Joint controlled blocks -- statements of the obvious

 

One of the Kjove Land giant erratics in East Greenland.  To call this a "joint controlled block" might or might not be accurate, but the term tells us nothing of importance about its origins.

To call a small clast or larger boulder a "joint controlled block" is to make a statement of the obvious, since the term can be applied to virtually all lumps of rock in nature.  I have come across the term several times recently, in the context of the bluestone monoliths of Stonehenge and the Newall Boulder......

https://nora.nerc.ac.uk/id/eprint/502372/1/WD-94-7.pdf

When bedrock breaks up, under the influence of weathering and erosive forces, it will always break along the planes or lines of weakness within the rock mass -- and these may coincide with bedding planes, faults, shear fractures  or simple joints caused by internal stresses such as drying out, cooling, contraction or compaction.  The term "brittle failure" is often used. Geologists also refer to fractures, fissures and planar or sub-planar joints, and also "cleavage" to describe splitting in metamorphic rocks such as slate.   Pressure release is one process that can create both vertical and horizontal jointing and slope failure.  I have talked about this in association with glacial erosional processes.  On a large scale regional tectonic forces can also be involved.  Whatever the scale, tensile stress is the thing......

Bedding planes and foliations are not the same as joints, but differences in texture and mineral arrangements might be sufficient for stresses to develop within the rock and to result in parallel or sub-parallel joints.

The only chunks of rock that can NOT be referred to as natural "joint controlled blocks" are massive rocks split with the ancient "wedge and feathers" method (involving the drilling of lines of holes) or rocks cut with a diamond saw -- so we can forget about those with respect to the Neolithic!!

Glacially eroded facets may or may not coincide with joints in a clast.  It all depends how a clast is positioned with respect to the flowing or abrading medium -- in this case, moving ice.

So to refer to a clast -- whether in a Stonehenge context or not -- as a "joint controlled block" tells me virtually nothing about it, although it might sound terribly scientific.  When I see a faceted or striated clast with abraded edges, gouges and chatter marks or other percussion fractures, I will continue to call it a glacial erratic if it is not locally derived, paying due respect to the processes that are at work in our environment. Geology gets us so far, but not far enough for a geomorphologist.

 Let's just call a spade a spade.........

The Newall Boulder -- to call it a "joint controlled block" is to miss almost everything significant about its appearance and its origins