Page 137 - Fundamentals of Geomorphology

120 STRUCTURE

Plate 5.2 Whin Sill, a dolerite intrusion in Northumberland, with Hadrian’s Wall running along the top.
(Photograph by Tony Waltham Geophotos)

the Colorado Plateau. Eroded bysmaliths and laccoliths
may produce relief features. Traprain Law, a promi-
nent hill, is a phonolite laccolith lying 32 km east
of Edinburgh in Scotland. However, the adjacent tra-
a
chyte laccolith at Pencraig Wood has little topographic ( ) Phacoliths
expression.
Phacoliths are lens-shaped masses seated in anticlinal
crests and synclinal troughs (Figure 5.3a). They extend
along the direction of anticlinal and synclinal axes and,
unlike laccoliths, which tend to be circular in plan, are
elongated.Erodedphacolithsmayproducerelieffeatures.
Corndon Hill, which lies east of Montgomery in Powys,
Wales, is a circular phacolith made of Ordovician dolerite
(Figure 5.3b).
b
() Corndon Hill phacolith
Ashes and Dolerite Sediments
Volcanoes andesites
Volcanoes erupt lava on to the land surface explosively
and effusively. They also exhale gases. The landforms
built by eruptions depend primarily upon whether rock
NW SE
is blown out or poured out of the volcano, and, for
effusive volcanoes, upon the viscosity of the lava. Explo- Figure 5.3 Phacoliths. (a) Occurrence in anticlinal crests
sive or pyroclastic volcanoes blow pyroclastic rocks and synclinal troughs. (b) Corndon Hill, near
(solid fragments, loosely termed ash and pumice) out Montgomery in Wales, an eroded phacolith.
of a vent, while effusive volcanoes pour out lava. Source: Adapted from Sparks (1971, 93, 94)