Lake Monongahela was a huge proglacial lake that formed during the Pre-Illinoian ice age when the Laurentide Ice Sheet blocked north-flowing river valleys near Pittsburgh. Water backed up through interconnected valleys in what are now western Pennsylvania, West Virginia, and Ohio, disrupting the ancestral Monongahela River system. As the lake rose, overflow near New Martinsville helped reorganize regional drainage and began the process that created the modern Ohio River valley, replacing the old north-flowing system with a new outlet to the south and west. Its lasting legacy lies in widespread sediment deposits, including the Carmichaels Formation, which preserve evidence of ancient environmental change and glaciation.