Considerazioni sulle problematiche proprie delle correlazioni cronologiche tra forme e processi

Abstract

The paper considers some aspects of chronological and strati graphical correlations between Quaternary processes and forms typical of glacial valleys and terraced plains, which may involve a certain amount of difficulty of interpretation, especially when a given process is no longer in progress, or when a sequence of processes took place in a given region. With regard to the glacial valley environment, a comparison is made between processes, present active and inactive forms of debris deposition along long portions of the floor of two Himalayan vaileys (upper Langtang and Khumbu valleys, Nepal) and their foreseeable future state if the present ice tongue would almost or completely disappear. The comparison shows that this kind of reconstructions may markedly differ from actual situations owing to the retreating complex dynamics of glacier tongues of "Himalayan" dimensions. There are two reasons for this: (1) the dead snout of a decaying glacier decreases in size by reduction of its thickness; thus a pause in its retreat is not necessarily highlighted by end-moraines; (2) it is possible that moraines lying across the main valley derive from the snout of a tributary tongue, which partly occupied the valley floor even after the retreat of the main tongue; this situation may render uncertain the identification of which tongue was responsible for their formation. Problems related to terraced alluvial plains are examined by making reference to the Piedmontese plain. Problems arise from the fact that: 1) a fluvial terrace as a form is the combination of a depositionai form (representing the alluvial element), an erosionai form (the scarp bordering the terrace), and a residue (terrace remain surface); 2) erosive episodes leading to land terracing may start from several, not necessarily time-related causes such as climatic changes, base level changes owing to a river capture, etc.; 3) watercourses that shaped the Piedmontese terraced plain flowed down from the Alps valleys, which means that their flow patterns in Pleistocene times had to be influenced by the periodic presence of large ice-tongues in the valleys. Thus, it is doubtful that correlations: anaglacial phase = increased flow rate, increased load transport, tendency of the river to evolve to alluvial dynamics; and cataglacial phase = reduced flow rate, reduced load transport, tendency of the river to evolve to erosive dynamics, are of general application. Pedological studies may be of help, although - in the writer's opinion - any reconstruction of the evolutionary trend of a series of forms (whether the expression of an erosion or deposition process) can be regarded as reliable unless its consistency with the dynamics of processes which generated them, is verified.