With the help of probing, relief photointerpretation and altimetry, the formation of an organic basin located between São José dos Campos and Caçapava was studied. This basin probably was originated by slow settlement through accomodation of layers on the tertiary substract below alluvial plains. This phenomenon favored the organic accumulation under the present level of the river. The preservation of the southern basin of the river without being destroyed by meandric oscillation, is due to the tendency of the river-bed to dislocate toward the north, as can be proved by the terraces. These conditions should prevail in other areas of the same river and which may have deep organic layers.