Geology Reference
In-Depth Information
Near the surface, the Chalk is frequently weathered into thin, platy fragments and a
calcareous “paste,” or colluvium, often mantles lower slopes. For over a century, these
and other phenomena were attributed to the action of frost (Bull, 1940; Reid, 1887;
Williams, 1987). Recent experimental studies confi rm that the chalk is highly frost-
susceptible (Murton et al., 2001). The Chalk outcrops are also veneered by loess and
coversand (“brickearth”) that locally reach a thickness in excess of 3-4m. Most loess
deposits date from the last glacial, the Late Devensian, but more extensive records in the
Somme basin, northern France, indicate that accumulation began as early as 900ka
(Antoine et al., 1998, 2003).
A further characteristic is that the near-surface 5-10 m of Chalk bedrock is frequently
brecciated (Murton, 1996a; Murton et al., 2003) (see Figure 13.2). This is attributed to
frost heave within and below an ice-rich layer in chalk that formed when permafrost was
present during the cold periods of the Pleistocene (Murton and Lautridou, 2003). The
brecciated chalk has been reworked as a diamicton that is known locally as “coombe rock”
(Reid, 1887), or more generally as “head” (Bates et al., 2003; Geikie, 1894).
The Chalk landscapes are also characterized by a network of valleys, many of which
are either dry or occupied by misfi t streams. Many are asymmetrical with steeper slopes
facing west or southwest. The apparent freshness of the valleys, the density of the network,
and the absence of large headwater catchments favor the explanation that they were
incised when the surface of the chalk was perennially-frozen and when semi-perennial
snow banks would have nourished fl ow (Bull, 1940). Relatively rapid valley incision
would have been facilitated by brecciated bedrock. Wind, snow, and differential mass
wasting at the time of incision probably produced the asymmetry (French, 1972a; Ollier
and Thomasson, 1957). When permafrost degraded, the Chalk regained its permeable and
porous nature to leave the valleys mostly dry.
In summary, the lithology of the chalk favored not only modifi cation by frost action
but also subsequent preservation of landscape. The modifi cations are recorded by brecci-
ated bedrock, surfi cial (“head”) materials, and dry valleys. As permafrost degraded, the
cold-climate landscape elements became largely fossilized.
2.4.2. Pine Barrens, Southern New Jersey, Eastern USA
The Pine Barrens of Southern New Jersey (latitude 39-40° N) lie south of the southern
limit of Late-Pleistocene (Wisconsinan) ice, and earlier glaciations did not extend into the
region. Today, the area is a preserved wilderness tract of over 1.4 million acres of heavily-
wooded lowland that extends through the center of the Outer Coastal Plain (Figure 2.8).
It consists of fl at, sandy, and gravely terrain with maximum elevations of 25-30 m a.s.l.
The area is underlain by sand and gravel of Cretaceous and Tertiary age. A surface cover
of fl uvial sand and gravel, known locally as Bridgeton Formation, is regarded as Late
Miocene to Early Pleistocene in age. In many respects, the geology is not unlike the
Beaufort Plain of Banks Island, described earlier.
Denudation rates since the Late Miocene on the Atlantic coastal plain have averaged
10 mm/year (Stanford et al., 2002). Slope retreat has replaced low-level uplands with low-
angled pediments, thereby preserving relict topography at higher elevations. Surfi cial
mapping (Newell et al., 2000) indicates the lower slopes are mantled by several sequences
of cold-climate colluvium.
Broad shallow valleys contain terraces that grade into wetlands in the valley bottoms.
Although the dense forest cover makes landforms diffi cult to identify, air photographs
taken in the early 1930s, when the region had been largely deforested by nineteenth-century
lumbering activity, reveals the current drainage to be misfi t and located within large
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