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BGS Technical Report WA/97/03 15 December 1997
of sand and gravel. Along many minor tributary valleys, canalising or piping of streams has
allowed a superficial cover of colluvium (Head) to form.
Details:
A trench [2650 30861 along the route of the Derby Southern Bypass to straighten out the course
of Etwall Brook was cut into c. 2 m of reddish brown stoneless clay. Grey clay fiom a lower level
was also dug out.
5.4 Head
This is a variable deposit formed on slopes by a combination of colluvial, solifluction and
gelifluction processes. In modern times, with intensive arable cultivation of the steeper slopes,
colluviation has become an important depositional agent. In former times, most head was
deposited as a result of periglacial gelifluction and related activity. Head generally underlies
distinctive, nearly planar to gently concave ' solifluction terraces' which steepen gradually
uphill from the flat alluvium and fluvial terrace deposits in the bottom of the valleys to the foot
of convex bedrock outcrops. These surfaces are widespread along the Trent and Dove valleys
and their tributary streams.
Head lithology depends on the nature of the bedrock and drift deposits up slope. It typically
comprises slightly reddish brown, sandy to clayey, stony silt or silty clay. The stones, like
most of the sand content, are derived from reworking of the Anglian glacigenic deposits and
river terrace sands and gravels. Soliflucted mudstone of the Mercia Mudstone Group is a firm
red and variegated clay which would be difficult to distinguish from weathered in situ bedrock
were it not for its geomorphological form and possible stone content. The upper part of the
deposit is generally colluvium and is sandier and lighter in texture. Numerous minor degraded
sections in head occur in ditches across the area. The deposit is up to a few metres thick.
The colluvium or surface hill wash was largely produced during the Flandrian by the
vegetation stripping caused by arable farming. The processes of solifluction, and in particular
gelifluction, mainly operated during the last severe periglacial environment of the late
Devensian. Similar deposits dating to earlier cold phases are considered to have been
extensively removed or modified by the slope erosion which then occurred. They are found
deeply involuted into the gravels of the Etwall and Egginton Common terrace deposits.
Characteristic periglacial soil features, formed from the growth and thawing of ground ice,
which can be developed in more or less in situ pre-late Devensian deposits, include cryogenic
involutions, ice wedge casts and vertical stone orientation. Locally, these superficial contorted
deposits were noted extensively by Deeley (1886; in Bemrose and Deeley, 1896, pp. 508-509)
as 'underplight'. They have also been referred to locally as 'Valley Drift' (Fox-Strangways,
1905; Lamplugh et al., 1908). If the superficial deposits are thin enough, the structures
penetrate down into the underlying bedrock. Cryogenically involuted terrace gravels are
widespread in the southern part of$he area and many examples were seen during the survey
where the underlying bedrock was also affected. For example, the Etwall Sand and Gravel and
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