Slopes in arid zones
331
obtaining of these data are very variable (Oberlander, 1997a). On the South Sinai
Peninsula, Yair and Gerson (1974) calculated rates of 0.1 to 2 mm/yr from fault free face
retreats. The presence of a thin lava cover over a retreating free face in Northern
Arizona, dated by K/Ar, let Lucchitta (1975) obtain values of 6.7 mm/yr. Schmidt (1980,
1989b), analysing the geometry of the captured consequent valleys of the Colorado
Plateaus, obtained values of 0.3 mm/yr and 0.5 to 6.7 mm/yr. Other methodologies,
much criticized by several authors, are based on the pack-rat accumulations of Neotoma
existing in the mouths of Arizona caves and dated by C-14. The values obtained are
0.45 mm/yr (Cole and Mayer, 1982). Young (1985) studied the free face retreat from
the Lower Eocene erosion surface in northeastern Arizona, from a deduced position of
the scarp based on the location of the straight valleys and obtained values of 0.16 to
0.17 mm/yr. Schmidt (1987, 1988, 1996) calculated the retreat by graphical constructions
starting from talus flatiron profiles. In his 1996 publication he correlated the talus
flatirons with Illinoisian and Wisconsinan glaciations and the resulting values were 0.2 to
0.35 mm/yr. Most of these works were based upon supposed ages. Sancho et al. (1988)
fixed the date from archaeological remains in slopes and talus flatirons, obtaining free
face retreat rates of 0.3 mm/yr in the Tertiary formations from the Ebro Depression of
Spain. Temporal values are more accurate when different slope talus flatirons are dated
with C-14, in the central Ebro Depression, which have ash and carbon remains in their
contents (Arauzo et al., 1996b; Guti6rrez et al., 1998b). The rates obtained by these
authors are 0.9 to 1 mm/yr for the last 35,000 years. All these data reflect that the retreat
rates in arid zones of different world parts are about 0.1 to 7 mm/yr.
It has already been indicated that most of these values were obtained with simplified
indirect methods and with supposed ages. All together these constitute an important but
inconvenient method for correlation of these values, as long as there does not exist a clear
reliability of the methods and, therefore, in the results. Another problem is related to the
sinuosity of the free face. Generally, it supposes a straight front, which is more common as
much of the scarp thickness is; despite that it is not a universal rule. When using the
method of extrapolation of talus flatiron segments (Sancho et al., 1988) until their
intersection with the prolongation of the top of the free face, intersections are obtained at
different distances, probably due to the free face sinuosity (Figure 14.18). Nevertheless,
while having many intersection points, an arithmetic mean can be obtained that is much
closer to the actual retreat rate.
One of the most important characteristics, relative to the major or minor retreat
velocity, is related to the lithologic and structural features of the constituent material of the
free face, which at the same time controls the resistance degree to the erosive processes
(Schumm and Chorley, 1966; Nicholas and Dixon, 1986; Schmidt, 1989a). The
mineralogical composition of the rock is fundamental as it determines the reaction to
the weathering processes dominant in the area. The erosion rate is also influenced by the
dip of the layers (Howard and Selby, 1994), so that the higher the dip, the smaller the
volume of eroded rock, and vice versa. Equally, the thickness of the rock of the free face is
in inverse ratio to the retreat rate (Schumm and Chorley, 1966; Schmidt, 1987, 1989b).
The presence of important thicknesses of constituent rocks of the free face facilitates the
application of the acyclic model of the origin of talus flatirons of Koons (1955), supported
by Schipull (1980) and Schmidt (1987). With this origin, free face retreat takes place
starting from strong impulses, unconstrained by big rock falls, between which alternate the
331
obtaining of these data are very variable (Oberlander, 1997a). On the South Sinai
Peninsula, Yair and Gerson (1974) calculated rates of 0.1 to 2 mm/yr from fault free face
retreats. The presence of a thin lava cover over a retreating free face in Northern
Arizona, dated by K/Ar, let Lucchitta (1975) obtain values of 6.7 mm/yr. Schmidt (1980,
1989b), analysing the geometry of the captured consequent valleys of the Colorado
Plateaus, obtained values of 0.3 mm/yr and 0.5 to 6.7 mm/yr. Other methodologies,
much criticized by several authors, are based on the pack-rat accumulations of Neotoma
existing in the mouths of Arizona caves and dated by C-14. The values obtained are
0.45 mm/yr (Cole and Mayer, 1982). Young (1985) studied the free face retreat from
the Lower Eocene erosion surface in northeastern Arizona, from a deduced position of
the scarp based on the location of the straight valleys and obtained values of 0.16 to
0.17 mm/yr. Schmidt (1987, 1988, 1996) calculated the retreat by graphical constructions
starting from talus flatiron profiles. In his 1996 publication he correlated the talus
flatirons with Illinoisian and Wisconsinan glaciations and the resulting values were 0.2 to
0.35 mm/yr. Most of these works were based upon supposed ages. Sancho et al. (1988)
fixed the date from archaeological remains in slopes and talus flatirons, obtaining free
face retreat rates of 0.3 mm/yr in the Tertiary formations from the Ebro Depression of
Spain. Temporal values are more accurate when different slope talus flatirons are dated
with C-14, in the central Ebro Depression, which have ash and carbon remains in their
contents (Arauzo et al., 1996b; Guti6rrez et al., 1998b). The rates obtained by these
authors are 0.9 to 1 mm/yr for the last 35,000 years. All these data reflect that the retreat
rates in arid zones of different world parts are about 0.1 to 7 mm/yr.
It has already been indicated that most of these values were obtained with simplified
indirect methods and with supposed ages. All together these constitute an important but
inconvenient method for correlation of these values, as long as there does not exist a clear
reliability of the methods and, therefore, in the results. Another problem is related to the
sinuosity of the free face. Generally, it supposes a straight front, which is more common as
much of the scarp thickness is; despite that it is not a universal rule. When using the
method of extrapolation of talus flatiron segments (Sancho et al., 1988) until their
intersection with the prolongation of the top of the free face, intersections are obtained at
different distances, probably due to the free face sinuosity (Figure 14.18). Nevertheless,
while having many intersection points, an arithmetic mean can be obtained that is much
closer to the actual retreat rate.
One of the most important characteristics, relative to the major or minor retreat
velocity, is related to the lithologic and structural features of the constituent material of the
free face, which at the same time controls the resistance degree to the erosive processes
(Schumm and Chorley, 1966; Nicholas and Dixon, 1986; Schmidt, 1989a). The
mineralogical composition of the rock is fundamental as it determines the reaction to
the weathering processes dominant in the area. The erosion rate is also influenced by the
dip of the layers (Howard and Selby, 1994), so that the higher the dip, the smaller the
volume of eroded rock, and vice versa. Equally, the thickness of the rock of the free face is
in inverse ratio to the retreat rate (Schumm and Chorley, 1966; Schmidt, 1987, 1989b).
The presence of important thicknesses of constituent rocks of the free face facilitates the
application of the acyclic model of the origin of talus flatirons of Koons (1955), supported
by Schipull (1980) and Schmidt (1987). With this origin, free face retreat takes place
starting from strong impulses, unconstrained by big rock falls, between which alternate the