ized ▁by ▁a ▁flat ▁relief ▁with ▁sand y ▁hills . ▁ ▁Within ▁the ▁plate au , ▁the ▁main ▁bre eds ▁are ▁C ret ace ous ▁s ands , ▁mar ls , ▁lim estone ▁and ▁ch alk , ▁which ▁occur ▁in ▁the ▁form ▁of ▁pro tr us ions ▁in ▁the ▁valle ys ▁of ▁rivers , ▁l ined ▁with ▁ancient ▁cry st all ine ▁rocks , ▁mostly ▁gran ites , ▁and ▁covered ▁with ▁a ▁thick ▁layer ▁of ▁t ert i ary ▁s ands , ▁cl ay , ▁mar ls ▁and ▁shell fish . ▁Qu atern ary ▁form ations ▁are ▁represented ▁by ▁flu vi og la cial ▁s ands ▁and ▁lo ams , ▁as ▁well ▁as ▁for ests ▁in ▁which ▁fert ile ▁gray ▁forest ▁so ils ▁and ▁black ▁earth s ▁are ▁developed . ▁ ▁Within ▁the ▁Pol iss ya , ▁m ora ine ▁depos its ▁are ▁commonly ▁found , ▁usually ▁lo ams ▁with ▁different ▁composition ▁of ▁b ould ers , ▁flu vi og la cial ▁s ands ▁and ▁forest - like ▁lo ams ▁with ▁surface ▁cover ▁of ▁sod - pod z ol ic ▁so ils ; ▁large ▁areas ▁are ▁occupied ▁by ▁pe at lands . ▁Gr ound water ▁belongs ▁to ▁Dev onian ▁and ▁C ret ace ous ▁depos its , ▁and ▁within ▁the ▁cry st all ine ▁mass if ▁they ▁are ▁bound ▁to ▁cra cks ▁in ▁cry st all ine ▁rocks . ▁Within ▁the ▁limits ▁of ▁Pol iss ya , ▁the ▁depth ▁of ▁ground water ▁is ▁ins ign ific ant . ▁ ▁Most ▁of ▁the ▁surface