A series of bentonite beds are found in the stage 4b, Caracloc (and possibly stage 4a, Llandvirn-Llandeilo), at Sinsen in the Oslo-Asker district. They are clescribed from a number of localities. From chemical analyses, X-ray data, D.T.A., determination of cation exchange capacities and staining tests, it appears that the bentonites from Sinsen probably contain 3 different types of mixed layered minerals. Most of the Sinsen bentonites have a rather high content of potassium (ca. 7, 5 % K2O), they have therefore been classed as K-bentonites.
Correlations with the Middle Ordovician of central and S-E Sweden inclicate that the sequence is more complete in the Oslo Region.