isthmus land bridge An isthmus or other land connection between what at other times are separate land masses which allows animals and plants to cross and colonise new lands. Land bridges are commonly created by regression, in which sea levels fall exposing previously submerged sections of continental shelf. Land bridges are also formed by: (a) upthrust at the edge of continental plates; and (b) glacial retreat alleviating pressure on shallow marine formations. ENVO ENVO:00000391 TGN:21478 Wikipedia:Land_bridge