▁The ▁hem is pher ical ▁void s ▁and ▁p its ▁result ▁from ▁the ▁select ive ▁dissol ution ▁of ▁carbon ate ▁con cret ions ▁within ▁the ▁gre yw ack e . ▁The ▁gre yw ack e ▁is ▁ident ifiable ▁by ▁its ▁low ▁met am orph ic ▁grade ▁and ▁the ▁ 1 0 – 4 0 % ▁rock ▁fragments , ▁distinct ive ▁vol can ic ▁cl asts , ▁and ▁spher ical ▁carbon ate ▁con cret ions ▁that ▁it ▁contains . ▁Om ars ▁are ▁typically ▁rounded ▁and ▁range ▁in ▁size ▁from ▁p eb bles ▁to ▁b ould ers . ▁Their ▁rounded ▁shape , ▁whether ▁found ▁in ▁gla cial ▁till s ▁or ▁gla cial - fl uv ial ▁( out w ash ) ▁gra vel s , ▁indicate ▁that ▁they ▁were ▁er oded ▁from ▁pre - ex isting ▁litt oral ▁or ▁fl uv ial ▁depos its . ▁Om ars ▁are ▁typically ▁found ▁associated ▁with ▁gran ules ▁and ▁p eb bles ▁of ▁o olit ic ▁j as per ▁that ▁were ▁transport ed ▁from ▁the ▁Bel cher ▁Islands ▁in ▁Hudson ▁Bay , ▁Canada . ▁ ▁The ▁name ▁given ▁these ▁gla cial ▁er rat ics ▁refer ▁to ▁their ▁source , ▁which ▁is ▁the ▁Pro ter oz o ic ▁O mar oll uk ▁Form ation ▁in ▁the ▁Bel cher ▁Islands ▁in ▁sout heast ▁Hudson ▁Bay . ▁The ▁Laurent ide ▁Ice ▁She et ▁er oded ▁om ars ▁from ▁the ▁Bel cher ▁Islands , ▁an ▁arch ip el ago ▁limited ▁to ▁only ▁about ▁a ▁quarter ▁of ▁ 1 % ▁of ▁Hudson ▁Bay .