I collected this rock in 2008 in the Scottish Highlands where the Old Red Sandstone is cut and displaced by the Highland Boundary fault. I was on a field trip led by Midland Valley Exploration, Ltd. I like the fact that this pebble is a miniature thumbnail of the kind of faulting that can deform regions. Besides, how often can you carry a graben around with you in your pocket?
This pebble is a very fine grained volcanic rock (rhyolitic?), which was contained in a mechanically soft matrix of silty, sandy, and clayey material. The pebble responded to tectonic stress by fracturing and faulting. In particular, this rock is cut by inward-dipping normal fault zones, slip along which has lengthened and flattened the pebble overall. The central part of the pebble is a graben structure. Faint slickenlines are apparent on some of the fault surfaces.
Age & Formation
from the Old Red Sandstone, Devonian age, ~380 Ma.