Bedrock

Bedrock is the solid rock that lies beneath soil, clay, sand and gravel. How deep the rock lies, and what kind of stone it is, has a large effect on how you can lay the foundations of a building and what it costs.

In everyday Norwegian the word is fjell, while technical language and regulations often use berg. In many places the bedrock is exposed as bare rock. In other places it lies deep under clay and moraine, the loose masses left behind by ice and sea. The stone itself is described as rock type, and the bedrock in an area often consists of several rock types.

Depth to bedrock determines the foundation method. If the building stands on rock, it can be founded directly. Over thick loose masses it must be piled down to bedrock, or founded on the loose masses with a risk of settlement. Below the marine limit, the masses above the rock can be marine clay and quick clay. Basements and underground car parks in rock require blasting, and the blasted rock must be removed. These masses can be a resource or a problem, depending on the rock type. The bedrock map does not show depth. You find depth in ground investigations registered in NADAG. Check both in a site analysis.

Norges geologiske undersøkelse (NGU, the Geological Survey of Norway) manages the national bedrock database. Bedrock N250 at a scale of 1:250,000 covers the whole country, while Bedrock N50 at 1:50,000 is available where the bedrock has been mapped in detail. Each rock unit polygon has a main rock type, age of formation and main tectonic division. In addition there are rock type boundaries, lineaments and faults, and structural measurement points with strike and dip.

Broadly, Norway is divided into four main tectonic units. Precambrian basement with gneiss and granite dominates in the south and north. The Caledonian nappes make up the mountain chain. The Oslo Region has Cambro-Silurian sedimentary rocks such as shale, limestone and alum shale, and Permian volcanic and plutonic rocks such as rhomb porphyry and larvikite. In western Norway there are Devonian basins.

Lineaments and faults are zones of weakness. There the rock can be unstable, rock quality poorer and water ingress high in tunnels and excavations. Uranium-rich rocks such as alum shale and some granites and pegmatites give high radon susceptibility. Alum shale also becomes acid-forming when it is excavated, and the excavated masses often have to be delivered to an approved facility, which costs money. It is common in the Oslo area and in Hadeland. Groundwater and energy wells draw water from fractures in the rock, so rock type and fracture density determine how much water a well gives.

In Placepoint, the map layer Geology has the Bedrock layer with 29 sublayers from N250, as well as separate layers for N250 and N50. If you click in the map, you get the main rock type and age for the polygon. The NADAG layer shows known ground investigations with depth to bedrock, and Report: Risk and climate risk includes ground conditions.

How the dataset appears in the map

The dataset is part of the map layer Geology as Bedrock:

From Placepoint's glossary: Bedrock

More information: NGU: about bedrock, NGU: bedrock map, Store norske leksikon: bedrock

English: Bedrock.

Frequently asked questions

What is the difference between bedrock and loose masses?

Bedrock is the solid rock. Loose masses are clay, sand, gravel and moraine that lie on top of the rock. The thickness of the loose masses determines how deep you have to go to reach solid rock.

How do I find the depth to bedrock on a site?

The bedrock map shows which rock type lies underneath, but not how deep it lies. You find depth to bedrock in ground investigations registered in NADAG. If there are no investigations nearby, you need to drill on site.

Why is alum shale a problem in building projects?

Alum shale becomes acid-forming when it is excavated, and it can contain uranium. The excavated masses therefore often have to be delivered to an approved facility, which costs money. Alum shale also gives high radon susceptibility and is common in the Oslo area and in Hadeland.

What is the difference between Bedrock N250 and N50?

N250 is mapped at a scale of 1:250,000 and covers the whole country. N50 is mapped at 1:50,000 and gives more detail, but is only available where the bedrock has been mapped more closely.