Geology Reference
In-Depth Information
Paleo-sandur surfaces are not easy to recognize in the mid-latitudes today because of
vegetation growth and subsequent modifi cation by land-use practices. Where ancient
paleo-surfaces have been preserved, aerial photographs sometimes reveal palimpsests of
these large paleo-channel systems. This is the case for the Pine Barrens of southern New
Jersey, Eastern USA (Farrell et al., 1985).
The following discussion concentrates upon the fl uvial modifi cations that occurred
further from the retreating ice margins, that is, in the Pleistocene periglacial zone
senso-stricto.
13.4.1. Major Rivers
Brief mention of the large-scale paleo-hydrology of mid-latitudes is justifi ed because ves-
tiges of early non-periglacial landscapes complicate recognition of cold-climate features.
For example, in northwestern Arctic Canada and in Alaska, complex changes to the
present-day Yukon and Mackenzie drainage systems were effected by late-Cretaceous
tectonic activity, followed by the various Cordilleran and continental glaciations during
the Quaternary (Duk-Rodkin et al., 2004; see Figure 2.2). Likewise, in Western Europe,
the Pleistocene proto-Rhine drainage fl owed across broad lowlands now occupied by the
North Sea. The proto-Thames also drained eastwards towards the North Sea, probably
joining with the proto-Rhine.
Evidence for these former fl uvial systems can be found in fl ights of terraces and upland
surfaces. In northwestern Canada and Alaska, high-level benches and pediment-like sur-
faces refl ect these old landscape systems (Duk-Rodkin et al., 2004). In Europe, the best
known terrace systems are those of the Thames, Rhine, and Maas catchments (Gibbard,
1988; van Huissteden, 1990; van Huissteden and Vandenberghe, 1988). In the North Sea
basin itself, “cold” fl uvial sediments, present in boreholes within the Dutch sector, provide
proof of the major river systems that extended across this lowland when sea level was lower
than today during the glacial periods (Laban and van der Meer, 2004).
There is a possibility that many so-called “cryoplanation” terraces, described in the
Pleistocene periglacial literature, from the hard crystalline uplands of both Europe and
northwestern North America, relate to these early landscape systems. As such, they may
not have a cryogenic origin but are largely inherited features that were merely “trimmed”
by cold-climate conditions (see Chapter 9). It is signifi cant that there is little evidence
for the current formation of cryoplanation terraces and cryopediments in today's high
latitudes.
The cold-climate nature of many of the ancient fl uvial deposits that are found in the
mid-latitudes is inferred from the presence of frost-fi ssure pseudomorphs, involutions, and
faunal remains contained within them. Sedimentological studies suggest a cyclic pattern
of river incision and subsequent infi lling (Vandenberghe, 1993) that corresponds to the
start and end of each cold period (Figure 13.9A). In theory, fl uvial erosion would have
been favored at the beginning of the cold stages because slope and riverbank stability
would have been maintained by the degrading vegetation cover relative to a decline in
evapotranspiration and an increase in runoff. At the cold period maximum, sediment
supply and discharge would probably have peaked, and aggrading, braided-channel
systems would have been characteristic. Then, during the transition back to the warmer
interstadial or interglacial conditions, there would have been incision due to the reduction
in sediment load. Although somewhat speculative, this model is supported by stratigraphic
observations. For example, Figure 13.9B shows a typical channel-fi ll sequence from a Maas
river terrace in the Southern Netherlands. From the analyses of paleo-fl uvial systems in
