Superficial deposits
Superficial deposits are all the loose material that lies on top of the bedrock: clay, silt, sand, gravel, stones, boulders and peat. The superficial deposits under a plot decide how you can build foundations, how much the building will settle and whether the ground can cause landslides.
Most of the superficial deposits in Norway were laid down during and after the last ice age. The field of study is therefore called Quaternary geology. Geologists group superficial deposits by how they were formed, and the way they formed says a lot about how the ground behaves when you build on it:
- Moraine was laid down directly by the glacier. The material is unsorted and firm, and it usually gives good building ground.
- Glaciofluvial deposits are sorted sand and gravel with good drainage. They are also an important source of gravel and groundwater.
- Marine and fjord deposits are clay and silt laid down in the sea below the marine border. They are sensitive to settlement and can contain quick clay.
- Fluvial deposits are sand and gravel along rivers and streams. These areas are often exposed to flooding.
- Peat and bog are organic material. They are very sensitive to settlement and give poor building ground.
- Bare rock means that the cover of superficial deposits is missing or thin.
There is also weathered material, which is rock that has crumbled where it stands, and landslide material from earlier slides.
The type of superficial deposit controls the foundations. On moraine you can often build foundations directly. Through soft clay you usually need to drive piles down to bedrock, and peat is dug out and replaced with better material. Fine-grained deposits cause settlement over time and affect how easy it is to dig and how stable the excavation will be. Surface water is also handled differently: gravel lets the water soak in, clay does not. Coarse and porous deposits let radon into buildings more easily. Where fine-grained marine deposits lie below the marine border, you must expect a risk of quick clay. A geotechnical assessment under TEK17 is then required before a zoning plan or outline permission can be granted, and a landslide hazard zone may apply.
Norges geologiske undersøkelse (NGU) (the Geological Survey of Norway) manages the national map of superficial deposits at different scales. Each area in the map has the deposit type, how it was formed and a rough class for thickness, that is, how deep the layer is. The map shows only the surface. What lies below, and how deep it is down to the bedrock, you must clarify with ground investigations. You can find existing boreholes in NADAG. The dataset is described under Loose masses.
In Placepoint, the map of superficial deposits is in the map layer Geology, and a click in the map gives the deposit type for that point. Combine it with Marine border and Possibility of marine clay for a complete check of ground conditions before a purchase or zoning, and with NADAG for existing ground investigations. Report: Risk and climate risk includes superficial deposits as part of the ground conditions topic, and the findings fit naturally into a plot analysis.
How the dataset is shown in the map
The dataset is part of the map layer Geology as Loose masses:
From Placepoint's glossary: Superficial deposits
More information: NGU: Quaternary geological mapping, superficial deposits, NGU: map of superficial deposits, Store norske leksikon: superficial deposits
In English: superficial deposits (Quaternary deposits).
Frequently asked questions
What is the difference between superficial deposits and bedrock?
Bedrock is the solid rock. Superficial deposits are the loose material on top of it, such as clay, sand, gravel and peat. Most building projects stand on superficial deposits, and the depth down to bedrock often decides how the building must be founded.
Which superficial deposits are best to build on?
Moraine and bare rock usually give good building ground, because the material is firm and settles little. Glaciofluvial deposits of sand and gravel are also often good and drain well. Clay, silt and especially peat and bog need more extensive foundations.
Can I rely on the map of superficial deposits when I assess a plot?
The map of superficial deposits from Norges geologiske undersøkelse (NGU) shows only what lies at the surface, and the boundaries are rough. The map is suitable for a first assessment. What lies below, and how deep it is to bedrock, requires ground investigations.
Why is clay below the marine border especially important to check?
Clay and silt that were laid down in the sea below the marine border can be quick clay. This type of clay can collapse and turn liquid under load or when you dig. If you find such deposits on a plot, you must expect a requirement for a geotechnical assessment.