The Niobrara River drains nearly 12,000 square miles and cuts through four rock formations. Atop is the Ash Hollow formation, a grayish cap-rock best seen on hills north of the river. Underlying this is the Valentine formation. This deposit forms steep cliffs along both sides of the river and is composed of poorly cemented light-colored sandstone. The Valentine formation showcases an abundance of fossils, including ancient mammalian species such as beaver, horses, rhinoceros, and mastodons. Next lies the Rosebud formation. This more resistant rock accounts for many of the river's rapids between Borman and Norden bridges in the popular canoeing reach. The Rosebud also provides the erosion-resistant layer over which flows tributary or springbranch streams that often tumble dozens of feet to the valley floor. Many of these springbranch streams have some of the most beautiful waterfalls in the state. Over 90 waterfalls have been recorded along the Niobrara. Further downstream near Meadville, the river cuts through shales of the Pierre formation. An extraordinary deposit of groundwater known as the Ogallala Aquifer, formed over eons of time from precipitation that saturated underground sand and rock layers. In the Niobrara Valley the river has cut into the plains as much as 300 feet, allowing water from the aquifer to seep out of the valley walls into the river.