Geology Reference
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Figure 11.5.
Suggested development of split rock.
The process may be initiated in the subsurface though splitting cannot occur until after expo-
sure. Given a corestone set in grus, evacuation of the matrix exposes the corestone as a boulder.
Instead of being supported on all sides by grus the weight of the boulder rests only on the still
buried part, and the stress regime changes from one of compression to one of flexure, with tension
on the upper side of the boulder and compression on the lower (Fig. 11.5). Any latent steeply-
inclined fractures, or other planes of weakness, thus tend to be pulled apart.
Some blocks and boulders have been split twice or more ( Fig. 11.3a). For these, frost riving
affecting secondary, latent fractures offers the most satisfactory explanation, for all occur in areas
subject to frequent freeze-thaw alternations, either at present or in the recent past. Even so, many
sit on a foundation too even and stable for the separate blocks to fall apart.
Some blocks defined by steeply inclined fractures have apparently been joggled or moved
( Fig. 11.6) , in places, as at Kokatha, in the northernmost Gawler Ranges, South Australia, by as
much as 2 m and on Dartmoor, southwestern England, by more than a metre. Such parted blocks
originally consisted of two cubic or quadrangular blocks that were, presumably, closely juxta-
posed, but which are now separated by a considerable gap. In some instances, both gap and mar-
ginal blocks are surmounted by other blocks.
These forms cannot be accounted for by unbuttressing and the two adjacent blocks rotating out-
ward in opposed directions, as suggested in explanation of split boulders. The blocks in question
are angular and could not roll apart, even if the blocks to either side were eliminated. In some
instances, the parted blocks are surmounted by other large residuals, the weight of which would
surely prevent the underlying masses from moving. Worth (1953) mentions lightning in relation to
such dislodged blocks, though the mechanism is not detailed, and there is in any case no evidence
suggestive of such strikes.
The parted blocks are not due to the weathering and erosion of sills of weaker rock, for no sign
of such materials is found in association with them. They cannot be attributed to the slippage of
blocks under gravity, for the parted blocks characteristically stand on sensibly horizontal bases.
Swelling clays that are capable of moving considerable loads are rarely, and certainly not neces-
sarily, present at the appropriate sites, and massive frost wedging cannot be cited as an explanation
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