Remnants of the glaciers that receded more than 10,000 years ago are evident in this region's rolling hills, scraped depressions, glacial lakes and potholes and accumulations of soil and stone debris. Much of the region's plant and animal diversity can be attributed to its ice-covered past. The region's numerous wetlands and ponds provide foraging and nesting habitat for bald eagles and other aquatic birds.
A majority of Pennsylvania's swamps and marshes are found in these northwest counties, including the vast Conneaut Marsh and bald eagle-rich Pymatuning region. The abundant wetlands, lakes, woodlands and wet meadows of this region provide vital habitat for many animals, including a large inventory of breeding birds and those just passing through during migrations.
At the far northwest corner, Lake Erie looms as an obstacle for migrating birds and creates a stopping point for great numbers and varieties. The birds must rest and feed after crossing the lake on southbound flights in fall and northbound migrants must do the same before crossing in spring. This great lake influences much of the climate in the region.
To the east, the Allegheny Plateau rises with deeper river valleys and large tracts of northern hardwood forests.