12thFebruaryPages of earth’s Past: Sedimentary RocksDrilling into the bottom of the North Sea, we encounter:oSoft mud and loose sand, silt, pebbles, and shells. Then:oSimilar materials that are more solidified. Then:oFragments of solid rock.Cemented sand and silt.Pottery-like clay.Cemented shells.Crystalline masses of salt.The drill reveals a transition. oSoft sediment changes into cemented sedimentary rock.oThis change occurs as burial depth increasesSedimentary Rock:oForms at or near Earth’s surface in one of several ways.Cementing loose clasts (fragments) of preexisting rock.Cementing together loose shells and shell fragments.Accumulation of organic matter from living organisms.Precipitation of minerals dissolved in water.Sedimentary rocks form layers like the pages of a book.oThe layers record a history of ancient environments.oThe layers occur only in the upper part of the crust.Sedimentary rocks cover underlying basement rock.Geologists define four classes of sedimentary rock: oClastic—loose rock fragments (clasts) cemented together. oBiochemical—cemented shells of organisms.oOrganic—carbon-rich remains of once living organisms.oChemical—minerals that crystallize directly from water. Physical and chemical weathering provide the raw material for all sedimentary rocks.Detrital (or clastic) sedimentary rocks consist of:oDetritus (loose clasts).Mineral grains.