ▁the ▁Hat