Tentative comparison of planation surfaces occurring under warm and under cold semi-arid climatic conditions

Abstract

Some terrace-like planations were found to occur in the river-valleys of Middle Poland. Analyses both geomorphical and geological - show that these forms are of the type of pediment. This is evidenced by the lobe-like outline of the upper limit of these forms - which the writer calls slope planations - by the values of gradient, the presence of island-elevations occurring oridinarily in the prolongation of the denudational spurs as well as by the geological structure of the slope-planations, the surfaces of which resulted from the truncation of the bedrock material, that is predominantly glacial or glaciofluvial, but neither fluvial, nor terrace-like. These surfaces are coated with some discordant, mainly allochtonous covering formations, due to congelifluction or sheet-wash. The veneer of covering material being very thin, shows that these surfaces of planation were - like those of pediments are said to be - merely surfaces of transit.

 Planations of a similar type were found to occur also in upland, inter-valley areas. They are here surrounded by trunk remnants, residual hills and hillocks, their geomorphic features among which most characteristic is the one relating to the zone of contact between the plain and the commanding elevation - as well as their outstanding geological properties being analogous to those displayed by the valley slope planations.

 The writer believes the origin of pediments to be not entirely dependant on the qualitative differentiation of the processes operating upon the retreating slope and thereby modeling the surface of planation. He thinks the degree of concentration of different processes - belonging frequently to one general category to be of much greater importance. In the formation of the slope-planation  discussed, predominant was the role of rill-wash on the slopes and sheet-wash on the surface of planation as well as that of concentrated congelifluction, which carved corrasional troughs on the slopes and of sheet-congelifluction which failed to produce any visible morphological effects on the pediments.

 The resemblance of these slope-planations in Middle Poland to the pediments occurring in tropical arid sub-tropical regions affords evidence of far-reaching dynamical analogies between them. It thus seems to justify the conclusion that there is some close resemblance between semi-arid and periglacial morphogeny. Geomorphic aridity together with the resulting relationship in the character of soil and vegetation cover, constitutes a fundamental feature common to both these regions. Intensive development of slopes is conditioned by mechanical weathering - which is greatly effective with regard to both these morphogenetic types - by the lively course of mass-movement  and by the intermittent character of water-flow which is either ephemeral or seasonal.