What the Dry Creek Showed
1Cedar Flats is a farm town in a wide valley of wheat and alfalfa fields. Last year was the driest anyone there could remember: from January through June, the valley got only about a quarter of its usual rain. Cedar Creek shrank to a trickle, and in places its bed dried out, showing gray bedrock that had been underwater for decades. Seventh grader Wren Castellanos, whose family farms beside the creek, noticed that the bedrock was full of small fossil seashells.
2Wren took her questions to Siddharth Rao, a geologist with the state geological survey. He explained that the bedrock is limestone, a sedimentary rock made mostly of shells, the skeletons of sea animals and lime mud that piled up on a shallow sea floor that covered the region long ago. Buried under more layers, the material was pressed and cemented into rock. Layers like these usually form nearly flat. At a quarry on the valley floor, workers crush the same limestone into a powder called ag lime. Farmers spread it on their fields to make the soil less acidic, and over several years, rain and soil water slowly dissolve it.
3In the Ridge Hills east of the valley, an old quarry cuts into white marble. Mr. Rao said the marble began as the same kind of limestone. Long ago, as huge pieces of Earth's crust pushed together, some of the limestone was buried several kilometers deep. There, heat from Earth's interior and great pressure changed it into marble without melting it. The same pushing slowly raised the hills, while rain, frost and flowing water wore away the rock above. Along the creek near the hills, Wren saw that the limestone layers lean steeply instead of lying flat.
4The county measures how much sediment (sand, silt and mud) Cedar Creek carries past the Route 9 bridge each month. Wren compared the dry year with a normal year (Table 1).