In Iceland we flew over the spectacular Gullfoss waterfall.  This is a wonderful site, although it was pretty cold, as the frozen spray around the waterfall demonstrates:

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Just downstream is an amazing outcrop of columnar basalt in the gorge wall .  This cannot be easily viewed from the ground (it is on the wrong side), but from the air one gets a pretty good view:

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As well as being a genuinely increadible rock outcrop, this is a very nice illustration of the control that joints play in controlling rockfalls.  Let’s zoom into a part of the image above:

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To the right of the columns is a section of heavily-jointed rock.  Here (relatively) small pieces can detach – and indeed below this section of cliff there are some blocks sitting on a ledge. The result is a face that is comparatively planar.  The columnar basalt.itself yields rockfalls much less easily.  To detach, a block needs to topple, or to fracture, both of which are quite hard to achieve.  The result is a rockface that, at least along a horizontal line, has a great deal more relief.  Below the basalt is an almost unjointed section – this probably erodes primarily through abrasion of debris carried by the river when it is in flood.  Leaving a smooth but undulating, surface.

It is for this reason that we often consider that the discontinuities (joints) are more important than the rock itself when trying to understand potential rockfalls.