▁" f irth " ▁( e . g . ▁the ▁Min ch ▁and ▁L och ▁Tor rid on ); ▁instead , ▁these ▁are ▁often ▁called ▁sea ▁lo chs . ▁Before ▁about ▁ 1 8 5 0 , ▁the ▁sp elling ▁" Fr ith " ▁was ▁more ▁common . ▁ ▁A ▁f irth ▁is ▁generally ▁the ▁result ▁of ▁ice ▁age ▁gla ci ation ▁and ▁is ▁very ▁often ▁associated ▁with ▁a ▁large ▁river , ▁where ▁er os ion ▁caused ▁by ▁the ▁tid al ▁effects ▁of ▁incoming ▁sea ▁water ▁passing ▁up river ▁has ▁w iden ed ▁the ▁river bed ▁into ▁an ▁est u ary . ▁Dem arc ation ▁can ▁be ▁rather ▁vague . ▁The ▁F irth ▁of ▁C ly de ▁is ▁sometimes ▁thought ▁to ▁include ▁the ▁est u ary ▁as ▁far ▁up river ▁as ▁D umb art on , ▁but ▁the ▁Or dn ance ▁Survey ▁map ▁shows ▁the ▁change ▁from ▁river ▁to ▁f irth ▁occurr ing ▁off ▁Port ▁Glasgow , ▁while ▁locally ▁the ▁change ▁is ▁held ▁to ▁be ▁at ▁the ▁T ail ▁of ▁the ▁Bank ▁where ▁the ▁river ▁cross es ▁a ▁sand bar ▁off ▁Green ock ▁at ▁the ▁j unction ▁to ▁the ▁G are ▁L och , ▁or ▁even ▁further ▁west ▁at ▁G ou rock ▁point . ▁▁ ▁However , ▁some ▁f irth s ▁are ▁exceptions . ▁The ▁C rom art y ▁F irth ▁on ▁the ▁east ▁coast ▁of ▁Scotland , ▁for ▁example , ▁res emb les ▁a ▁large ▁lo ch ▁with ▁only ▁a ▁relatively ▁small ▁out let ▁to ▁the ▁sea ▁and ▁the ▁Sol way ▁F