THE GEOLOGY OF NORTHAMPTONSHIRE
England's Jurassic County
Source: Mike Lewis
Head of
Geography (with Geology), Northamptonshire Grammar School,
Northampton, UK.
The geology of Northamptonshire is
almost entirely made up of sedimentary strata pertaining to the
Jurassic period (195 to 135 million years BP). The exceptions to
this trend are patchy deposits of Quaternary glacial sands,
gravels and till from the Wolstonian glacial cycle and the
alluvial deposits found in the main tributary valleys of the
river Nene. Although the Jurassic period is associated in
palaeontology with the greatest abundance of dinosaur remains
('The Age of the Reptiles'), the above picture is somewhat
misleading in that no significant dinosaur fossils have ever been
recovered from the county.
The Jurassic system in the
British Isles runs almost continuously from the Cleveland Hills
in Yorkshire to Lyme Regis in Dorset. It is from the latter
deposits that much has been recovered in the way of Jurassic
fossils. In Northamptonshire, the Jurassic system comprises two
series of rocks: Lias clays (of which there are three divisions -
Lower, Middle and Upper) and the Oolitic series (comprising
limestone, ironstone and Northampton sands). Oxford clays also
form a minor component of Northamptonshire's geology and are
found towards the south-east.
Click here
for a geological map.
The Lower Lias tends to
dominate the north-western extremes of the county. It forms a
reduced, blue-coloured clay reaching average depths of 550ft and
supports a fertile soil, although suffering from waterlogging in
water and dessication and cracking in summer. The exposure of the
Lower Lias is attributed to the extensive fluvial erosion of the
river Nene. There are outcrops of Middle Lias within the county,
although this band is very thin. It has some importance
hydrologically by giving rise to springs beneath the more
permeable oolitic beds.
The Oolite rests on top of
the Lias and is subdivided into the Inferior Oolite and Great
Oolite. It forms the most significant component of
Northamptonshire's geology. The lowest oolite bed is the
Northamptonshire Sand reaching depths of around 80 ft and
outcropping significantly around Northampton. It is the
Northamptonshire Sand which contains the distinctive ironstone
with which so many buildings in Northamptonshire and surrounding
counties are constructed. The Northamptonshire Sand acts as an
aquifer, lying as it does between beds of limestone and clay,
and, together with the Middle Lias, is a source of tributary
springs. The relationship between the Northamptonshire Sands and
underlying clays has further predisposed the surface strata to
cambering, particularly so during periglacial times. Saturation
of the clays, together with the weight of overlying strata, has
resulted in their movement towards the incised tributary valleys
of the river Nene causing the sandstone to form deep clefts or
even blocks isolated from the main sandstone outcrops. Such
relationships are clearly discernible eastwards of the Brampton
valley, particularly between Brixworth and Maidwell.
Still within the Oolite
series and resting upon the Northamptonshire Sand is a layer of
Oolitic Limestone, the Lincolnshire Limestone which, in places,
can be cleaved and has been used extensively for roofing. This
can be seen in noticeably in the older houses of Collyweston and
surrounding villages, hence the term 'collyweston slate', which,
apart from its cleavage characteristics, has none of the
attributes of slate.
The upper-most layer in the
Oolitic series is the Great Oolite which rests on top of the
Lincolnshire Limestone or, where the latter is absent, directly
above the Northamptonshire Sands. The Great Oolite grades from a
sandy clay in its lower layers to limestone above. The Great
Oolitic limestone supports a moist soil formed of Oolitic clay.
A minor outcrop associated
with the Oolitic series is the ferruginous limestone known as
Cornbrash. It forms the youngest oolitic bed and can be found
locally supporting the Oxford Clay in eastern parts of the county
west of Oundle. It is of little importance in the geological
history of the area and does not influence greatly the surface
hydrology.
The most recent rocks in
Northamptonshire are the Oxford Clays which outcrop south of the
river Nene and form the south-eastern boundary of the county.
These clays rest directly on the Oolitic clay. The soils above
the Oxford Clays tend to suffer from severe alternate
waterlogging and dessication and have tended to be avoided by
farming. The comparatively few settlements and villages in this
part of the county reflects the poor nature of these soils. No
major tributaries rise from the Oxford Clay.
The drift sequences in
Northamptonshire are usually attributed to the Wolstonian glacial
cycle, as the county lay some distance south of the maximum
advance of the Devensian ice sheets. Some Devensian outwash
deposits are found, but the veneer is fairly thin. Wolstonian
till varies in thickness from as little as a few inches to as
much as 120 ft in places, especially in eastern parts, although
wherever it occurs it is discontinuous. The patchy nature of
these deposits allows both Quaternary and solid geology to be
depicted together in the available BGS maps of this area.
This page is still under
construction. Information on Jurassic events in Northamptonshire
and web links will follow.
Link
Northamptonshire
Slatestone - Dr D
Sutherland
Use of Northamptonshire's rocks as roofting material.
© Department of
Geography and Geology, Northamptonshire Grammar School