Hovedøya

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The Oslo group

On Hovedøya you can find a number of sedimentary rocks of Ordovician age (485.4 to 443.8 million years ago). The rocks form a relatively coherent time line from the nodular limestones in the Arnestad Formation (approx. 457 million years old) through shales/slates, limestones and sandstones in nine other formations before reaching the youngest, Silurian aged, shales and slates in the Solvik Formation (approx. 442 million years old).

Particularly interesting is the southeastern end of Hovedøya. There you can see parts of the Langøyene Formation. Cutting down in this formation is a layer of conglomerate and several newer silt and sandstones (the Kalvøya Member). This has been interpreted as traces of a river that flowed here in the late Ordovician, when the area was higher than sea level (Bockelie et al., 2017; Calner et al., 2021). The other sedimentary rocks on Hovedøya was formed in the ocean. 

The Caledonian Orogeny

For approx. 425–405 million years ago, the Baltic Plate (with Norway) collided into the Laurentia continent, today's North America and Greenland. This collision resulted in the formation of a massive mountain range, the Caledonides. The creation of this mountain range led to the folding of the sedimentary rocks on Hovedøya, both on a large and small scale. If we go from north-west to south-east, we will see that the layers, which were once horizontal, are now almost vertical at the ridges on the western and eastern ends of the island. The rocks on the island are folded in a large anticline, A-shape, the top of which has been eroded away during the ice ages. The softer shales in the underlying layers have eroded faster than the harder sandstone in the top layer, so the center of the island is lower. In the east of the island, the folding is so extreme that the layers are actually partly upside down. In several places on the island you can also find smaller folds, some of which are also less than a metre.

Rift time

The youngest rocks we find on Hovedøya are intrusive rocks that were formed 300-270 million years ago, when there was volcanic activity in the Oslo area. The island has a syenite passage and several diabase passages that have penetrated the sedimentary rocks. Several of the larger diabase passages can today be found as scars in the terrain, as the stone has been carved out for use as building stones for, among other things, the monastery on the island. 

During the faulting period, there were also movements in the earth's crust which probably caused earthquakes, traces of which we can find on Hovedøya.

References

Bockelie, J.F., Baarli, B.G. & Johnson, M.E. 2017: Hirnantian (latest Ordovician) glaciations and their consequences for the Oslo Region, Norway, with a revised lithostratigraphy for the Langoyene Formation in the inner Oslofjorden area. Norwegian Journal of Geology 97, 119–143. https://dx.doi.org/10.17850/njg97-2-01

Calner M., Bockelie J.F., Rasmussen C.M.Ø., Calner H., Lehnert O., & Joachimski M.M. (2021) Carbon isotope chemostratigraphy and sea-level history of the Hirnantian Stage (uppermost Ordovician) in the Oslo–Asker district, Norway. Geological Magazine 158: 1977–2008. https://doi.org/10.1017/S0016756821000546