Page 126 - Fundamentals of Geomorphology

LARGE-SCALE TECTONIC AND STRUCTURAL LANDFORMS 109

Figure 4.7 The chief morphotectonic features of a passive continent margin with mountains.
Source: Adapted from Ollier and Pain (1997)

where the valleys deeply incised into the escarpment, which fringe the west coast of peninsular India, are a great
although modified by glaciers, are still recognizable escarpment bordering the Deccan Plateau. The ridge
(Lidmar-Bergström et al. 2000). Some passive margins crests stand 500–1,900 m tall and display a remarkable
that lack great escarpments do possess low marginal continuity for 1,500 km, despite structural variations.
upwarps flanked by a significant break of slope. The Fall The continuity suggests a single, post-Cretaceous pro-
Line on the eastern seaboard of North America marks cess of scarp recession and shoulder uplift (Gunnell
an increase in stream gradient and in places forms a and Fleitout 2000). A possible explanation involves
distinct escarpment. Below great escarpments, rugged denudation and backwearing of the margin, which pro-
mountainous areas form through the deep dissection of motes flexural upwarp and shoulder uplift (Figure 4.9).
old plateaux surfaces. Many of the world’s large water- Shoulder uplift could also be effected by tectonic pro-
falls lie where a river crosses a great escarpment, as in cesses driven by forces inside the Earth.
the Wollomombi Falls, Australia. Lowland or coastal
plains lie seawards of great escarpments. They are largely Active-margin landforms
the products of erosion. Offshore from the coastal plain
is a wedge of sediments, at the base of which is an Where tectonic plates converge or slide past each other,
unconformity, sloping seawards. the continental margins are said to be active. They may
Interesting questions about passive-margin land- be called Pacific-type margins as they are common
forms are starting to be answered. The Western Ghats, around the Pacific Ocean’s rim.