Legend to the map. Map based on Whattam et al. (2024).
Alnsjø succession - Permian rocks
In the field from Grefsenåsen to Ammerud we find rocks from the Permian, preserved in the remnants of the Nittedals cauldron southern end. Calderas are depressions in the bedrock, formed when the rocks above a magma chamber sink as the magma chamber empties. This causes the surface rocks to lie lower than the corresponding surface rocks outside the caldera. Lavas and sediments commonly fill such a depressions in the terrain given the time. As these rocks are lower in the terrain, than the corresponding stratigraphic units outside the calderas, they lay protected from erosion during the subsequent ice ages.
The Nittedals cauldron is one of the least preserved calderas in the Oslo Rift. Oftedahl (1960) reports that the Nittedals cauldron is nearly 80 % destroyed by a process called magmatic stoping. Magmatic stoping is the process where magma intrudes into a rock complex and breaks loose boulders that sink into the magma. This occurred in the Nittedals cauldron about 260-250 Ma ago. At that time, the Nordmarka - Hurdal Syenite Complex intruded into the area. All the Nordmarkites and Grefsen syenites around the remnants of the Alnsjø succession date from this time. The Alnsjø succession itself, which may be a large block that did not melt, underwent contact metamorphism when the magma intruded and possibly during later intrusions. Whattam et al. (2024) report many green minerals such as epidote and chlorite in the rocks that make up the Alnsjø succession, so the degree of metamorphism has probably reached greenschist facies.
Rhomb porphyry in Grorud/Ammerud from an exposure in the road Ammerudgrenda, just outside off the map shown in the top image, compared with the sketches of RP9 and RP11 in Larsen (1978) approximately to scale. Mapped as most likely RP9 by, among others, Natterstad (1978) and Graversen et al. (2017).
Plagioclase basalt
Plagioclase basalt with a syenite vein, from the basalt near the base of the Alnsjø succession. Presumably what Whattam et al. (2024) call the Kringlefjell basalt K4 in the western part of the area.
Aphyric basalt
An exposure of some layers of aphyric basalt, in what Whattam et al. (2024) call the Breisjøhogdene basalt in the eastern part of the area.
Tuff
Ignimbrite
Ignimbrite
Part of what Whattam et al. (2024) call the Alnsjø tuff, an andesitic lapilli tuff.
Ignimbrite
Ignimbrite
Ignimbrite
Part of what Whattam et al. (2024) calls the Linderudkollen ignimbrite, a densely welded glassy-ash with many phenocrysts of K-feldspar and sparse small vesicles.
Diabase dike in Storhaug mudstone
A diabase dike intruding into the lower part of what Whattam et al. (2024) calls the Storhaug Mudstone. Diabase = dark grey, Mudstone = red. On the path between Storhaug and Alnsjøen (59°57′58.81905″ N, 10°50′23.62511″ E).
Ignimbrite
A close up of what Whattam et al. (2024) calls the Storhaug ignimbrite, an unwelded to moderately welded ignimbrite. “The ignimbrite is variably colored with fresh surfaces typically light pink to greywhite, dotted with green (altered) and brown slightly- to nonvesicular lapilli”(Whattam et al, 2024, p. 9 supplementary material)
Storhaug conglomerate
An exposure of the Storhaug conglomerate. Partially covered by moss (green) and lichen (light grey to white). On the path towards Ole Høiland's cave (named after a bank robber from the 1800s).
A close up of Storhaug conglomerate
A close up of Storhaug conglomerate. Just besides the coin, a clast of a aphyric vesicular basalt is visible. The vesicles i partly filed with chlorite and other green minerals. Dark grey to black clast are basalts, redish clasts ar likely ignimbrites.
Mining
Linderud Mines
In the 1880s, the Franco-Belgian mining company “Compagnie Miniere de Grorud” operated copper mines in the area. Linderud mines (This is the northern field (59°58′27.6528″, 10°50′28.9464″)) remain from that time (Pedersen, 2014, p. 81).
The old mining road, “Gamle gruvevei”
The old mining road, “Gamle gruvevei”, built around 1880 to transport raw copper from Linderud Mine down to Alnsjøen. From there, the ore was transported to the east side of Alnsjøen, where the Gotthalfske Copper Works was located. Unfortunately, this cultural heritage road was damaged during logging in 2020 (Langsholt, 2020).
Gothaltsche Kaaber-Werk
Gothaltsche Kaaber-Werk
Unknown foundation and basement of a building, (59°58′1.5204″ N, 10°51′36.6408″ E), possibly related to the Gotthalfske Copper Works. Gotthalfske Copper Works operated sporadically between 1704 and 1750, mining 10-ish prospects around Alnsjøen. The largest was perhaps the Rødtvet Mine south of the lake (Pedersen, 2014, pp. 79-80). In the area downside of the dam, slag from the copper works can be found, they appear as black glassy rocks, occasionally with reddish patterns.