Geology Reference
In-Depth Information
13
Periglacial Landscape
Modifi cation
During the cold periods of the Pleistocene azonal processes undertook signifi cant
landscape modifi cation in the ice-free mid-latitudes.
Frost action in soils and on bedrock produced disturbed horizons and frost-
derived debris. The latter was transported by mass-wasting processes towards lower
elevations. Strong katabatic and zonal winds transported and redeposited fi ne
sediment (loess) derived either from outwash plains adjacent to the ice margins or
from the cold deserts of central Asia. In some areas, defl ation activity and locally-
derived dunes and sand sheets were important. Large rivers crossed the periglacial
zone, and ice-marginal channels (pradolinas), sandur, and proglacial water bodies
paralleled the ice margins.
Landscape modifi cation was by fl uvial incision, modifi cation of valley-side slopes,
snowmelt-induced erosion, and sediment transport on slopes. Mass wasting caused
valley-bottom aggradation. Thermokarst processes, described in Chapter 12, further
modifi ed landscapes.
13.1. INTRODUCTION
Evidence for past frost action is often ambiguous because the frost-action effects of
present climate must also be considered. Even when present frost action is clearly not
involved, the “relict signifi cance” of the phenomenon concerned depends upon an evalu-
ation of the lithological susceptibility of the soil or rock in question, and site-specifi c
conditions. Further problems arise because certain phenomena can be produced without
invoking frost action. Finally, anthropogenic activities can modify and even obliterate
structures. For example, it is easy to mistake a series of old plough furrows for a sequence
of frost-disturbed structures and old post holes for ice-wedge pseudomorphs. For these
reasons, the recognition of relict frost-action phenomena requires caution. Only where
several different features occur either together or in close association can one assume a
frost-dominated environment.
It is also relevant to ask how much of the present mid-latitude landscape refl ects its
periglacial legacy. Unfortunately, there is no easy answer to this question. Not only did
the cold-climates vary in intensity and duration in different areas, but different rock types
would have produced different landscape responses.
