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