also ▁known ▁as ▁the ▁Mill ▁Creek ▁Form ation ). ▁The ▁contact ▁is ▁grad ational , ▁with ▁vol can ic ▁fragments ▁becoming ▁progress ively ▁more ▁common ▁toward ▁the ▁top ▁of ▁the ▁Ma ▁But te ▁Form ation . ▁The ▁lower ▁part ▁of ▁the ▁formation ▁is ▁tra ch yt ic ▁with ▁abund ant ▁san id ine ▁phen oc ry st s , ▁mel an ite ▁and ▁py ro x ene . ▁The ▁upper ▁part ▁contains ▁san id ine , ▁anal c ime , ▁mel an ite ▁and ▁rock ▁fragments . ▁It ▁is ▁un con form ably ▁over lain ▁by ▁the ▁sh ales ▁of ▁the ▁Black stone ▁Form ation ▁that ▁were ▁depos ited ▁during ▁a ▁marine ▁trans gression ▁in ▁the ▁Late ▁C ret ace ous . ▁ ▁De position ▁The ▁vol can ics ▁were ▁laid ▁down ▁on ▁an ▁in land ▁flo od plain ▁that ▁is ▁represented ▁by ▁the ▁underlying ▁Ma ▁But te ▁Form ation . ▁The ▁er u ptions ▁probably ▁occurred ▁to ▁the ▁west ▁near ▁what ▁is ▁now ▁C ran bro ok , ▁British ▁Columbia , ▁and ▁the ▁material ▁was ▁subsequently ▁moved ▁east ward ▁by ▁thrust ▁fault ing ▁during ▁the ▁Lar am ide ▁or ogen y . ▁It ' s ▁estimated ▁that ▁the ▁vol can ics ▁originally ▁covered ▁an ▁area ▁of ▁about ▁, ▁and ▁their ▁volume ▁is ▁estimated ▁at ▁. ▁ ▁Th ick ness ▁and ▁distribution ▁The ▁C rows nest ▁Vol can ics ▁are ▁exposed ▁along ▁a ▁series ▁of ▁fol ded , ▁west - di pping ▁fault ▁pl ates ▁in ▁the ▁Front ▁R anges ▁and ▁fo oth