GEOMORPHOLOGY 125 
The lateral erosion of the Rio La Troya has probably extensively 
destroyed this gravel formation and the levelled part has been 
completely covered with modern alluvions. At Carrizal the gravel 
is limited by a fault. 
It is very probable, that such a gravel also occurs in the Jaguel 
bolson (in the north), but the recent alluvial cover does not permit 
any observations, except in one place, in the immediate vicinity of 
the settlement of Jaguel, where there rise some low hills over the 
plain, which are composed of similar gravels, but slightly hardened 
and not much dislocated. One may assume, that a great part of 
the filling masses of the bolson consists of the same gravels, as 
the bolson was re-deepened also in posttertiary time. 
After the destruction of the young tertiary conglomerate cover 
the erosion pursued in the continental sandstones lying on the ba- 
sement block sides. This denudation process appeared in a suc- 
cessive formation of sheet flood terraces, on lower and lower levels. 
2) Sheet Flood erosion. 
This characteristic kind of mountain degradation was more closely 
explained by Mc Gee.! The author states with regard to the ge- 
neral conditions of sheet flooding as follows: „The first requisite 
for typical sheet flooding is a precipitation so rapid, as to exceed 
immediate absorption of the dry earth and immediate evaporation 
in the dry air. An attendant condition is, that the precipitation 
shall be rapid. A third condition is, that the soil shall be readily 
pervious only in limited degree. or to limited depths. And this 
condition is met with on the lightly veneered baselevels adja- 
cent to the mountains, where the mantle only is porous and the 
underrocks sound and hard; it is not met with in the deeper central 
portions of the valleys, where the permeable sands are of conside- 
rable depth.“ The gradient of the slope must be about 300 feet 
1 Mc Gee: Sheet flood erosion. Bulletin of the Geological Society of America 
Vol. VIII. 1897. Pages 87—112. 
