O Desequilíbrio do quadro natural de Franca, SP e a formação das voçorocas

Abstract

The disequilibrium of Franca city (S.P.) natural landscape and the formation of the gullies is a work that corresponds to one part of a doctoral thesis entitled: "Estudo Geomorfológico das Voçorocas de Franca (S.P.) "', presented by Dr. Neuza Machado Vieira from the Faculdade de Filosofia, Ciências e Letras de Franca.

There were studied all the natural factors that condition the formation of the gullies which are kinds of typical erosion from unbalanced areas.

The climate contributes to the formation of the gullies, due to the volume, duration, intensity and frequency of rain~. which are conditioned by the chronological distribution throughout the seasons of the Year.

Concerning the lithology, the gullies develop in recent and Mesozoic lands of sandy constitution, crossed by clay layers and based upon a more impermeable stratum that constitutes the level of local basis in the development of the erosion valleys.

The relationship which exists between the development of the gullies and the evolution of. the longitudinal profiles of the water streams is also intimate.

As for the topography there were considered: the degree of declivity, the shape and extension of the slopes of the hills. The erosion is greater in strong inclinations, convexly shaped and quite broad - as the ones that can be seen at Franca City.

Finally, it was considered the vegetation element changed by man's presence. The ancient protecting "cerrados" of the ground were destroyed, breaking the light equilibrium that existed between climate, ground, topography and vegetation.

The gullies are a kind of relief that reflects not only the disequilibrium phase of the landscape but also the transformation of the slopes from concave to convex in order to achieve a dynamic equilibrium.