Fabric - Introducing Tectonics, Rock Structures and Mountain Belts - page 65

the layer; that is, the individual rods are
boudins , formed by the process of bou-
dinage , as described in Chapter 4 ( see
Figure 4.8). Alternatively, the layer may
have been deformed by microfolding,
in which case the rods may represent
crenulation fold hinges, or the inter-
sections of a crenulation cleavage on a
foliation surface, as described above.
Mullions are so called because of
their resemblance to the architec-
tural features of that name on some
older grand buildings such as cathe-
drals. In their geological context, they

are typically formed at the interface
between two sedimentary layers of con-
trasting strength, such as sandstones
and shales (or their metamorphosed
equivalents). As in the case of rodding
lineation, both folding and extension
can create a rounded profile at such
an interface, which typically assumes
a cuspate form with the cusps point-
ing into the more competent material
(Figures 7.6D-G). A famous example
of mullion structure is recorded in
Moine psammites at Oykell Bridge
in Sutherland, northern Scotland.

It is obvious from the above that
care must be taken to establish the
origin of these types of lineation before
attempting to interpret their structural
significance. For example, an elonga-
tion rodding will be parallel to the
local maximum strain axis, whereas
crenulation hinges may well be perpen-
dicular to it. Of course, both rodding
and mullion structure, of whatever
origin, may rotate towards the shear
direction under high strain, in which
case they will become indistinguish-
able from the elongation lineation.

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