<P> Facies can be distinguished in a number of ways: the most common are by the lithology (for example: limestone, siltstone or sandstone) or by fossil content . Coral, for example, only lives in warm and shallow marine environments and fossils of coral are thus typical for shallow marine facies . Facies determined by lithology are called lithofacies; facies determined by fossils are biofacies . </P> <P> Sedimentary environments can shift their geographical positions through time . Coastlines can shift in the direction of the sea when the sea level drops, when the surface rises due to tectonic forces in the Earth's crust or when a river forms a large delta . In the subsurface, such geographic shifts of sedimentary environments of the past are recorded in shifts in sedimentary facies . This means that sedimentary facies can change either parallel or perpendicular to an imaginary layer of rock with a fixed age, a phenomenon described by Walther's Law . </P> <P> The situation in which coastlines move in the direction of the continent is called transgression . In the case of transgression, deeper marine facies are deposited over shallower facies, a succession called onlap . Regression is the situation in which a coastline moves in the direction of the sea . With regression, shallower facies are deposited on top of deeper facies, a situation called offlap . </P> <P> The facies of all rocks of a certain age can be plotted on a map to give an overview of the palaeogeography . A sequence of maps for different ages can give an insight in the development of the regional geography . </P>

Where are the earth's sedimentary rocks generally found