 Upon the line of Wyoming and Livingston counties, it breaks through a mountain barrier in a deep gorge and forms the Portage Falls.Below this point the course of the river is through a valley wide and bordered by banks high.At Rochester it flows over the precipitous edges of the Niagara limestone, forming the Upper Genesee Falls; and below it flows over the edge of the Medina sandstone, forming the Lower Genesee Falls.