▁recent ▁flow ing ▁water . ▁Many ▁are ▁present ▁in ▁the ▁G org on um ▁Cha os . ▁ ▁Gul lies ▁occur ▁on ▁ste ep ▁s lop es , ▁especially ▁cr aters . ▁Gul lies ▁are ▁believed ▁to ▁be ▁relatively ▁young ▁because ▁they ▁have ▁few , ▁if ▁any ▁cr aters , ▁and ▁they ▁lie ▁on ▁top ▁of ▁sand ▁d unes ▁which ▁are ▁young . ▁Usually , ▁each ▁g ul ly ▁has ▁an ▁alc ove , ▁channel , ▁and ▁apr on . ▁Although ▁many ▁ideas ▁have ▁been ▁put ▁forward ▁to ▁explain ▁them , ▁the ▁most ▁popular ▁involve ▁liquid ▁water ▁either ▁coming ▁from ▁an ▁aqu ifer ▁or ▁left ▁over ▁from ▁old ▁gla ci ers . ▁ ▁There ▁is ▁evidence ▁for ▁both ▁theories . ▁Most ▁of ▁the ▁g ul ly ▁alc ove ▁heads ▁occur ▁at ▁the ▁same ▁level , ▁just ▁as ▁one ▁would ▁expect ▁of ▁an ▁aqu ifer . ▁Vari ous ▁measurements ▁and ▁calculations ▁show ▁that ▁liquid ▁water ▁could ▁exist ▁in ▁an ▁aqu ifer ▁at ▁the ▁usual ▁depth s ▁where ▁the ▁g ul lies ▁begin . ▁One ▁variation ▁of ▁this ▁model ▁is ▁that ▁rising ▁hot ▁mag ma ▁could ▁have ▁m elt ed ▁ice ▁in ▁the ▁ground ▁and ▁caused ▁water ▁to ▁flow ▁in ▁aqu if ers . ▁Aqu if ers ▁are ▁layer ▁that ▁allow ▁water ▁to ▁flow . ▁They ▁may ▁consist ▁of ▁por us ▁sand stone . ▁This ▁layer ▁would ▁be ▁per ched ▁on ▁top ▁of ▁another ▁layer ▁that ▁prevents ▁water ▁from ▁going ▁down ▁( in ▁ge ological ▁terms ▁it ▁would ▁be ▁called ▁imper me able ). ▁The