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Chapter 1: The Geological
Context
Steep-sided landscape of Coniacian
type close to Les Eyzies-de-Tayac-Sireuil.
Gently sloping topography of
Santonian type, Grande Beune.
The Black Perigord, the north-easternmost
region of the Aquitaine Basin, backs on to the crystalline platform of
the Massif Central. The area of this study, the lower valley of the
Vezere and its tributary valleys, covers two thirds of the region.
Archaeological and geological considerations (ill. 4) dictated its
boundaries.
The area, which is essentially limestone,
extends from Condat-Le-Lardin in the east to Limeuil in the west, where
the Vezere River joins the Dordogne River. The Lascaux Cave lies to
the northeast, a fair distance from the concentration of decorated caves
found around Les Eyzies-de-Tayac in the southwest - a region that
effectively boasts the greatest density of prehistoric remains in
Western Europe. There are no fewer than thirty-seven decorated caves
and rock shelters [3] around Les Eyzies-de-Tayac, and an even greater
number of Palaeolithic habitation sites, both under rock shelters and in
the open, including Le Moustier, La Madeleine, Tayac and La Micoque.
Location of the cave of Lascaux and
the studied karst region.
A succession of parallel strata follow
the course of the Vezere from the northeast to the southwest, extending
from the primary crystalline bedrock to rocks that were formed during
the Jurassic and Cretaceous periods. The Cretaceous period includes the
Coniacian, Santonian and Campanian stages (ill. 5). Two geological
faults have had a major effect on the area: the La Cassagne fault to the
east and the Campagne Saint-Cyprien fault to the west. Both have
an Armorican alignment (they run noticeably from the southeast to the
northwest). The Sarlat syncline -- a fold of stratified rock in
which the strata slope up from the axis was formed between these two
features of tectonic origin and was itself interrupted by several
secondary folds or anticlines. The latter correspond with valleys such
as the Manaurie and the Petite Beune, dictating their relative positions
and causing a dip in the course of the rivers flowing through the
valleys.
Around Montignac, marly limestones at the
base of the Santonian and a greater heterogeneity of the Coniacian
layers contributed in widening the valley. Along the western edge of the
drainage basin, the exposed Campanian layer also softened the landscape.
Between these two poles the scenery is more spectacular. Steep-sided and
narrow meanders coincide with the more resistant outcrops of the Middle
and Upper Coniacian. As a result, rock faces with an average height of
around 40 metres (ill. 2) were formed. These cliffs [4] developed
characteristically, both along the central course of the Vezere River
and in the valleys of its tributaries, particularly those of the Beune
rivers. The gentler landforms of the Santonian and the Campanian, which
are more varied and give the landscape a more subdued appearance (ill.
3), have developed above these formations.
5 Geological map of the lower Vezere
drainage basin.
The cliffs demonstrate a certain
uniformity. In general, they are split into two units of roughly equal
thickness, separated by a deep, almost horizontal linear niche marking
the boundary between the Middle and Upper sub-stages of the Coniacian.
This geological discontinuity stretches from La Roque-Saint-Christophe,
[5] the great terrace dominating the Vezere (ill. 6), to Pech-Saint-Sour,
downstream from Les Eyzies-de-Tayac, on the right bank. The niche is a
deep cavity, with an even floor and a convex ceiling, which sometimes
reaches over 10 metres in height.
6 The great terrace of La Roque-Saint-Christophe,
Peyzac-le-Moustier. This long horizontal incision marks the boundary
between the middle and upper layers of the Coniacian.
7 Stretch of the Grande Beune close
to the chateau of Commarque, Les Eyzies-de- Tayac-Sireuil. This flat-
bottomed valley was filled by peat, breccia and travertine deposits,
formed during the Holocene period.
This hollow played a major role in the
settlement of communities, not only during the Middle and Upper
Palaeolithic but also more recently, right up to the sixteenth century.
The role of the overhanging formation was fundamental for the
Palaeolithic population of the Black Perigord. It shelters the majority
of the important sites of the region, such as Le Moustier, La Madeleine,
Laugerie-Haute. Laugerie-Basse, Pataud and the terrace of the National
Museum of Prehistory at Les Eyzies-de-Tayac. Only a few remnants of the
Palaeolithic floor have survived the effects of subsequent activities on
the latter terrace, but we can assume that there was an important human
presence here during different periods because it is located at the
confluence with the Grande Beune and faces southwest.
Along the Vezere, the altitude of this
long niche differs by some 20 metres. It follows the warp of the
strata that form the Sarlat syncline, which were shaped by tectonic
movement. The height of the niche ranges from 35 metres above the valley
floor at La Roque-Saint-Chrisrophe to 15 metres near Tursac, a third of
the way along its course, before rising once again to 35 metres on the
terrace of the National Museum of Prehistory.
In the valleys of the tributaries,
particularly those of the Benne rivers, the rock-face formations follow
a slightly different pattern. Their profiles consist of a series of
steps, which make access to the different terraces much easier.
During the course of the past millennia,
several geological phenomena have altered the landscape by increasing
the erosion of slopes and the movement of sediments. This has affected
both the discovery and the preservation of prehistoric sites,
settlements or sanctuaries. A number of rock shelters and cave entrances
have no doubt disappeared, having been filled in and concealed.
These processes are an important factor
in the study of the region's archaeology. Along the Grande Beune, a
survey of the valley floor revealed 14 metres of sedimentary deposits
close to its confluence with the Petite Beune. Sampling showed the
presence of limestone gravels and peat, an accumulation of deposits
generated by the formation of downstream travertine obstructions
composed of a mixture of calcite and vegetation. The formation of these
deposits seems to have occurred at the beginning of the Holocene period,
well after the Ice Age and the passage of man at Lascaux. Observations
made close to the chateau of Commarque (ill. 7) show that aggradation
(the deposition of material by a river, stream or current) was equally
prevalent along the Grande Beune during the same period. Traces of
medieval activity on the rock walls, particularly alignments of
scaffolding holes, are well below the present level of the Beune. This
suggests that the surface of the land at the beginning of the second
millennium was some 3 to 5 metres lower than it is today.
As a result of the altered profile of the
tributary valleys, the long sub-horizontal niche marking the transition
from Middle to Upper Coniacian has been almost entirely covered up (ill.
8). In fact, it is only visible downstream along the first 2 kilometres
of the valley of the Beune rivers. Above Les Combarelles it is buried by
Holocene peat and breccias; the northern Grand Abri, one of the springs
fed by this decorated cave, marks the last point at which it can be
identified. The change in this tributary's profile also means that the
majority of traces of Palaeolithic occupation are hidden today,
undoubtedly buried below several metres of sediment.
This could explain the relatively low
number of inhabited rock shelters discovered in these sections of the
tributary valleys: the filling-in process only applies to them, while
the Vezere itself remains unaffected by these geological events. The
particular distribution of habitation sites in the Black Perigord, with
a significant number of prehistoric sites in the Vezere Valley but
comparatively low numbers in the valleys of its tributaries, especially
the Beune, is thus largely dictated by phenomena operating on a
geological scale over the course of the past millennia.
8 Schematic representation of the
different levels of sedimentation in the valley floors of the Vezere,
the Grande Beune and the Petite Beune rivers.
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