Surficial sedimentary and volcanic rocks are not distributed uniformly through time. The masses lying between restored surfaces have been classified in several ways. Long, laterally lenticular bodies in “mobile” or “orthogeosynclinal” belts contrast with areally more equidimensional forms of “stable” regions and cratons. The forms of these great surficial rock masses change with time.

The geometric shapes are reconstructed from thicknesses measured from surface outcrops or from wells, or interpreted from geophysical observations. Measurements suggest magnitude of subsidence though differences can be significant, particularly where rocks are thin. Rates of deposition are to be distinguished from rates of subsidence. Subsidence controls deposition in that great thickness cannot much exceed subsidence prior to or during deposition, but subsidence can so exceed deposition as to produce sediment of deeper-water origin. Deposition in some environments progresses continually and gradually, but much is quite spasmodic. Thickest sections tend to lie in belts of greatest mobility during deposition, which are sites of later deformation and intrusion. Estimates in the belts of maximum thicknesses have errors through misinterpretations of structures and effects of metamorphism and intrusion.

Rates of subsidence depend also on judgments of spans of time. Few moments in geologic history are dated accurately; estimates of intervening horizons are judgments from physical and biological records. Present chronologic scales have been influenced by knowledge of thicknesses. Even if the dating of standard sections were precise, methods of correlation from other sections are inadequate. Thus estimates of spans of time as well as those of thicknesses have inaccuracies that affect conclusions on rates of subsidence.

Few estimates of maximum rates of subsidence have been compiled, and very few of the average thickness of surficial rocks for parts or the whole of continents. Most estimates in geosynclinal belts are for sections of later systems, particularly of the Tertiary, because older rocks are deeply buried or deformed or metamorphosed if preserved at all. For spans as long as periods, deformation at rates more than 500 feet (150 m) in 1 million years is uncommon; for shorter spans, parts of periods or epochs, rates occasionally exceed 1000 feet (300 m) but have greater probable error because of relatively greater doubt in measurement of time spans. Maximum recorded rates are 2000 feet (600 m) or more, represented not only in orthogeosynclinal troughs, but in fault-bounded geosynclines and foredeeps in the margins of stable cratons. Subsidence in basin form within the cratons occasionally approaches 500 feet (150 m) for spans of a few million years, but subsidence tends not to be long-continued or extensive.

That the rates of maximum subsidence have increased through time is not conclusively shown, for there are records of very rapid subsidence in the Paleozoic, and the greater number of later records should give apparent increasing number of thick maxima. Estimates of average thickness of surficial rocks for continents are strongly influenced by interpretations of continental genesis.