Dinosaur National Monument is a time machine that we can use to visit the long lost world of dinosaurs. The great fossil bone quarry at Dinosaur National Monument is world-renowned as a place for ongoing scientific and interpretive discovery. The monument was established in 1915 specifically to protect and preserve this great paleontological site. The quarry is an ancient river deposit containing the abundant remains of twelve species of fossil reptiles entombed in a 149 million year-old sandstone. Between 1909 and 1924 the remains of over 450 dinosaurs were excavated from the quarry. During those historic excavations, the exact positions of each of the thousands of bones uncovered were recorded on quarry maps, which are now preserved in the monument's archives. The image shown here is a small part of one of those maps. It records the position of a skeleton of Allosaurus fragilis, the apex predator of its time, as it was unearthed. The grid lines represent four feet on a side. Collected in 1924, the nearly three foot long skull at the top center of the map can be seen on exhibit in the Quarry Exhibit Hall at Dinosaur National Monument.