Geology Reference
In-Depth Information
Figure 11.
Lozère part of the Truyère river catchment (700 km
2
, Massif Central,
France): thickness of the saprolite (increasing thickness from yellow to red
and black) and the fissured layer (green colours) (from Lachassagne et al.,2001).
Figure 11 displays an example of such a map obtained in France, Truyère
river catchment, Massif central, 730 km
2
(Lachassagne et al., 2001), in a
region where no data from wells was available. The interface between the
saprolite and the fissured layer was systematically sought along roads and
trails, following routes perpendicular to the slopes. About 700 km of roads
and trails, distributed as evenly as possible over the study area (about one
linear km per km²), were reconnoitred. These field studies were accomplished
in two weeks. Based on observed or inferred interfaces, as well as on available
surveying data, the contour lines of the bottom of the alterites were plotted.
The observation grid was tightened in the zones where the bottom of the
saprolite is cut by vertical slip faults subsequent to the weathering (for the
most part these faults were contemporary to the formation of Oligocene
grabens). The contour lines of the bottom of the saprolite were then digitized,
turned into a figure field and interpolated in order to create a grid that can
be overlain on the DEM. The thickness of the saprolite layer was then
calculated by subtracting the altitude of the bottom of the saprolite from that
