30 NORTH AMERICA/Continental Interior of the North American Continent today, these rates are high and probably are not representative of the past Another approach to denudation rates is to determine how much material was present and now evoded away and to divide that amount by the time interval Neither approach is totally satisfactory, but both give a fair estimate The Sedimentary Sequence Figure 10 Other distinctive rock types that occur in the Contin ental Interior (A) Lignite from Cretaceous of central Kansas; (B) thin bituminous coal (Upper Pennsylvanian); (C) Pliocene volcanic ash, a distinctive geochemical signature makes it an excellent marker bed for correlation; (D) layered chert in limestone in the Flint Hills of central Kansas as floods or devastating wind storms More ‘normal’ rates generally prevail in between catastrophic events Rates of denudation can be determined by measuring the amount of material carried by streams and rivers in a unit of time This usually is reported as so many centimetres reduction in the land surface per millennium Because of the relatively high elevation With special conditions, unique rock sequences occur The North American Craton is one place where these special sequences – rhythmites and cyclothems – are preserved Rhythmites are alternations of two sediment types, such as the chalk and bentonite that developed in the Cretaceous (Zuni), or siltstone and the shales that are present in the Pennsylvanian (Absaroka) Cyclothems are repetitious sequences that occur repeatedly in the Permo-Pennsylvanian (Absaroka) The classic cyclothem records a single advance and retreat of the sea in an area, giving rise to a symmetrical rock cycle of non-marine to marine to nonmarine sediments; however, the ideal seldom occurs Because the transgressive part of the cycle usually is better represented (or better preserved) than the regressive part, most cycles are asymmetrical and many are incomplete or have components repeated Bundles of cyclothems have been termed megacyclothems and an example is given in Figure 11 Origin of cyclothems has been proposed as being the result of: (i) change of sea-level; (ii) tectonic movements; or (iii) climatic factors; the truth is that all three are probably responsible to some degree In Kansas alone, in just the Middle and Upper Pennsylvanian (Absaroka) section, there are parts or all of about 40 cyclothems Their formation probably was the result of changes in sea-level as a result of the waxing and waning of southern-hemisphere glaciers Sediments were accommodated spatially by the craton gradually sinking in relation to sea-level, perhaps partially as a result of the weight of the added sediments and the glaciation was caused by changes in the climate Structural Development The development of the structural elements on the craton to their present disposition has not been spectacular, but has been incremental and persistent The movement is recorded, not only by the sediments, but by the gaps in the record – the unconformities Much of the regional structural development on the craton has been by epeirogenesis; that is, up-and-down movement and tilting of the area in response to orogenic (tectonic) activity elsewhere Four major unconformities indicate four major changes in the structural regimen – the end