Strata are layers of sedimentary rock that form beds or bands of colored or textured material. From smooth black shale to rough white limestone, rock strata are most easily identiﬁed in cliffs, canyons, quarries, road cuts, or the exposed banks of rivers. Strata vary in thickness from a centimeter to a kilometer or more. Each bed contains fossils set down in a speciﬁc sequence with a deﬁnite mode of deposition—river silt, beach sand, coal swamp, sand dune, or lava. When John Price explored Badlands National Park in his book Not Just Any Land, he “followed a bison trail up to a high, hot place” and viewed “the impressive, serrated edge of the distant Pinnacles, their strata bleached white in the sun.” Hikers elsewhere might have a view of many strata from above. For example, at the Grand Canyon, erosion has exposed rock strata ranging from the 1.7-billion-year-old Vishnu Schist at the bottom of the inner gorge, through brilliantly colored limestone, shale, and sandstone formations, to one-million-year-old black lava ﬂows in the western canyon.