We have examined quantitatively the effect of large (>200 m) long-period (>50 m.y.) sea level changes on the development of the landscape of old mountain belts. On the basis of the available data we have assumed that the rate of denudation of these old landscapes is proportional to the average regional elevation Ȳ by a constant (K ≈ 10-4/1000 years). The results are as follows: (1) In the absence of large sea level changes it will take in excess of 300 m.y. and more likely more than 450 m.y. to degrade a Himalayan size mountain belt (Ȳ=5000 m) to a peneplain (Ȳ=10 m). For an old mountain belt that has been eroded to a regional elevation of 500 m, it will take an additional 220 m.y. and more likely 390 m.y. to degrade to a peneplain. (2) In a system in which the river valleys are graded with respect to base level, the rate of downcutting is enhanced by the rate of uplift caused by the isostatic response to regional denudation. With sea level (base level) rising the rate of downcutting is decreased by the rate of rise. However, transgression of the river valleys cannot take place until the rate of river valley uplift caused by isostatic response to the denudation is equal to the rate of sea level rise. Under usual conditions of sea level rise (nonglacial) this will not occur until the landscape has been degraded to a regional elevation of 60 m or less. However, under these conditions the decrease in the rate of downcutting in the river valleys greatly enhances the rate of degradation. As a consequence, older mountain bells already eroded to a regional elevation of 500 m or less may be further degraded to a peneplain (Ȳ≤10 m) during an episode of persistent sea level rise at rates of several tenths of a centimeter per 1000 years for several tens of millions of years. With sea level falling, the landscapes evolve toward an equilibrium surface at which point the rate of degradation is equal to the rate of sea level fall. (3) We have applied these concepts to the post Triassic evolution of the ridge and valley province of the Appalachians. Sea level probably rose 300 m during the interval from 205 to 95 Ma, which was sufficient to degrade those areas with a regional elevation of 500 m or less to a peneplain. Subsequent to this time, from 80 to 10 Ma, sea level has fallen approximately 300 m, sufficient to cause emergence of the former peneplain to a regional elevation (Ȳ) of 215 m which is approximately equal to the present-day regional elevation of the AppalachianS since the ridge and valley province near Harrisburg. We have made a further test of this by attempting to restore the remanent peneplain surface (i.e., the Schooley Mountain surface) to its former elevation in the upper Cretaceous and thus estimate the height that sea level had to be at that time. By infilling the valleys with country rock up to 457 m, which is the present elevation of the presumed former peneplain remanent surface, and making the appropriate isostatic adjustments, we may conclude that at a minimum, sea level was 277 m above present in the upper Cretaceous. Finally, we conclude that large-scale peneplanation can only take place during episodes of significant sea level rise (>250 m) over long periods of time (>50 m.y.). Thus episodes of extensive peneplanation are indicative of sea level changes of this magnitude.