Geology Reference
In-Depth Information
steeply inclined terrain and generally occur close to the surface, typi-
cally at less than 30m depth. They can often be traced laterally for
hundreds of metres. Most sheeting joints are young, geologically, and
some have been observed to develop explosively and rapidly as tensile
fractures in response to unloading (Nichols, 1980). Others are propa-
gated to assist in quarrying, using heat or hydraulic pressure
(Holzhausen, 1989). Their recent origins and long persistence without
rock bridges differentiates them from most other joints and from most
bedding, cleavage or schistosity-parallel discontinuities.
Sheeting joints are common in granite and other massive igneous
rocks but also develop more rarely in other rock types, including
sandstone and conglomerate
(Figures 3.44
and
3.45)
. Some sheeting
joints develop at shallow dip angles, for instance, during quarrying,
where high horizontal compressive stresses are locked in at shallow
depths. In Southern Ontario, Canada, for example, high horizontal
stresses locked in following glacial unloading, often give rise to quarry
floor heave and pop-up structures accompanied by opening up of pre-
existing incipient discontinuities such as bedding planes and schistose
cleavage (Roorda et al., 1982). Where there are no pre-existing weak-
ness directions, new sub-horizontal fractures may develop in otherwise
Figure 3.44
Sheeting joint
through arkosic
sandstone, Ulu
r
u
(Ayers Rock),
Australia.