About 34 million years ago, volcanic eruptions near Guffey (18 miles to the southwest) produced volcanic mudflows (water-saturated mass of ash and rock debris). These mudflows—looking and flowing like concrete—buried a lush valley and petrified the bases of huge redwood trees that grew there. These mudflows also created a dam in the valley, forming a lake about one mile wide and 12 miles long. Volcanic ash from subsequent eruptions formed fine-grained sediments at the bottom of the lake. Plants, insects, and other organisms were entombed in this material. Over millions of years these sediments were compacted into layers of shale. The delicate details of these organisms were preserved as fossils and provide a look at the life and the prehistoric ecosystem of the Florissant valley during the end of the Eocene Epoch.