▁which ▁ir rig ated ▁agricult ure ▁is ▁possible . ▁The ▁valle ys ▁of ▁the ▁Cas ma ▁River ▁and ▁its ▁trib ut ary , ▁the ▁Se chin ▁River , ▁are ▁one ▁of ▁the ▁linear ▁o ases . ▁The ▁ancient ▁area ▁of ▁the ▁Cas ma / Se chin ▁culture ▁extends ▁about ▁ ▁in land ▁from ▁the ▁sea . ▁The ▁width ▁of ▁the ▁ir rig able ▁valle ys ▁range ▁from ▁. ▁ ▁Loc ated ▁along ▁about ▁ ▁in ▁the ▁Se chin ▁River ▁valley , ▁just ▁up stream ▁from ▁its ▁j unction ▁with ▁the ▁Cas ma , ▁is ▁a ▁complex ▁of ▁arch ae ological ▁ru ins ▁that ▁includes ▁sites ▁such ▁as ▁Se chin ▁B ajo , ▁Se chin ▁Al to , ▁Cer ro ▁Se chin , ▁and ▁T au k achi - K on kan , ▁all ▁of ▁them ▁except ▁Se chin ▁Al to ▁in ▁the ▁desert ▁just ▁beyond ▁the ▁edge ▁of ▁the ▁ir rig ated ▁river ▁valley . ▁The ▁Cas ma ▁valley ▁was ▁populated ▁long ▁before ▁the ▁monument al ▁construction ▁began . ▁The ▁oldest ▁radio ▁carbon ▁date ▁found ▁at ▁Se chin ▁B ajo ▁is ▁ 4 5 0 0 ▁BC ▁although ▁other ▁signs ▁of ▁human ▁occupation ▁of ▁the ▁Cas ma ▁valley ▁date ▁back ▁to ▁before ▁ 6 , 0 0 0 ▁BC . ▁ ▁Se chin ▁B ajo ▁is ▁located ▁about ▁ ▁north ▁of ▁the ▁Norte ▁Ch ico ▁civil ization , ▁believed ▁to ▁be ▁the ▁oldest ▁in ▁the ▁Amer icas . ▁Given ▁the ▁short ▁distances ▁contact ▁and ▁transmission ▁of ▁cultural ▁tra its ▁between ▁the ▁two ▁areas