A sandstone layer near the bases of these hills has yielded one of the richest concentrations of fossilized mammal remains ever discovered. Since 1904, paleontologists have been uncovering fossil bones here that have greatly contributed to knowledge of prehistoric mammals. Many of these fossils are now studied and displayed at leading museums and universities, though thousands of bones have been removed and many more still lie undisturbed within the fossil hills. The trail leads to the main fossil quarry sites at University and Carnegie Hills, where small sections of the fossil layer are displayed as they were found.