You will learn
What is under the surface, and where is the water?
Lesson 3 told you what people did to the site. This lesson is about what nature put there: the soils and rocks, the groundwater moving through them, and the water that can flood the surface.
- Describe the ground as three layers: made ground, superficial deposits and bedrock.
- Find geology maps, old borehole records, groundwater information and flood maps for a site.
- Explain what the ground and water mean for the project and for the investigation.
Why it matters
The ground decides, the water complicates
The ground decides
How deep the foundations go, whether a trench can be dug with a normal excavator, and whether the dug soil can be reused or must be taken away.
The water complicates
Groundwater makes excavations collapse and fill up. It carries contamination off site. And flooding decides where you can build at all.
Unexpected ground and water are a common reason for delays and extra cost on construction projects. The good news is that in the UK, much of the geology and water information you need is free.
The idea, in plain words
The ground in three layers
Geologists describe the ground in many ways. For a desk study, three layers are enough to start with:
Fill, rubble, ash and waste. You found the clues for this in Lesson 3. Its thickness and quality are hard to predict.
Laid down by rivers, ice, wind and slopes in roughly the last two and a half million years. For example, alluvium (soft river clays, silts and sands), river terrace deposits (old river sands and gravels) and glacial till (clay with stones, left by ice).
The solid rock underneath, such as mudstone, sandstone, limestone or chalk. Weathered near the top, so “rock” is not always hard.
Key words
Geology map: a map showing which deposits and rocks lie at or near the surface.
Borehole record (log): a description of the layers found when a hole was drilled, with depths.
Water strike: the depth at which water was first found in a borehole.
Aquifer: a layer of soil or rock that holds and lets through useful amounts of groundwater.
The idea, in plain words
Finding the geology
The British Geological Survey (BGS) shows bedrock and superficial deposits free online, at 1:50,000 scale, in GeoIndex and the BGS Geology Viewer. Note what lies under the site and what is nearby, including any faults.
FreeGeoIndex gives free access to over a million scanned borehole, shaft and well records. Open the ones closest to your site and note the layers, their depths and any water strikes.
FreeEarlier reports for the site or its neighbours, from the client, the planning files or the previous owner. Often the best information you will get. Before you use one, check it matches your site, and whether it is a draft or final report.
Usually freeBGS also produces data on natural hazards such as shrinking and swelling clays, dissolving rock and landslides. The detailed data is usually bought, often as part of a commercial data report.
Usually paidReading an old borehole record
Old logs are gold, but read them with care. Descriptions may use older terms. The water level is a single reading on a single day. And a hole 150 m away tells you what is likely on your site, not what is certain. Always say how far away a record is.
The idea, in plain words
Finding the water
Groundwater
The Environment Agency classes the ground by how important it is for groundwater. These are called aquifer designations, and they are given separately for superficial deposits and bedrock:
| Designation | In plain words |
|---|---|
| Principal aquifer | Very important for water supply, often at a regional scale |
| Secondary A | Can support local water supplies and feed rivers |
| Secondary B | Mostly low permeability, storing limited water in cracks or thin layers |
| Secondary undifferentiated | Not possible to say whether A or B applies |
| Unproductive | Very little groundwater |
Around drinking water wells and boreholes, the Environment Agency also draws Source Protection Zones (SPZs). The inner zone, SPZ1, is the area where groundwater takes up to 50 days to reach the supply, or 50 m around it, whichever is larger. Both aquifers and SPZs can be viewed free on Defra’s MAGIC map.
Flooding
On GOV.UK, the Flood map for planning shows Flood Zones 1 (low), 2 (medium) and 3 (high) for rivers and the sea, updated in 2025. The Check your long term flood risk service also shows surface water flooding. Note both: surface water flooding often catches people out.
Dry in August, soft in November
A site walked after a long dry summer can look firm and easy. In winter, the same fields can be soft, waterlogged and impossible for a drilling rig. Spring lines, where groundwater comes out at the surface, stay wet all year. Say this in the desk study, so the investigation contractor can plan access, and so nobody can later claim they were not told.
Aquifers change how you dig
If a site sits on a principal aquifer or in an SPZ, the investigation and the works need extra care: boreholes sealed so they cannot carry contamination down into clean water, and controls on any water pumped out of excavations. Flagging it early saves a difficult conversation later.
On our site
Brindlemere Yard: ground and water
Here is the next section of our report. Notice how each finding ends with what it means.
Brindlemere Yard: Desk Study Report (fictional example)
4. Ground and water
Geology. The geology map shows alluvium along the River Brindle, over river terrace deposits (sand and gravel), over mudstone bedrock. Made ground is expected across the site from its industrial history (Section 3).
Nearby borehole records.
| Record | Layers found | Water |
|---|---|---|
| BH-A, 1960s, 120 m north | Made ground to 1.2 m; soft clay to 3.1 m; sand and gravel to 6.8 m; mudstone below | Struck at 2.4 m |
| BH-B, 1980s, 200 m west | Topsoil to 0.3 m; firm clay to 2.0 m; sand and gravel to 5.5 m; mudstone below | Struck at 3.0 m |
Groundwater. The river terrace deposits are a Secondary A aquifer and the mudstone is Secondary B. The site is not in a Source Protection Zone. Groundwater is likely at about 2 to 3 m depth, in the sand and gravel, and may be linked to the river.
Flooding. A strip along the eastern boundary is in Flood Zone 3, with a narrow band of Flood Zone 2 on its landward (western) side. The rest of the site is in Flood Zone 1. One low area in the centre is shown at low risk of surface water flooding.
What this means. Soft clay is likely at shallow depth near the river. Excavations deeper than about 2 m are likely to meet groundwater and need support and pumping. Any contamination could reach the river through the sand and gravel. The homes should be kept out of the Flood Zone 3 strip.
Your turn
Describe your site’s ground and water
Wherever you are
- Your national geological survey usually publishes geology maps, and often borehole records too.
- Your environmental regulator or water authority may map groundwater resources, protected wells and flood risk.
- Universities, previous investigations for nearby buildings, and local wells can fill gaps.
- Look at the landscape on a map: rivers, low ground, springs and marshes are clues to water, wherever you are.
Task (20 minutes)
For your own site, write four short paragraphs, like ours:
- Geology: made ground, superficial deposits and bedrock.
- Nearby records: at least one borehole record or previous report, with its distance from the site.
- Groundwater: aquifer type, any protection zone, and a likely depth.
- Flooding: from rivers or the sea, and from surface water.
End with one or two sentences on what this means for the project.
Quick check
Five quick questions
This is not a test. It simply shows you whether you are ready for Lesson 5. Choose an answer to see the explanation.
Key points
What to take away
- Describe the ground as made ground, superficial deposits and bedrock.
- In the UK, BGS geology maps and over a million borehole records are free to view online.
- Record aquifer designations, Source Protection Zones, flood zones and surface water risk.
- Think about the seasons: ground that is dry in summer may be soft and wet in winter.
- End every finding with what it means for the project.
Where to look (free)
- BGS Geology Viewer (British Geological Survey)
- BGS borehole records (British Geological Survey)
- MAGIC map, for aquifers and Source Protection Zones (Defra)
- Flood map for planning (GOV.UK)
- Check your long term flood risk (GOV.UK)
Up next: Lesson 5
Constraints and hidden hazards. Protected nature and heritage, landfill, mining, radon, unexploded bombs and buried services.
Halfway check-in
How is the course going so far?
You are halfway through. Please tell me how it is going. It takes about a minute, and your answers help me make the course clearer for everyone.
This is optional. You can go straight on to Lesson 5 if you prefer.
Thank you, your check-in has been sent. Carry on to Lesson 5 when you are ready.