▁stem ▁of ▁San ▁Diego ▁Creek . ▁ ▁References ▁ ▁Category : R ivers ▁of ▁Orange ▁County , ▁California ▁Category : R ivers ▁of ▁Southern ▁California <0x0A> </s> ▁The ▁Mod oc ▁Rock ▁Sh elter ▁is ▁a ▁rock ▁cl iff ▁at ▁the ▁edge ▁of ▁the ▁Mississippi ▁River ▁Valley ▁that ▁was ▁under cut ▁by ▁Ice ▁Age ▁flo ods . ▁This ▁site ▁is ▁significant ▁for ▁its ▁arch ae ological ▁evidence ▁of ▁human ▁habit ation ▁during ▁the ▁Arch a ic ▁period ▁in ▁the ▁Eastern ▁United ▁States . ▁ ▁It ▁is ▁located ▁on ▁the ▁n ort he astern ▁side ▁of ▁County ▁Road ▁ 7 ▁( Bl uff ▁Road ) ▁sout heast ▁of ▁Pra irie ▁du ▁Ro cher ▁in ▁Rand olph ▁County , ▁Illinois , ▁United ▁States . ▁ ▁It ▁was ▁declared ▁a ▁National ▁Historic ▁Land mark ▁in ▁ 1 9 6 1 . ▁ ▁Description ▁The ▁site ▁has ▁over ▁ 2 8 ▁feet ▁of ▁sed iment ▁that ▁contain ▁artifact s . ▁Ev idence ▁from ▁the ▁site , ▁including ▁four ▁separate ▁periods ▁of ▁Arch a ic ▁occupation ▁and ▁one ▁of ▁a ▁later ▁period , ▁suggests ▁that ▁the ▁cult ures ▁of ▁the ▁Eastern ▁Wood lands ▁may ▁have ▁been ▁compar able ▁in ▁age ▁to ▁the ▁big ▁game ▁hunting ▁cult ures ▁of ▁the ▁Great ▁Pla ins . ▁ ▁Based ▁on ▁the ▁analysis ▁of ▁artifact s , ▁arch ae olog ists ▁discovered ▁that ▁ 9 , 0 0 0 ▁years ▁ago ▁this ▁rock ▁shelter ▁was ▁used ▁as ▁a ▁short - term ▁camp ▁by ▁small ▁hunting ▁groups ; ▁by ▁ 6 , 0 0 0