NGT37-3/4-05
1957
Pollenanalytisk datering av et myrfunnet ildsted fra Tryvasshøgda i Oslo.
37
3, 4
415-428

During the ground work for the Tryvann Stadium in Oslo 1933/34 a hearth consisting of 5-6 burned stones standing on edge with a handful of ash between them, was found at the bottom of a 3 m deep bog. Hoping that this find at same time might be dated pollen-analytically Dr. Gunnar Holmsen, geologist at the Norwegian geological Survey, on the 6th of January 1934 took out a profile with a sampling interval of 20 cm. After the results of my pollen-analytical investigations on the late Quaternary development in the inner Oslofjord area had been published in 1956, Holmsen sent these samples to me, hoping that I would be able to date the find.

By comparing the pollen diagram prepared on the basis of this profile (see Fig. 1) with the standard pollen diagram for the inner Oslofjord area (see Fig. 2) the hearth (located at the bottom of the diagram) may be coincident with the Sub-Boreal minimum of the mixed oak-forest occurring in a series of the diagrams from the Oslo area. As this minimum is supposed to represent a comprehensive late Neolithic clearance phase referring to the Flint Dagger period, the hearth find from the Tryvann Stadium probably dates to about 1500 B.C. The decrease of the amount of non-arboreal pollen (see the TOTALdiagram) at the Sub-Boreal-Sub-Atlantic (VIII-IX) transition is assumed to be a consequence of the climatic deterioration, allowing the Norway spruce, with its very spreading and shallow root system, to colonize even Tryvasshøgda. This syenitic-granitic hill district is namely covered by a rather sparse ground- and ablation-moraine, poor in plant nutrients and having a small water-holding capacity. Because hunting and fishing played an important part in the economy of the farmers during the Younger Stone Age and the Bronze Age, agricultural activity could take place during the Sub-Boreal period even in this high-lying, unfavourable area. This dating also seems to agree very well with the stratigraphy, as the increasing wetness represented by the transition from wood peat into Sphagnum peat at 230 cm below surface probably corresponds to Granlund's RY IV, ca. 1200 B.C.

The find of a typical pollen grain of holly (Ilex aquifolium) is of special palaeobotanical interest, as only one subfossil pollen grain of this oceanic species has formerly been recognized from eastern Norway, viz. from a Sub-Boreal sample in Vålertjern in the Mjøs region (cp. Hafsten 1956) . There is now reason to assume that even holly grew in the inland districts during the Post-Glacial warmth period. It is, as a matter of fact, most likely that holly grew in the favourable localities in the lowland, at the foot of the hills, and that pollen and pollen-bearing insects have been carried up in the hills by the strong convection currents which aften occur on hot summer days. Using the methods adopted by Iversen (see Fig. 3) the Ilex find indicates that the temperature for the coldest month (January) must have been ca. 3.5oc or more higher than at the present time.