How are fossils preserved at Fossil Butte? Objective Students will understand how fossils are preserved at Fossil Butte. Background Fossil Butte National Monument protects an extraordinarly high amount of fossils that are well-preserved. These fossils are here thanks to Fossil Lake which existed 52 million years ago. The lake's bottom was quite salty and lacked oxygen while the surface was oxygenated freshwater. Because of this, when a fish died and sank to the bottom of the lake, there were no scavengers or decomposing bacteria to disturb the carcass. Mud and algae from the lake's surface san Duration: 30 Minutes.