The question is raised as to whether multiple erosion surfaces that have been described in a number of recent papers actually exist, and, if so, whether they have been correctly interpreted as products of repeated partial peneplanations.
Analysis of the criteria used for determining such surfaces indicates that some are not adequate. Particularly, the use of ridge crests, isolated hills, and spurs as markers of former erosion levels is unsafe. Too little correlation has been made between surface form and rock resistance.
Analysis of what happens after an old-age surface is rejuvenated indicates that stream-bed grading is very rapid relative to inter-stream degradation, so that the “treppen” concept of stair-step-like benches with bordering scarps retreating across a region following successive rejuvenations of a master stream cannot ordinarily be valid.
Multiple erosion surfaces at small vertical intervals are difficult to explain as products of sub-aerial erosion working toward intermittently lowered base levels, but they are entirely expectable results of another process that has been given too little attention. Progressive burial of surfaces planed by the waves of an intermittently advancing sea, or of pediment surfaces bordering a mountain range in an arid land, followed by uplift and stripping of the blanketing sediments, is not only a competent cause of the production of multiple surfaces but so inevitably leads to their production that such an explanation should receive first consideration where multiple surfaces can be definitely proven.
Application of this principle to the interpretation of the physiography of the Front Range of the Rocky Mountains is suggested.