A Day at Calder Quarry
1In October, the Fernwood Middle School geology club spent a day at Calder Quarry, where workers cut limestone for roads and buildings. Wearing hard hats and staying well back from the quarry wall, seventh grader Ravi Chandrasekar and the club collected fossils from loose piles of rock with Ms. Henrike Vos, the quarry's geologist. They found crinoids, animals with long stems made of small discs, and brachiopods, animals with two shells, both of which lived on a shallow sea floor. The rocks of the wall are about 370 to 380 million years old.
2Ms. Vos described the layers from top to bottom. Each limestone and shale layer formed from lime mud or clay mud that slowly settled on the sea floor. Over many millions of years, slow movements of Earth's crust lifted the region, and the sea drained away. Using fossils and other clues, geologists have estimated how long the thick layers took to build up (Table 1). Layer A is found under much of the state, not only at the quarry. The flat tops of some shale beds in Layer D are covered with ripple marks a few centimeters apart, like the ones waves make in the sand of a shallow beach, which can form and change within minutes.
3Two thin layers are different. Layer B is a jumble of broken shells with a sharp, flat bottom. The biggest shell pieces lie at the bottom of the layer and the smallest at the top, the way pieces settle when churned-up water calms down. The same layer has been found in quarries 50 km apart. Layer C is soft gray clay made from volcanic ash. Geologists have matched the crystals in it to one eruption of a volcano hundreds of kilometers away, and the same ash layer is found in rock across several states.
4Above Layer A, there is no more limestone. Instead, a layer of jumbled clay, pebbles and boulders, 3 m thick, lies right on top of it. Some boulders are granite, a rock that does not crop out within 600 km of the quarry, and each has a weathered rind about 1 cm thick. Where workers scraped this layer away, the top of Layer A is smooth and scratched with long, straight, parallel grooves. Ms. Vos said a sheet of ice more than 1 km thick covered this region until about 15,000 years ago. Most of the time between Layer A and the jumbled layer is not recorded in the wall at all.