Geology Reference
In-Depth Information
9.6.2
Origin
Before concluding that he was unable to account for rock doughnuts, Blank (1951) considered a
number of possibilities. He wondered whether waters held in the pit had impregnated and
indurated the surrounding rock. He could find no supporting evidence from his study area in
Te xas, and the low porosity and permeability of the crystalline rocks in which doughnuts are
developed renders the proposal inherently unlikely.
Blank (1951) speculated whether the flow of water over the surface of the granite domes he
studied had been disturbed by the standing water in the basins in such a way as to protect the imme-
diately adjacent rock surface, thus allowing the development of a raised rim. Experimental work
and field observations concerned with both wind and water confirm that such obstacles indeed
interfere with flow and induce further turbulence, but the effect is to increase, rather than decrease,
(c)
(d)
Figure 9.20.
(c) Twin fonts on granodiorite from Altar de Cabrões, Serra do Gêrez, northern Portugal. (d)
Half doughnut from Río Vilamés, Serra do Xurés, southern Galicia, Spain. The surface on
which these forms have developed is more than 300 Ky old.
