The oldest geologic feature of the San Juan River basin are the San Juan Mountains, which consist largely of Precambrian and Paleozoic crystalline rock and of outcrops of Tertiary volcanic rock. During the Paleocene and Eocene (about 66–34 million years ago), streams draining off the southern flank of the San Juan Mountains terminated in the San Juan Basin around present-day Farmington, slowly filling it with thick layers of sediment. Over millions of years, the burial of organic material under sedimentary layers created the abundant coal, oil, and natural gas deposits found in the area today.