Landscape
Geology
Much of the landscape character of North Wiltshire is dominated
by underlying geology of the middle to late Jurassic period,
with elements of rock from the later Cretaceous period. The
age of the rock bed moves roughly from the older Middle Jurassic
to the west through the Upper Jurassic to the Lower and Upper
Cretaceous in the southeast.
In the west of the district the landscape is underlain by
limestone, more specifically the 'Great Ootite Series'. This
forms the eastern edge of the Cotswolds. Forest Marble limestone
dips southeast, gradually giving way to Cornbrash limestone
which yields well drained soil for corn production.
The deeply eroded and incised valleys to the south, By Brook
and its tributaries, expose the underlying Fullers Earth and
pockets of Lower Jurassic Sands are revealed.
The limestone gives way to the undulating landscape of Kellway
day and the higher strata of the Oxford Clays towards the
east. The dominance of the underlying Upper Jurassic geology
is disrupted by alluvium deposits and river terraces of the
Avon and Thames and their major tributaries.
The Thames crosses the northern fringes of the district from
west to east and the Avon runs north to south both cutting
through the underlying clay. Within the Kellway Beds are pockets
of Kellway sand, providing free draining ground in this heavy
clay landscape.
To the east of the Avon Valley, the landscape rises in a
significant scarp slope, where thin beds from the Corallian
Series of the Upper Jurassic period are present. The higher
ground is dominated by Coral Rag [stone] which in places
gives
way to the more fertile lower Calcareous Grits.
Further east, the Corallian series gives way to an undulating
Landscape as the underlying fine mudstone of Kimmeridge Clay
becomes dominant. The presence of this clay follows low tying
ground below the Cretaceous chalk scarp, rising in the south
east of the district.
The southeast corner of North Wiltshire is exclusively Cretaceous.
These include the Lower Greensands of the Lower Cretaceous
period (a thin bed in which silicaceous iron-rich sands and
sandstones prevail) which area evident at the base of the
Cretaceous Scarp. Gautt Clay forms the next layer and Chalks
overlie this forming the more resistant layer, which forms
the higher ground of plains and downs. Lower Middle and Upper
Chalk are both present. This Cretaceous landscape forms part
of the extensive Wessex Downs, which extends both to the south
and east.
Lyneham Geology
The western and southern areas of the parish are situated
on the Corallian ridge which runs southwestwards from Wheatley
(Oxon) to Calne. Within an area bounded to the north by the
Chippenham Swindon road, to the east by the Hilmarton Lyneham
Road, and to the south by the Preston Lyneham road, beds of
Red Down Clay alternate with beds of Red Down Iron Sand.
East of a line from Church End to Trow Lane the clay gives
way to the Coral Rag of the ridge again. In the extreme south-eastern
corner of the parish around Thickthorn Farm a belt of Red
Down Clay, which runs south-westwards from Greenway to the
boundary with Hilmarton, is succeeded by a bed of Red Down
Iron Sand. In the most south-easterly corner of the parish
Thickthorn Farm stands on an extensive bed of Kimmeridge Clay.
The northern limits of the Corallian ridge determine the
northern, western, and part of the southern, boundaries of
Lyneham. Bradenstoke, Lyneham, and Preston all lie on the
Coral Rag of the ridge, while West Tockenham and Shaw Farm
are situated on the Red Down Iron Sand.
In the west and south of the parish the Corallian ridge
reaches a height of c. 400 ft., and rises gradually to over
475 ft. west of Bradenstoke. The dip slope of the ridge falls
gently away south-eastwards to the clays and sandy soils in
the east of the parish, where, at Thickthorn, on the Kimmeridge
Clay, the land drops away to 350 ft.
For the most part, by virtue of its somewhat exposed position
on the Corallian ridge, the parish presents an open and treeless
landscape, except in the north, where the spring action of
Lilly Brook has caused the erosion of sand beneath the Coral
Rag at Blind Mill. Here this process has resulted in the incision
of a steep-sided and thickly-wooded gully. North and cast
of Preston the parish is traversed by a network of streams
and the soil there is wet and heavy. These streams are tributaries
of Cowage Brook and meet above Littlecott (Hilmarton).
One stream has gorged a narrow, curving valley, now flanked
by trees and known as the Strings, through which it flows
southwards from Freegrove. Another tributary forms the eastern
part of the southern boundary of the parish, while a third
flows southwestwards from Middle Hill, past Preston, and thence
to Littlecott. Most of the land was under pasture in 1968,
although there was some arable cultivation on the lighter,
sandier soils, especially around Shaw Farm.
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