al il and ▁is ▁built ▁on ▁more ▁than ▁ 7 0 0 ▁million ▁year ▁old ▁ igne ous ▁and ▁met am orph ic ▁cry st all ine ▁bas ement ▁rock .. ▁These ▁ancient ▁units ▁are ▁covered ▁in ▁thick ▁layers ▁of ▁sed iment ary ▁rock ▁formed ▁in ▁the ▁last ▁ 2 0 0 ▁million ▁years ▁and ▁influenced ▁by ▁the ▁rif ting ▁apart ▁of ▁the ▁Som ali ▁Pl ate ▁and ▁the ▁Arab ian ▁Pl ate . ▁ ▁Str at ig raph y ▁& ▁Ge olog ic ▁History ▁The ▁oldest ▁rock ▁units ▁in ▁Som al il and ▁date ▁back ▁over ▁ 7 0 0 ▁million ▁years ▁to ▁the ▁Pro ter oz o ic , ▁forming ▁cry st all ine ▁bas ement ▁rock . ▁Many ▁rock ▁units ▁date ▁to ▁the ▁Mes oz o ic ▁and ▁the ▁current ▁C en oz o ic ▁e ons . ▁ ▁Pro ter oz o ic ▁Som al il and ▁pres erves ▁good ▁examples ▁of ▁Pro ter oz o ic ▁rocks . ▁As ▁bas ement ▁rocks , ▁they ▁form ▁a ▁composite ▁ter rane , ▁with ▁younger ▁rocks ▁in ▁the ▁east ▁and ▁older ▁rocks ▁in ▁the ▁west . ▁The ▁older ▁rocks ▁pre - date ▁the ▁Pan - A fr ican ▁or ogen y ▁and ▁were ▁affected ▁by ▁both ▁ igne ous ▁and ▁met am orph ic ▁activity ▁ 8 4 0 , ▁ 8 0 0 ▁to ▁ 7 6 0 ▁and ▁ 7 2 0 ▁million ▁years ▁ago . ▁▁ ▁The ▁early ▁ge ology ▁of ▁Som al il and ▁is ▁divided ▁into