Matching Drill Cores Across the River

Before the Boardwalk

1The Pine River Watershed Council is turning an old cornfield on the river's north bank into a wetland, with a boardwalk that will cross the river to a park on the south bank. Ines Valcourt's eighth-grade class helps the council plant reeds and pull trash out of the shallows every spring. This year the class also got to watch something new: before the boardwalk posts go in, a drilling crew pulled long cylinders of rock, called cores, out of the ground on both banks so engineers could see what the posts would rest on.

2Site A is on the north bank, and Site B is on the south bank. The two sites are less than 1 km apart. Each core came up in sections about 1 m long, and the crew laid the sections in wooden boxes in order, from the surface down, with the depth written on each box. Dr. Obi Nwachukwu, a geologist working with the council, helped the class describe every layer and the fossils in it. The class wore safety glasses and only handled the core sections she handed to them. The tables below show their notes.

3At both sites the coal seam is a thin black layer full of flattened bark from scale trees, plants that grew in swamp forests long ago. The gray shale below it holds the same small trilobite species at both sites. At Site A, a tan sandstone with pieces of dinosaur bone lies on top of the coal. At Site B, the river gravel sits directly on the coal, and the top of the coal there is uneven and dotted with pebbles.

4Dr. Nwachukwu explained that geologists organize Earth's 4.6-billion-year history by using layers like these. In undisturbed layers, the deepest layers formed first. Fossils show what lived when each layer formed, and some kinds of life appear in rocks of only one span of time. Where the fossils in a stack of layers change sharply, geologists often draw a boundary between two eras. She showed the class the time scale below, which uses ages measured in other places around the world.