The Art of Field Collection Fossil excavation is often a mix of force and finesse. Once the overburden (loose rock and soil) are bulldozed away, the "capping-layer" (a layer of hard, black limestone) is removed with sledgehammers to expose the finely laminated, fossil-bearing rock. Thin strips of steel are carefully hammered into the edges of these blocks to take advantage of naturally occurring separations between layers. Once a slab is lifted, scientists check for fossils on its underside, as well as on the surface of the newly exposed layer below. A very thin layer of rock (the matrix) generally covers the fossils, which can be both a blessing and a curse. The matrix will protect the fossil as it is sent back to the Museum, but it can also hide small fossils if the sunlight is directly overhead; for this reason, fieldwork is usually suspended between 11:30 am and 3:30 pm. Any fossils that are found are marked and cut out from the surrounding rock using a rock saw. Fossils are then carefully packed into wooden crates and brought back to The Field Museum.