The Quaternary Geology of Ås, southeastern Norway

Sylvi Haldorsen and Rolf Sørensen
Department of Plant and Environmental Sciences, P.O.Box 5003, N-1432 Aas-NLH, Norway sylvi.haldorsen@umb.nlh.no and rolf.sorensen@umb.no

Norwegian University of Life Sciences is located in the municipality of Ås, 30 km south of Oslo (Figs. 1 and 2).

The Oslofjord area was deglaciated during the Younger Dryas/Early Holocene (Fig. 1). During the ice retreat during the retreat two prominent ice marginal ridge systems were formed in the Ås area (The Ås-Ski ridges, Fig. 2); of which the Ås moraine is the southernmost one. Smaller ice marginal ridges are found between the two major ones (Fig. 3 ). The other dominating Quaternary deposits in the Ås district are marine clays and silts, which make up the sediments between the moraine ridges. The district is as a whole posited below the marine limit (Fig. 4). The fine-grained sediments are partly of glaciomarine origin, with dropstones, deposited during the Younger Dryas, and partly Holocene clays (Fig. 4). Basal till is almost entirely lacking between the ice marginal deposits, and the bedrock is exposed in the highest areas.

The Ås Moraine is dominated by mud-rich till and poorly sorted debris flows. Hydrogeological investigations indicate that glaciofluvial material occurs, but no large bodies of sorted material are identified. The small share of sorted sediments makes the basis for the farm wells and are the only permeable sediments in the area.

The Agricultural University is built on a ground which belonged to some of the largest farms in the area, and on the best agricultural land. The best farmland was the sandy soil of the ice marginal deposits, which was easy to cultivate and which became dry and warm early in the spring compared to the marine silt and clay. A main part of the University is located along the slopes of the Ås Moraine (Fig. 5)

The soils of the moraine ridges have been cultivated for more than 4000 years (Fig. 6) and the oldest farms were located on the ice marginal ridges. Graves from the Viking Period are found along the crest of the Ås moraine (Fig. 7) The southern slopes of the moraine ridges are still the most favourable locations for growing vegetables and fruit.

The main features of the Quaternary geology has been known since the middle of the last century, when Th. Kierulf, who introduced the glacial theory in Norway, used the moraine ridges as one of the arguments for an extensive glaciation (Fig. 8). In 1862 he introduced for the fist time the glacial theory in a Norwegian geology text-book, and this book was written for the students at the new Agricultural University. Maybe he found it easier to convince the Ås students that his theory was right, than to convince the more critical colleagues at the Oslo University. Anyway, the Quaternary geology of Ås must have been a very good help for this argumentation.

References:

Haldorsen, S. and Sørensen, R. 1986; Distribution of tills in southeastern Norway. In: Meer, J.v.d. (ed.): Tills and Glaciotectonics, 31-38. Balkema, Rotterdam.

Nordahl-Olsen, T. 1990: Ski 1914 III. Map of the Quaternary Geology, scale 1: 50 000 with description. Norges geologiske undersøkelse Skrifter 95, 33 pp.

Sørensen, R. 1979: Late Weichselian deglaciation in the Oslofjord area, south Norway. Boreas 8, 241-246.

Sørensen, R. 1983: Glacial deposits in the Oslofjord area. In: Ehlers, J. (ed.): Glacial deposits in North-West Europe, 19-27.