The Summer the River Dropped
1Last summer was the driest that the wheat-growing town of Corliss had seen in decades. Cattle ponds shrank to mud, and the river that runs past the town dropped so low that a long stretch of its bed dried out. One August evening, eighth grader Beatriz Halvorsen walked the dry riverbed with her uncle, who farms wheat and raises cattle on the land above it. On a flat shelf of gray limestone, she found a line of large three-toed footprints, each longer than her forearm.
2A geologist from the state geological survey, Dr. Winifred Osei-Brandt, came out the next week to measure and photograph the tracks before the river rose again. She explained that footprints are trace fossils. They do not preserve the animal's body, but they record something it did. The tracks show that a large three-toed dinosaur once walked across soft, limy mud here. Oyster shells in the same limestone suggest that the mud lay near the edge of shallow, salty water. Later the mud hardened and was buried under more layers.
3Above the riverbed rises a bluff about 16 m high. Dr. Osei-Brandt described its layers from the top down (Table 1). The layers lie flat, one on top of another, and are not folded, tilted or broken. In a quarry 5 km away, the same track limestone sits on top of a darker limestone that does not crop out at the river, so she added it as the last row of the table. She split open loose chunks of each layer with a rock hammer, wearing safety glasses, and searched them for fossils. In layer C she found ripple marks but no fossils.
4Dr. Osei-Brandt also brought a chart of age ranges for some fossils (Table 2) and the main divisions of the geologic time scale (Table 3). Scientists divide Earth's 4.6-billion-year history into long spans of time. She told Beatriz to look closely at where the eras begin and end, and at which fossils first appear or last appear near those times. In October, heavy rains finally came, and the river covered the tracks again, but the survey's photographs keep a record of them.