In
geology, a mullion describes a
linear,
cylindrical structure comprised of
elongated rods or
columns 20
millimetres to 2
metres across and up, to 100 metres long.
The surface may be smooth or corrugated and define a lineation parallel to fold axes. Mullion structures are most common in strongly deformed metamorphic rocks, and may form by buckling of the surface between strong and weak rock beds.