21 research outputs found
Some aspects of the denudation of the chalk in the County of Wiltshire
The concern of the investigation described in
this thesis has been to examine certain drift deposits in
southern Wiltshire and to consider their distribution in
relation to the accepted interpretation of the landscape.
The area examined has been discussed by Wooldridge
and Linton (1955) but lies on the periphery of the tract
with which they deal, and their treatment is essentially
theoretical and not circumstantial.
In the course of the investigations which are
described, samples were collected from over two hundred
sites, surface indications of the geology were examined at
a substantially larger number of sites and more than thirty
sections were specially excavated in particularly interesting
locations.
The work is based not on the application of
refined geological techniques but on the ability to
distinguish, in hand specimens, common and distinctive rocks.
In important sections, samples of gravel of from 6 lbs to
2O lbs weight were secured and broken down into three
fractions (1⁄16"-5⁄32"; 5⁄32"-1⁄2"; 1⁄2" upwards); for
correlative purposes exact counts of the component materials were carried out for the two coarser fractions. Although
some of the individual examples encountered are problematic
the conclusions have invariably been based on a significant
volume of unmistakable material. The use of refined
correlative techniques to confirm the results has not been
attempted. This omission to some extent reflects the
nature of the investigation, which is best regarded as a
reconnaissance of a large area, describing evidence not only
in Wiltshire but on the Chalk and Eocene outcrops elsewhere
in We ss ex, for the most part on ground lying to the West
of the Salisbury Avon.
The results of the work are described in a
comparative study of landscape features using, so far as
possible, the whole geographical context of the ground
examined: the dissection and weathering of the surface,
the drifts and soils which rest upon it, and the relation
of the ground to relief both above and below it.
In Part One of the thesis an attempt is made to
establish the nature of the sub-Eocene unconformity.
A study of the Pleistocene river gravels about
Salisbury suggests that Eocene formations may have been
preserved on the western part of Salisbury Plain at the
beginning of the Pleistocene period. This result is inconsistent with the views adopted by Wooldridge and Linton.
The composition of the gravels examined suggests that the
Eocene formations in question did not resemble formations
now recognised in the main Eocene outcrops adjoining
Salisbury Plain.
A study of the Palaeogene outlier on Salisbury
Plain, at Clay Pit Hill, shows the gravels in the outlier to
be closely similar in terms of petrology to the distinctive
gravels in the Bagshot Beds about Dorchester in Dorset, and
a correlation with these beds is suggested.
A distinctive gravel from another Palaeogene outlier,
at Cley Hill, near Warrninster, is described. This gravel
at a height of about 800' O.D. appears to be preserved at
or near the level of its original deposition, and is
tentatively referred to the Oligocene period.
The evidence at Cley Hill and Clay Pit Hill is
taken to show that the Chalk suffered extensive erosion
during the Eocene and Oligocene periods.
A study of sites on the ground intervening between
the Palaeogene outliers on Salisbury Plain and the Bagshot
outcrop in Dorset provides new evidence on the nature of the
Reading Beds, which tends to confirm the results of the
investigations on Salisbury Plain.
In Part Two of the thesis drifts on the mid-
Tertiary surface of Wooldridge and Linton (1955) are
described.
Drifts on the Upper Greensand outcrop in the Vales
of Wardour and Warminster are believed to be of periglacial
origin and Pleistocene age. Drift on the Portland outcrop
in the Vale of Wardour appears to be the debris of Lower
Cretaceous formations and probably rests on an intra-Cretaceous surface.
On the Chalk a contrast is discerned between
ground occupied by deep very flinty drifts of residual
aspect and ground occupied by shallower flint drifts, also
of residual aspect. The shallow drifts are shown to be
associated with the relics of gravels incorporating Upper
Greensand, Lower Cretaceous and Jurassic debris.
The deep very flinty drifts are believed to
confirm the views of Wooldridge and Linton concerning the
mid-Tertiary surface, but the shallower drifts and
associated gravels are thought to rest on a separate surface,
and the Cretaceous and Jurassic debris is believed to imply
the competence of transverse gradients during the dissection
of the mid-Tertiary surface.
The evidence described in Part Two of the thesis is difficult to reconcile with the concept of marine
transgression across the Wiltshire Chalk. In Part Three
of the thesis drifts on the supposed marine plain are
described and the marine origin of the surface is rejected.
Specific evidence of a 'Pliocene' transgression
in Wessex is reviewed and a study of supposed 'Pliocene'
material refers the material in question to the Eocene period.
Problematic aspects of Pliocene stratigraphy and
the concept of superimposition from a Pliocene surface are
reviewed. Pinchemel's objections (1954) to a depositional
surface are accepted but his views on fluvial aggradation
are rejected in a study of gravel at Alderbury, near
Salisbury, referred by him to this aggradation. Difficulties
inherent in the concept of marine planation are recognised
and ground referred by Wooldridge and Linton to the Pliocene
marine plain is shown to be essentially indistinguishable
from their mid-Tertiary surface and is therefore believed
to be correlative with it. This view implies post-mid-Tertiary deformation of the Chalk.
In Part Four of the thesis a theory of drainage
evolution is developed to replace the hypothesis based on
a concept of late Tertiary marine planation.
Drifts, landforms and drainage morphology in the Vale of Wardour are shown to demonstrate a deformed,
subaerial planation surface, bevelling the outcrop of the
Chalk and earlier formations at a level below the mid-Tertiary
surface of Wooldridge and Linton. The present drainage
pattern is believed to have originated during the
deformation of this surface, probably in the Plio-Pleistocene interval.
This subaerial surface is also identified on
Salisbury Plain and the origin of the drainage pattern there
and in the Alderbury syncline is described.
The available evidence suggests that the subaerial
surface was elaborated during the Pliocene period following
the deformation of the Oligo-Miocene (mid-Tertiary) surface.
The character of the late Tertiary (Pliocene) surface
suggests comparisons with the surfaces described by King
(1962) and termed by him pediplains, although the nature of
the process involved remains doubtful.
In Part Five of the thesis some aspects of
Pleistocene erosion are treated.
Adjustments to the drainage pattern in the basin
of the Avon above Salisbury during the Pleistocene period
are discussed. This account is based on analyses of
gravels of the Avon and its tributaries about Salisbury.
The concept of successive stages of marine
planation in the Hampshire Basin daring the Pleistocene
period is rejected in a study of the composition and
morphology of the Pleistocene gravels.
An alternative scheme of subaerial planation is
suggested and some morphological evidence of this planation on the Wiltshire Chalk and on the Eocene outcrop in the Hampshire Basin is described.
The problems of scarp recession and the dessication of the dry valleys are reviewed and the concept of scarp recession is abandoned.. The Chalk escarpments are believed to have evolved where they now stand; the bearing of this idea on the dessication of the dry valleys is discussed.
Evidence of the catastrophic nature of Pleistocene
erosion is described and a concept of a 'clinatic cycle'
reflecting Pleistocene glacial fluctuations is tentatively
suggested.
In the conclusion to the thesis it is stressed
that the main importance of the work should be recognised not
in the chronological treatments of the results but in the
evidence that some of the fundamental concepts which have
guided geomorphological research in southern England have in the past been applied without a proper recognition of
the complications which may impair their value.</p