Such flows also produce thick sequences of hummocky beds by sustained bidirectional flow.
Concentrated sediment flows produce thick strata sequences by abrupt deposition from liquefied suspension or evenly bedded strata by flow transformation to a tractive current.
The third and fourth steps become much more difficult since lithologies and fossils cannot be traced across continents and from continent to continent.
Twentieth-century geologists taught the familiar maxim: "The present is the key to the past." But now that catastrophic processes are widely employed to describe the strata record, twenty-first century geologists are wondering whether "marine flood sedimentation is the key to the past." Geological strata and their contained marine fossils provide critical evidence that the ocean once covered the continents, even the highest continental areas.
Isochron methods avoid the problems which can potentially result from both of the above assumptions.
Isochron dating requires a fourth measurement to be taken, which is the amount of a different isotope of the same element as the daughter product of radioactive decay.
In addition, it requires that these measurements be taken from several different objects which all formed at the same time from a common pool of materials.The Tippecanoe Sequence also covers much of North America and may well extend into Europe and Africa.There are also intercontinental redbed sequences, intercontinental tuff beds, and coal-bearing strata cycles.Scientists divide the Earth into a number of periods - the "Geological time-scale", according to the rock types and sort of fossils found in each one.These divisions are pretty arbitrary, like all such man-made classifications, but they at least can serve as useful labels, so we can orientate ourselves in our journeys through deep time.