Geology Reference
In-Depth Information
The reason why the upland surfaces of the Pine Barrens preserve periglacial structures
relates to hydrogeology. Although the current regional groundwater table is within 1-2 m
of the surface, this has not always been the case. Similar to chalk, the underlying sand
and gravel is highly porous and permeable. During the cold periods of the Pleistocene,
the regional water table would have dropped in accordance with the fall in Atlantic Ocean
sea level. At the same time, a lack of vegetation on the well-drained sandy and gravely
terrain would have allowed deep frost penetration. Under these conditions, moisture
would have migrated upward from the depressed water table towards downward-
advancing frost or permafrost. Although modern geotechnical wisdom (see Chapters 4
and 14) suggests that coarse-grained sediments are not usually associated with high ice
content, there is fi eld experience from present-day permafrost regions that demonstrates
ice volumes and porosities in such sediments can exceed 30% (see Chapter 7).
Following thaw-degradation and eventual disappearance of permafrost, the terrain of
the Pine Barrens regained its highly permeable nature. This has resulted in the low-level
upland surfaces being largely preserved from Holocene fl uvial landscape modifi cation.
The lack of post-colonial agriculture on the infertile sandy soils has been a further factor
that has allowed sand-wedge structures to be preserved in the near-surface sediments and
for palimpsests of the paleo-drainage to be visible on aerial photographs.
2.5. CONCLUSIONS
Three general conclusions can be drawn from these four regional examples.
First, a basic advance in our understanding of periglacial environments is to recognize
that the geomorphic footprint of periglacial conditions is not always achieved, and most
periglacial environments possess some degree of inherited paraglacial or proglacial
characteristics.
Second, the importance of lithology should not be underestimated in any consideration
of periglacial landscapes. Lithology determines not only the susceptibility of the landscape
to cold-climate modifi cation but also the speed of modifi cation and the degree to which
it can be preserved. Ground ice dynamics appear crucial. For example, the growth and
thaw of segregated ice near the top of permafrost and in seasonally-frozen ground is
largely a function of (i) frost susceptibility of bedrock and surfi cial deposits, and (ii)
hydrogeology. It is the eventual thaw of ground ice that largely controls periglacial land-
scape modifi cation because this infl uences the supply of sediment to hillslopes and streams.
If ground ice were not a factor, the similarities in landscape evolution between the hot
and cold deserts of the world would be striking.
Third, it appears that the preservation of relict periglacial features in now-temperate
regions relies largely upon their current desiccation and removal from runoff and fl uvial
activity.
ADVANCED READING
(a) General
Büdel, J. (1977).
Klima-Geomorphologie
. Gebruder-Bortraeger, Berlin. English translation, 1982,
by Lenore Fischer and Detlef Busche,
Climatic
Geomorphology
, Princeton, NJ, Princeton Uni-
versity Press, 443 pp. (especially sections 2.2 “The polar zone of excessive valley-cutting (the
active periglacial zone)” and 2.3 “The peritropical zone of excessive planation”).
