Sedimentary Rock Fossiliferous Limestone

Associated Smithsonian Expert: Cara M. Santelli, Ph.D.

Dr. Cara Santelli visits a cave in eastern Tennessee (USA) to take samples of fungi that live in subterranean environments.

Photo by Sarah Carmichael, Appalachian State University

Dr. Cara Santelli is a research geologist and curator with the Department of Mineral Sciences at the Smithsonian National Museum of Natural History. Santelli grew up on the “Iron Range” of northeastern Minnesota, a region famous for its banded-iron formations. As a child, she loved digging in the dirt outdoors and bringing home pockets full of rocks, each of which was unique and beautiful. She always knew she wanted to be a scientist or engineer, but did not realize that her future lay in geology until she took a mineralogy course as an undergraduate at the University of Wisconsin. In 2007, Santelli earned a doctorate in marine geomicrobiology through a joint program of the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institute, and she joined the Smithsonian staff in 2010. She studies the impact of microbial activity on mineral formation, rock-weathering processes, and remediation of contaminated environments. One of the biggest thrills of her career was her dive 2.4 km (1.5 miles) underneath the Pacific Ocean in the deep-submergence vehicle Alvin. She still comes home from field excursions with pockets full of rocks.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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About Biochemical Sedimentary Rocks

When snails, corals, shellfish, and marine microorganisms die, their empty exoskeletons, rich in calcium carbonate or silica, pile up at the bottom of oceans and seas. The shells break up or decay into small pieces, called bioclasts or biochemical sediments. Over long periods of time, the upper layers of debris compress the lower layers, squeezing out excess water or air trapped between the shell fragments and breaking them down even further. Eventually, silica or calcite cements the individual fragments together to form a rock. The chalk you use to draw on the sidewalk comes from the shells of microorganisms that lived millions of years ago.

View of the red-sandstone Smithsonian Castle at dusk, Washington, D.C., USA
Photo by Ken Rahaim, Smithsonian Institution

Rocks as Building Materials

Humans have long prized granite, an igneous rock rich in the minerals quartz and feldspar, as a building material because it does not crack easily and can be polished. The Smithsonian's National Museum of Natural History building was built out of four different types of granite from four different states on the East Coast of the United States. Other rocks that make up the building include roof tiles of slate, and museum floors made of terrazzo, which is crushed marble and granite with cement filler. Elsewhere in Washington, D.C., marble, granite, and bluestone gneiss went into the Washington Monument; marble was used inside and outside the Lincoln Memorial; and red sandstone makes up the Smithsonian Castle on the other side of the National Mall. The nearby National Archives, Department of Justice, and Internal Revenue Service buildings have limestone facades and granite foundations.