Geology Reference
In-Depth Information
(a)
(b)
Figure 7.10.
(a) A conical residual, the Meda de Rocalva, Serra do Gerez, northern Portugal-southern
Galicia. (b) A meda, known as the Sugarloaf, in the Organ Mountains, southern New Mexico
(Seager, 1981).
where water table fluctuations caused repeated cycles of wetting and drying, and where organic
inputs were optimal. The rock just below the surface was rapidly weathered and the rate of weath-
ering diminished gradually in depth. Shallow groundwaters evidently did not stand and cause
intense weathering a metre or two below the surface for flared forms are subdued. The surface
exposure was reduced to a small area. The effectiveness of weathering evidently diminished grad-
ually with depth beneath the surface to produce inclined, rectilinear slopes, and thus a conical
form overall (
Fig. 7.11)
.
Whether the larger medas are of similar origin is not known. The medas of Galicia, NW Spain
and northern Portugal are developed in a fine-grained leucogranite with well-developed steeply
