1. Prior to 18,000 years ago, the entire area was covered by an ice sheet up to 400 meters thick. 2. As the ice sheet retreated north of Norley/Cuddington, it deposited boulder clay and then stopped, unable to retreat further west due to pressure from ice flows from the Welsh highlands. 3. Outwash from the static ice front deposited glacial sands and gravels north of Norley/Cuddington and through gaps in the sandstone ridge. 4. Final retreat of the ice sheet left further extensive deposits of boulder clay across the area.

1. 1. Prior to the start of the final
retreat of the ice sheet ~
18,000 years ago.
Whole area covered by ice up
to >400m thick.
2. Ice retreats to north of
Norley/Cuddington, leaving
deposits of Boulder Clay
(blue), then stops.
Unable to retreat west of
Ridge because of pressure
exerted by ice-flows from
Welsh highlands.
Retreat of ice-
sheet halted here.
3. Outwash from the static
ice-front to north of
Norley/Cuddington and
through gaps in the Ridge
(blue arrows).
Leave deposits of glacial
sands and gravel.
4. Final total retreat of the
ice-sheet leaves further
extensive accumulations of
Boulder Clay.
Sandstone Ridge
high ground

2. Hydrogeology Workshop
Present Geology (1)
• And it has all resulted in this:
Go to British Geological Survey (BGS) ‘Geology of Britain Viewer’ @
http://www.bgs.ac.uk/discoveringGeology/geologyOfBritain/viewer.html
(Follow instructions at rear of handout)

3. Hydrogeology Workshop
Present Geology (2)
Note: Relative thickness of the Glacial Clay,
Sands and Gravel and Alluvium is greatly
exaggerated

4. Hydrogeology Workshop
Part 2
Introduction to Hydrogeology

5. Hydrogeology Workshop
Introduction to Hydrogeology (1)
What is groundwater and its place in the hydrological cycle?
Ocean
Evaporation
Heat/Radiation
Clouds form
Circulation in the
atmosphere
Precipitation
Runoff
Transpiration and
evaporation
Recharge
Water table
Groundwater flow
Discharges
Groundwater discharges:
•River/lakes
springs/wetlands;
•Sea/estuaries
•Abstractions
(wells/boreholes)
Saline
water

6. Hydrogeology Workshop
Introduction to Hydrogeology (2)
Some basic terminology
Aquifer:
• Rock which allows water to flow in
‘significant’ quantities (Ridge area examples:
Terrace/Glacial Sands and Gravel, Sherwood
Sandstones)
Aquiclude:
• Rock through which virtually no water
flows. (Ridge area examples rare strictly speaking
but could include Mercia Mudstones, Halite
formations, Glacial Boulder Clay)
• Can be a layer above a ‘confined’ aquifer.
Aquitard:
• Rock which allows ‘small’ amounts of
water to flow through it. (Ridge area examples
Mercia Mudstones, Halite formations, Glacial
Boulder Clay)
• Can be a layer above a ‘semi-confined’
aquifer.
Porosity; Void space, expressed as
% of rock volume.
• Primary porosity – intergranular;
• Secondary porosity –
fractures/fissures;
• Effective porosity – space in which
groundwater flow is ‘active’ (also
related to ‘storage’).
Permeability (Hydraulic
Conductivity)
• Property which allows fluid (water)
to move through rock/soil.

7. Hydrogeology Workshop
Introduction to Hydrogeology (3)
Some basic terminology
• Water table:
– Surface of groundwater, interface between the saturated aquifer and
the unsaturated zone.
• Piezometric Level:
– ‘Pressure’ (head) of water in a confined or semi-confined aquifer.
Effectively where the water level will be in a borehole drilled into the
aquifer.
• Aquifer conditions:
– Unconfined: Aquifer which contains the water table. Water
can enter it from recharge.
– Confined: Fully saturated aquifer with piezometric surface
(e.g. level of water in a borehole) above its top. Sometimes known
as ‘artesian’. Strictly speaking, no water enters as recharge from
above.
– Semi-confined: Fully saturated aquifer with piezometric
surface (e.g. level ,of water in a borehole) above its top. BUT water
can leak into it from above.
– Perched aquifer: An upper unconfined aquifer (i.e. It
contains a water table) which is separated from a lower one , with an
intervening unsaturated zone.
Based on Brassington, R. (1988) Field Hydrogeology, Geological Society Handbook, Open
University Press, Milton Keynes/Halstead Press, New York – Toronto, P.4, fig. 1.2
Confined or semi-confined Unconfined

8. Hydrogeology Workshop
Introduction to Hydrogeology (4)
Groundwater Flow
Bath Tubs and Black Boxes
Where does it want to go?
• In at the top (recharge/leakage) and out at the bottom (discharge to
rivers, lakes, sea etc).
• From high groundwater level/pressure (higher ground) to low
level/pressure (lower ground/lower pressure).

9. Hydrogeology Workshop
Introduction to Hydrogeology (5)
Groundwater Flow
Bath Tubs and Black Boxes
How does it get there?
• Driven by hydraulic gradients
– Lateral hydraulic gradients
– Vertical hydraulic gradients
• Downwards
• Upwards
– How do they work (e.g from recharge to discharge at rivers, springs etc).
• ‘Controlled’ by Permeability
• Primary – intergranular – flow tends to be fairly slow
• Secondary - fractures/fissures/bedding planes and geological faults – flow can be very fast.
Sherwood Sandstone aquifers benefit from Primary AND Secondary
porosity/permeability:
– Lots of water ‘stored’ in intergranular primary pores;
– Rapid flow of groundwater through fractures/fissures;
– (very good for water supplies)
And...........

10. Photographs from CCTV down a
public water supply borehole in
the Sherwood Sandstone.
• Primary porosity in the rock-mass;
•Clear fissures showing secondary porosity;
•Nice clear water under these ‘static’
conditions.