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Climatic Geomorphology
The importance of climate changes has been discussed at great length for several types
of landscapes. In desert regions with scarce land cover, the variations of vegetation can
produce important modifications of the hydrologic conditions. Climate characteristics in
the centre of the Ebro Depression in Spain agree with maximum sediment yields of the
curve proposed by Schumm (1965). According to this graph, small variations of
temperature and annual precipitation can modify the percentage of land cover and, so, the
sediment yield.
On the other hand, in periods with more humidity (pluvial) the processes that generate
sediments are more active on the free face, mainly undercutting and slumping. This is due
to the increase of infiltration and groundwater flow (Ahnert, 1960). When a decrease in the
temperature occurs, the number of frost days per year raises and, therefore, the sediment
yield in the scarp will be higher due to frost action.
The climatic interpretation of alluviation and trenching stages leads to the establishment of a correlation with known climate sequences. The two most recent accumulation
stages are well represented in all the slopes in northern Spain (Guti6rrez and Pefia, 1989,
1992, 1998) and have been dated in many places by means of archaeological and
radiometric techniques. The most recent, post-medieval accumulation can be correlated
with the Little Ice Age (1450 to 1850) (Grove, 1988). The accumulation corresponding to
the stage previous to the latter was formed during the Cold Period of the Iron Age (2900 to
2300 BP) (Lamb, 1977; Gribbin and Lamb, 1978; Grove, 1979; Harding, 1982). Guti6rrez
and Pefia (1989, 1992, 1998) correlated these two accumulation stages with those cold
periods for the northeastern Iberian Peninsula (Figure 25.9).
At present there are only two dates for accumulations older than the above mentioned,
both obtained by means of C-14 at Mezalocha and Chalamera in Zaragoza province and at
Monteagudo, Sofia province in Spain (Guti6rrez et al., 1998a; Guti6rrez and Mart/nez,
2001). In principle, there is a good correlation with the Heinrich events established from
the analysis of deep-sea cores in the North Atlantic (Guti~rrez et al., 2005). Ages obtained
for the accumulations could correspond to H3 (27 ka BP) and H4 (35 ka BP) (Heinrich,
1988; Bond et al., 1993, 1997). A Heinrich event is represented by glacial sediments
coming from the melting of icebergs with scarce foraminifera. The stages of formation of
these events are related to movements of a great abundance of icebergs, resulting from the
detachment from northern ice sheets. These circumstances produce a widespread cooling
of the superficial waters of the ocean. As a result, cool winds from the Atlantic came into
the Ebro Depression. This fall in temperature produced a decrease in evapotranspiration
and therefore, an increment in the availability of water in soils, causing an increase in land
cover. These conditions seem to have been suitable for the accumulation of sediments on
slopes. These Heinrich events occur in periods of extreme atmospheric cooling and are
followed by net and short interstadials (Dansgaard et al., 1993).
From the above, it may be deduced that accumulation stages coincide with cool-climate
epochs, whereas the stages of trenching correspond with warm ones in which talus
flatirons are formed. Therefore, the sequences of talus flatirons record climate changes.
In semiarid environments, those changes in slope systems that generate talus flatirons,
are produced in relatively small time spans. In dryer areas (Figure 25.10), however,
climatic changes on the order of 100 kyr are needed to generate these relict slopes (Gerson,
1982; Gerson and Grossman, 1987; Bull, 1991; Schmidt, 1994, 1996).
Climatic Geomorphology
The importance of climate changes has been discussed at great length for several types
of landscapes. In desert regions with scarce land cover, the variations of vegetation can
produce important modifications of the hydrologic conditions. Climate characteristics in
the centre of the Ebro Depression in Spain agree with maximum sediment yields of the
curve proposed by Schumm (1965). According to this graph, small variations of
temperature and annual precipitation can modify the percentage of land cover and, so, the
sediment yield.
On the other hand, in periods with more humidity (pluvial) the processes that generate
sediments are more active on the free face, mainly undercutting and slumping. This is due
to the increase of infiltration and groundwater flow (Ahnert, 1960). When a decrease in the
temperature occurs, the number of frost days per year raises and, therefore, the sediment
yield in the scarp will be higher due to frost action.
The climatic interpretation of alluviation and trenching stages leads to the establishment of a correlation with known climate sequences. The two most recent accumulation
stages are well represented in all the slopes in northern Spain (Guti6rrez and Pefia, 1989,
1992, 1998) and have been dated in many places by means of archaeological and
radiometric techniques. The most recent, post-medieval accumulation can be correlated
with the Little Ice Age (1450 to 1850) (Grove, 1988). The accumulation corresponding to
the stage previous to the latter was formed during the Cold Period of the Iron Age (2900 to
2300 BP) (Lamb, 1977; Gribbin and Lamb, 1978; Grove, 1979; Harding, 1982). Guti6rrez
and Pefia (1989, 1992, 1998) correlated these two accumulation stages with those cold
periods for the northeastern Iberian Peninsula (Figure 25.9).
At present there are only two dates for accumulations older than the above mentioned,
both obtained by means of C-14 at Mezalocha and Chalamera in Zaragoza province and at
Monteagudo, Sofia province in Spain (Guti6rrez et al., 1998a; Guti6rrez and Mart/nez,
2001). In principle, there is a good correlation with the Heinrich events established from
the analysis of deep-sea cores in the North Atlantic (Guti~rrez et al., 2005). Ages obtained
for the accumulations could correspond to H3 (27 ka BP) and H4 (35 ka BP) (Heinrich,
1988; Bond et al., 1993, 1997). A Heinrich event is represented by glacial sediments
coming from the melting of icebergs with scarce foraminifera. The stages of formation of
these events are related to movements of a great abundance of icebergs, resulting from the
detachment from northern ice sheets. These circumstances produce a widespread cooling
of the superficial waters of the ocean. As a result, cool winds from the Atlantic came into
the Ebro Depression. This fall in temperature produced a decrease in evapotranspiration
and therefore, an increment in the availability of water in soils, causing an increase in land
cover. These conditions seem to have been suitable for the accumulation of sediments on
slopes. These Heinrich events occur in periods of extreme atmospheric cooling and are
followed by net and short interstadials (Dansgaard et al., 1993).
From the above, it may be deduced that accumulation stages coincide with cool-climate
epochs, whereas the stages of trenching correspond with warm ones in which talus
flatirons are formed. Therefore, the sequences of talus flatirons record climate changes.
In semiarid environments, those changes in slope systems that generate talus flatirons,
are produced in relatively small time spans. In dryer areas (Figure 25.10), however,
climatic changes on the order of 100 kyr are needed to generate these relict slopes (Gerson,
1982; Gerson and Grossman, 1987; Bull, 1991; Schmidt, 1994, 1996).