Page 256 - Fundamentals of Geomorphology

FLUVIAL LANDSCAPES 239

this valley floor then leaves remnants of the former valley Terrace formation and survival
floor on the slopes of the deepened valley as rock-floored Four groups of processes promote river terrace formation:
terraces. Rock-floored terraces are pointers to prolonged (1) crustal movement, especially tectonic and isostatic
downcutting, often resulting from tectonic uplift. The
rock floors are cut by lateral erosion during intermissions movements; (2) eustatic sea-level changes; (3) climatic
changes; and (4) stream capture. In many cases, these
in uplift.
factors work in combination. River terraces formed by
stream capture are a special case. If the upper reach of a
lower-lying stream captures a stream with a high base-
Alluvial terraces level, the captured stream suddenly has a new and lower
baselevel and cuts down into its former valley floor. This
Alluvial or accumulation terraces are relicts of allu- is a one-off process and creates just one terrace level.
vial valley floors (Plate 9.9). Once a valley is formed by Crustal movements may trigger bouts of downcutting.
vertical erosion, it may fill with alluvium to create a flood- Eustatic falls of sea level may lead to headward erosion
plain. Recommenced vertical erosion then cuts through from the coast inland if the sea-floor is less steep than
the alluvium, sometimes leaving accumulation terraces the river. Static sea levels favour lateral erosion and valley
stranded on the valley sides. The suites of alluvial ter- widening. Rising sea levels cause a different set of pro-
races in particular valleys have often had complicated cesses. The sea level rose and fell by over 100 m during
histories, with several phases of accumulation and down- the Pleistocene glacial–interglacial cycles, stimulating the
cutting that are interrupted by phases of lateral erosion. formation of suites of terraces in many coastal European
They often form a staircase, with each tread (a terrace) river valleys, for instance.
being separated by risers. A schematic diagram of the ter- Climatic changes affect stream discharge and the grain
races of the upper Loire River, central France, is shown size and volume of the transported load (Figure 9.12).
in Figure 9.11. The classic terrace sequences on Rivers Iller and Lech,

Plate 9.9 Alluvial terraces along the Broken River, Castle Hill, New Zealand.
(Photograph by David Knighton)