▁Geography ▁ ▁Land ▁Sed iment ary ▁rock ▁such ▁as ▁all u vi um , ▁cl ay , ▁lim estone , ▁and ▁sand stone ▁forms ▁the ▁greatest ▁share ▁of ▁the ▁Far ▁North ' s ▁ge ology . ▁These ▁depos its ▁follow ▁the ▁province ' s ▁rivers , ▁such ▁as ▁the ▁Log one ▁and ▁May o ▁Ts an aga , ▁as ▁they ▁empty ▁into ▁Lake ▁Ch ad ▁to ▁the ▁north . ▁At ▁the ▁province ' s ▁south , ▁a ▁band ▁of ▁gran ite ▁separ ates ▁the ▁sed iment ary ▁area ▁from ▁a ▁zone ▁of ▁met am orph ic ▁rock ▁to ▁the ▁south west . ▁This ▁latter ▁region ▁includes ▁depos its ▁of ▁g ne iss , ▁m ica , ▁and ▁sch ists . ▁The ▁Rh ums iki ▁Valley , ▁a ▁mountain ous ▁field ▁l itter ed ▁by ▁the ▁cores ▁of ▁ext inct ▁vol cano es , ▁constit utes ▁a ▁small ▁area ▁of ▁vol can ic ▁rock , ▁such ▁as ▁tra chy te ▁and ▁r hy ol ite . ▁ ▁The ▁Far ▁North ' s ▁so ils ▁are ▁a ▁bit ▁more ▁complex . ▁Much ▁of ▁the ▁province ▁is ▁composed ▁of ▁young ▁so ils ▁rich ▁in ▁raw ▁min er als . ▁This ▁is ▁true ▁of ▁much ▁of ▁the ▁land ▁south ▁of ▁Lake ▁Ch ad , ▁and ▁of ▁the ▁Mand ara ▁Mountains ▁on ▁the ▁western ▁border ▁with ▁Niger ia . ▁So il ▁here ▁is ▁black ▁cl ay ▁( all uv ial ▁soil ). ▁The ▁season al ▁flo oding ▁of ▁the ▁Log one ▁River ▁gives ▁rise ▁to ▁a ▁north -