2 6 15 Lateral continuity the same bed can are continuous but can change along the length of it example a bed of sand might be mostly mud 5 or 10 miles away from where it is mostly sand Facies a zone of related rock characteristics Facies grade laterally but are all the same age Walther s law vertical change comes from lateral shift in facies What is a fossil Preserved remains of an organism Also a trace of an organism s activity examples bones and teeth or footprints Body fossil bones and trace fossil footprint Three key factors help something become a fossil 1 Have a part that can be fossilized a Shells teeth bones 2 Rapid burial a Burial protects from scavengers erosion b Vast majority of dead plants and animals are scavenged c In marine environments there are abundant benthic scavengers d Burial also protects remains from physical destruction by natural events e The chance for rapid burial is high where sediments are collecting but almost impossible where eroding 3 Become preserved turned into rock a Organic material is fragile b Usually turned into replaced by something durable Several processes of fossilization 1 Unaltered remains stuck in tar pits amber bog bodies frozen in ice 2 Altered replaced remains A Permineralization fill voids with minerals B Recrystalization replacement as tissues dissolve or rot they are replaced by minerals example a log s wood is replaced by quartz it is a quartz log Mineral almost always come from the circulating groundwater C Carbonization complex proteins and carbohydrates decompose leaving behind carbon which remains as a stable film as in leaf fossils A fossil is the same age as the rock that holds it Trilobites lived 521 Ma to 250 Ma Perch live 80 Ma to present Ages of specific organisms provide more precise age control