I collected this in 2009 in the vicinity of the tiny village of Lykios, west of Megalopolis on Mt. Lykaion in the Peloponessos. It is truly a special display of ideal joint properties. Rare to find such features so well displayed.
When this rock was buried, strain energy produced by stretching/extension of this rock layer caused it to break in tension along parallel planar joint faces. A joint surface is not produced all at once, but instead propagates (at half the speed of sound) through the brittle rock layer in which it is found. The patterns of plumes and fringe markings can be interpreted in terms of direction of propagation of individual joints. The feathery plumes, for example, converge in a direction that is opposite to the direction of propagation.
Age & Formation
Joint fringes and plumose markings on perfect joint faces in radiolarian chert, Peloponessos, Greece. Rock is Jurassic. Jointing probably occurred in late Cretaceous and early Tertiary.