|
|
|
Surface features of the cobbles
Quartzite cobble showing a strongly pock-marked surface. The pockmarks are flat, circular to elongate depressions grazed by countermovements of cobbles in contact. The whitish colour results from micro-fracturing in the contact zone (see below: halo). The distinct sub-parallel fracturing, WNW - ESE trending, is associated with a completely smooth surface. Not any shearing has acted on the cobble.
Cratered surface of a quartzite cobble. Note the strong radial fracturing and the central mounds in the miniature craters.
Single crater with a distinct central mound.
Stereo photograph of the crater in the image above.
A 4-cm diameter crater with a central mound. Note the sharp contours of the crater.
A quartzite cobble with two opposite craters. Again note the sharp contours indicating brittle fracturing. For the origin of the craters from spallation see below.
Photograph by courtesy of Tilman Bucher
Quartzite cobble showing two collision contacts with strong radial fracturing. Note the widely open tensile fractures without any shearing of the cobble. This proves short-time deformation and excludes tectonic compression, which would have sheared the component within the conglomerate.
The strongly squashed quartzite cobble, however not broken to pieces, is incompatible with long-lasting tectonic stress in the conglomerate, but indicates short-time deformation under high confining pressure.
[top] |
(Copy to the e-mail address bar please)