22 NORTH AMERICA/Continental Interior Figure Index map of North American physiographic provinces showing location of Interior Lowlands and Great Plains of Continental Interior wedged in between the Appalachian Mountains on the east and the Rocky Mountains on the west (From Kay and Colbert (1965), John Wiley & Sons.) This part of the Earth was relatively stable during the next several hundred million years, and as the seas advanced and retreated over the low relief surface, they deposited fairly uniform marine sediments, layer upon layer, over extensive areas The type and distribution of the sediments responded to the global change in sea-level as well as to changes in local conditions All of these events were recorded in the sedimentary section, which, as the history unfolded, was sprinkled liberally with long intervals of no record Because of the sensitivity to conditions in the accumulation of sediments in this type of environment, much of the history is not chronicled; in fact, it has been estimated that as much as 85% of time in any one place is not represented by the rock record This situation is aptly described as layer-cake geology The Foundation The foundation is the craton, which was formed by adding on (accretion of) material to the original