Reading a geological map for a development site

A published geological map tells you what to expect under a site before anyone puts a rig on it, and it tells you within limits that matter. The British Geological Survey's mapping shows bedrock and superficial deposits at a mapping scale that was never intended for individual plots, so a boundary drawn on the map can be tens of metres from the boundary on the ground. Read it as a hypothesis to test rather than as a survey, and pair it with the archived borehole records, which are site-specific and often more useful than the map itself.

For an architect deciding whether a scheme needs a ground investigation, and how much of one, this half hour of desk work changes the brief more than any other single input.

What the viewer shows

The online geology viewer presents two principal layers and a set of supporting ones. Understanding which you are looking at is the first skill.

Bedrock geology is the solid rock beneath any loose cover, described by formation name and lithology. Superficial deposits are the younger, unconsolidated materials above it, such as alluvium, river terrace gravels, glacial till, head and peat. A site can show bedrock only, superficial only, or both depending on how the mapping is displayed.

Supporting layers worth switching on include artificial ground, linear features such as faults, and mass movement deposits indicating landslip. Where available, the viewer also links through to indicative hazard ratings and to the borehole record index.

The formation name is not decoration. It tells a geotechnical engineer roughly what strength and stiffness to expect, whether the material is likely to be shrinkable, and whether it will hold water or shed it.

Bedrock against superficial deposits

This distinction drives the foundation decision more than anything else the map shows.

Founding in competent bedrock at shallow depth is straightforward. Founding in variable superficial deposits, particularly alluvium or peat, is not, and the thickness of those deposits is the number you need. The map shows their presence but rarely their depth, which is precisely why the borehole records matter.

Shrinkable clays deserve their own attention. Where the superficial or bedrock geology includes a clay with a high plasticity, foundation depths driven by seasonal moisture change and by nearby trees become the governing constraint, and the interaction with any retained tree needs designing rather than assuming. On those sites the tree survey and the ground investigation should be commissioned together, because the Arboricultural Impact Assessment and the foundation design are answering the same question from different ends.

Peat is the other flag. Its presence raises compressibility, ground gas, and carbon questions, and on some sites it will make a scheme unviable.

Finding borehole records nearby

The archived borehole record index is the part of this exercise that produces hard numbers. Records are lodged from previous investigations, water wells and site works, and many are available to view.

Search a radius around the site rather than only within it. A record 150 metres away on the same terrace tells you more than nothing, and three records around a site let you sketch the layer thicknesses. Read the log critically: note the date, the drilling method, the logger's descriptions, whether any testing was done, and whether water was recorded and when.

Older records use older terminology and less consistent description conventions, so treat descriptions from several decades ago as indicative. What they are reliably good for is depth to a named stratum and the presence of made ground.

Record what you find in a schedule with borehole references, distances and depths. That schedule goes straight into the investigation brief. Finding borehole and geology records for a site covers the search mechanics in more detail.

Made ground and why it matters

Made ground is material placed by people, and it is mapped inconsistently because much of it was never recorded. On any site with an industrial or urban history, assume it is present until an investigation says otherwise.

It matters for four reasons at once. Its bearing capacity is unpredictable and usually poor. It may contain contaminants, which brings the Contaminated Land Assessment into the scope. It generates ground gas where it includes degradable material. And it frequently contains obstructions, from old foundations to buried tanks, which affect both cost and programme.

The map will not find made ground on a small infill plot. Historic mapping will, and using historic mapping in a contamination desk study is the companion exercise to reading the geology.

Mapping scale and its limits

Geological mapping in England is published at scales intended for regional and district understanding. The consequence is that a line on the map is an interpreted boundary carrying real positional uncertainty.

Three limits to keep in mind. Boundaries are interpretations drawn from exposures, boreholes and landform, and between those data points they are inferred. Thin superficial deposits may not be shown at all where the mapping convention sets a minimum thickness. And a site can sit on a boundary, in which case both units should be assumed present until proven otherwise.

None of that makes the map unreliable for its purpose. It makes it unsuitable for a purpose it never claimed, which is telling you what is under a specific corner of a specific plot.

When a desk study replaces the map

A geological map is one input to a desk study, not a substitute for it. A proper desk study also draws on historic mapping, the borehole index, records of mining and extraction, radon and ground gas information, flood and groundwater data, and any previous site investigation reports.

Where the site has any of the following, go to a full desk study rather than relying on the map: a former industrial use, proximity to a coalfield or other mineral working, a filled pond or quarry visible on historic mapping, a watercourse or culvert on or adjacent to the site, or a neighbouring structure whose foundations your works will affect.

Feeding the result into a site investigation brief

Turn the desk work into instructions rather than a summary.

1. State the expected bedrock and superficial sequence, with the depths suggested by nearby records.
2. Name the specific uncertainties the investigation must resolve, such as the thickness of alluvium or the presence of made ground.
3. Set a minimum depth for exploratory holes that gets through the superficial deposits into bedrock where that is the founding stratum.
4. Specify the testing needed for the parameters your structural engineer will use, rather than a generic schedule.
5. Require groundwater monitoring installations where the records show water, with a monitoring period long enough to be meaningful.
6. Ask for a contamination scope where made ground or an industrial history is indicated.

A brief written that way produces a Geotechnical Assessment that answers your design questions instead of describing the ground in general terms. It also costs less, because a targeted investigation costs less to procure than an open one, and the map is what lets you target it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and interpretation of ground conditions needs a qualified geotechnical engineer.