120
CLAUDE J. GRENOT
Fig. 4. Typical evidence of the weathering effects: ruiniform rock formations
sculptured by erosion (in one of the regs of the Tanezrouft).
relatively even relief is subjected to very high temperatures as well as to
extreme variations between diurnal and nocturnal temperature ranges, resulting in the breaking, cracking, and progressive crumbling of the rock
formations. The relief is also subjected to a nearly constant mechanical
action of the wind which, by blowing particles of sand with great force,
causes extensive erosion. The wind transforms granite and sandstone rock
formations into tortuous-appearing features (Fig. 4 ) . However, in certain
protected areas (shallow basins), an important sedimentation process does
occur, resulting in the formation of wind-blown sand dunes (Fig. 5 ).
The velocity of the wind may reach and even exceed 100 km/hour and
its effects on the ground surface are of great amplitude. At Reggan, the
wind mean monthly intensity is 14-28 km/hour. However, even more violent winds can occur each month reaching an intensity greater than 56
km/hour, and this is not uncommon. A maximal wind intensity of 104
km/hour was recorded in March, 1964.
Sandstone disaggregation owing to fluvial erosion is sufficient to account,
not only, for the presence of large dune massifs, but also for less important
sand formations scattered on the desert surface (Bellair, 1940, 1953).
The old concept which claimed that the Sahara was a "dried out ocean
floor" has long been rejected. Even the sand origin, just mentioned, is no
CLAUDE J. GRENOT
Fig. 4. Typical evidence of the weathering effects: ruiniform rock formations
sculptured by erosion (in one of the regs of the Tanezrouft).
relatively even relief is subjected to very high temperatures as well as to
extreme variations between diurnal and nocturnal temperature ranges, resulting in the breaking, cracking, and progressive crumbling of the rock
formations. The relief is also subjected to a nearly constant mechanical
action of the wind which, by blowing particles of sand with great force,
causes extensive erosion. The wind transforms granite and sandstone rock
formations into tortuous-appearing features (Fig. 4 ) . However, in certain
protected areas (shallow basins), an important sedimentation process does
occur, resulting in the formation of wind-blown sand dunes (Fig. 5 ).
The velocity of the wind may reach and even exceed 100 km/hour and
its effects on the ground surface are of great amplitude. At Reggan, the
wind mean monthly intensity is 14-28 km/hour. However, even more violent winds can occur each month reaching an intensity greater than 56
km/hour, and this is not uncommon. A maximal wind intensity of 104
km/hour was recorded in March, 1964.
Sandstone disaggregation owing to fluvial erosion is sufficient to account,
not only, for the presence of large dune massifs, but also for less important
sand formations scattered on the desert surface (Bellair, 1940, 1953).
The old concept which claimed that the Sahara was a "dried out ocean
floor" has long been rejected. Even the sand origin, just mentioned, is no