The Fundamental Laws of Geology Several principles or “laws” are fundamental to the geologic interpretation of a sequence of events

Rock Cycle Review

Uniformitarianism  “The past is the key to the present”  Basically states that geological processes and natural laws that occur today have been occurring throughout time in the same manner and with the same intensity  Ex. Depositional and environmental conditions affecting bivalves (clams) today likely affected bivalves during the Cretaceous (80 MYA)

Original Horizontality  Most sedimentary rocks are formed by particles that settle to the bottom of rivers, lakes and oceans under the influence of gravity, and form essentially in horizontal layers.  Therefore, if sedimentary rocks are found in an inclined or folded attitude, they must have undergone movement after deposition

Superposition  Sedimentary rocks are formed by the accumulation of numerous layers  In any undisturbed sequence of layers, the oldest should therefore be on the bottom and the youngest or last- deposited layer would be on the top  This is assuming that the layers have not been overturned or inverted by folding or faulting

Lateral Continuity  Most thick layers of sediment were originally deposited over geographically extensive areas  Subsequently, they are covered with overlying layers and lithified into sedimentary rocks  Uplift, erosion, faulting and folding have n many cases disturbed the original lateral continuity  Ex. Grand Canyon – even though the layers have been eroded, the rock formations found on the north rim can still be found on the south rim

Crosscutting Relations Crosscutting Relations  Any rock unit or fault that cuts across other rock units is younger than the rock units through which it cuts  This applies to faults that cut rock layers or igneous intusions that cut through rock layers

Components  The components of a layer of sedimentary rock are older than the date of the deposition of that rock layer  Since sedimentary rocks are made of weathered older rocks, the components of a sedimentary rock must be older than the rock itself

Fossil Succession  Each layer in a succession of rock strata can be identified by the distinctive fossils it contains  Therefore, plant and animal fossils succeed one another in a recognizable order through the geologic record