4 relative dating principles accommodating type 2 diabetes in the chinese american family
Law of Superposition: The principle of superposition states that in a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect.
Thus, the layer at the bottom of a sequence is the oldest, and the layer at the top is the youngest.
Lateral continuity: The principle of lateral continuity states that sediments generally accumulate in continuous sheets within a given region.
If today you find a sedimentary layer cut by a canyon, then you can assume that the layer once spanned the area that was later eroded by the river that formed the canyon.
Thus, once a fossil species disappears at a horizon in a sequence of strata, it never reappears higher in the sequence or, put another way, extinction is forever.
Smith’s observation has been repeated at millions of locations around the world, and has been codiﬁed as the principle of fossil succession.
Uniformitarianism: The principle of uniformitarianism states that physical processes we observe operating today also operated in the past, at roughly comparable rates, so the present is the key to the past.
Canal digging provided fresh exposures of bedrock, which previously had been covered by vegetation.Once the relative ages of a number of fossils have been determined, the fossils can be used to determine the relative age of the beds containing them.For example, if a bed contains Fossil F (from the succession speciﬁed above), geologists can say the bed is older than a bed containing Fossil A, even if the two beds do not crop out in the same area.They then go further by interpreting the formation of each feature to be the consequence of a speciﬁc geologic event.Examples of geologic events include: Deposition of sedimentary beds; erosion of the land surface; intrusion or extrusion of igneous rocks; deformation (folding and/or faulting); and episodes of metamorphism.