ae ▁( Is men ius ▁Lac us ▁quad rangle ) ▁region , ▁but ▁it ▁occurs ▁in ▁other ▁places ▁as ▁well . ▁ ▁The ▁rem n ants ▁consist ▁of ▁sets ▁of ▁di pping ▁layers ▁in ▁impact ▁cr aters , ▁in ▁de press ions , ▁ ▁and ▁along ▁mes as . ▁S ets ▁of ▁di pping ▁layers ▁may ▁be ▁of ▁various ▁sizes ▁and ▁shapes — some ▁look ▁like ▁Az te c ▁py ram ids ▁from ▁Central ▁America . ▁ ▁This ▁unit ▁also ▁de gr ades ▁into ▁brain ▁terrain . ▁ ▁Bra in ▁terrain ▁is ▁a ▁region ▁of ▁ma ze - like ▁rid ges ▁ 3 – 5 ▁meters ▁high . ▁ ▁Some ▁rid ges ▁may ▁consist ▁of ▁an ▁ice ▁core , ▁so ▁they ▁may ▁be ▁sources ▁of ▁water ▁for ▁future ▁colon ists . ▁ ▁Some ▁regions ▁of ▁the ▁upper ▁pla ins ▁unit ▁display ▁large ▁f ract ures ▁and ▁tr ough s ▁with ▁raised ▁r ims ; ▁such ▁regions ▁are ▁called ▁rib bed ▁upper ▁pla ins . ▁ ▁F ract ures ▁are ▁believed ▁to ▁have ▁started ▁with ▁small ▁cra cks ▁from ▁str esses . ▁ ▁Str ess ▁is ▁suggested ▁to ▁initi ate ▁the ▁f ract ure ▁process ▁since ▁rib bed ▁upper ▁pla ins ▁are ▁common ▁when ▁de br is ▁apr ons ▁come ▁together ▁or ▁near ▁the ▁edge ▁of ▁de br is ▁apr ons — such ▁sites ▁would ▁generate ▁compression al ▁str esses . ▁ ▁Cra cks ▁exposed ▁more ▁surfaces , ▁and ▁consequ ently ▁more ▁ice ▁in ▁the ▁material ▁sub lim ates ▁into ▁the ▁planet ’ s