23 July 2013
Last week I showed a time lapse video of the large rock topple failure at Saint Jouin Breneval in Normandy, Northern France on 18th July. It turns out that there is an even better video on Youtube now:
Apparently the volume of the collapse was about 30,000 cubic metres. I have taken this screenshot of the rock topple as it developed:
This answers my question about the dust cloud just before failure – it was caused by the impact of material detaching from the top of the block as the rock topple developed. However, note that at the foot of the block the high stress levels during the rotation is causing the block to fragment. Note also in the video the secondary topple a few seconds after the main event.