During the Pleistocene Epoch, over 15-thousand years ago, a huge ice sheet covered the ground all the way from Canada down to the Ohio River. On the edges of this ice sheet, great herds of giant mastodons, wooly mammoths and ground sloths were attracted to the warm salt springs that still bubble from the earth at Big Bone Lick State Park. The salty marsh that attracted these prehistoric visitors sometimes proved to be a fatal attraction. Animals became trapped and perished in what the early pioneers called "jelly ground," leaving skeletons and interesting clues about life in prehistoric Kentucky. The fossilized remains of these prehistoric animals were discovered in 1739 and displayed extensively at museums throughout the world. Notable Americans such as Thomas Jefferson and Benjamin Franklin personally examined the fossils, many of which are on display today at Big Bone Lick Museum. The scientific community recognizes the site as the "Birthplace of American Vertebrate Paleontology."