Faulted pebble from conglomerate, Scottish Highlands

This pebble is a very fine grained volcanic rock (rhyolitic?), which was contained in a mechanically soft matrix of silt, sand, and clay. The pebble responded to tectonic stresses by fracturing and faulting. In particular, this rock is cut by a series of subparallel faults, movement along which lengthened and flattened the pebble. Faint slickenlines are apparent on some of the fault surfaces.

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 this rock because it is part of the classic “Old Red Sandstone,” which I had never seen in outcrop.   I find it intriguing that this pebble does not simply fall apart, given all of the faults that cut it.  Instead, there must be ‘cement’ sealing the faults, a cement deposited during deep ground water circulation or even syntectonic precipitation of vein material.

Age & Formation
Pebble from the Old Red Sandstone, Devonian age, ~380 Ma.