Gullying has been studied in the Piracema basin, a tributary of Bananal river which drains toward the middle course of Paraíba do Sul river (boundary of São Paulo and Rio de Janeiro states). Most gullies developed along the topographic hollow axis of structurally-controlled amphitheater-like landforms, in connection with the expansion of the regional channel network (83%, for N=117), among which 52% are stabilized and 31% are active; the other 17% of observed cases developed in separately, along minor topographic hollows of the headwater zones. Gully extension is controlled by topographic aspects: gradient and hollow density within the amphitheater or above channel heads.

A typical case has been measured at Bela Vista amphitheater since 1982 showing an average retreat rate on the order of 1,300 m³/year. This gully progressed through the axis of a major topographic hollow, growing finger-tips toward the upper, minor hollow axis. The main trunk and its minor tips developed parallel to the underlying sub-vertical joints. Average monthly erosion rates tend to increase linearly with average monthly rainfall. One of the gully-tip is retreating faster and faster as it gets closer to the watershed divides.

Gully formation and network growth follow the "headward spring sapping" model stated by Dunne (1980,1990). Local, temporary subsurface flows exfiltrate at the base of thick, loose and highly permeable Late Quaternary fills; recharge is favored by vegetation-fauna-soil interactions near the surface: grass-roots and the network of pipes constructed by "Saúva" ants provide higher hydraulic conductivity within the dense-root zone of about 30 cm thick (1.26 x 10^-4 cm/sec); at 30 cm and 60 cm it decreases to around 3.52 x 10^-5 and 4.45 x 10^-5 cm/sec, respectively. Once seepage erosion removes the sediment fill, retreat rates tend to accelerate due to the interference of the regional aquifer; as shown by a piezometer network. Gullying is progressing toward to the watershed divides and may cross it, reproducing the previous joint-controlled routes of the Pleistocene-Holocene transition.