The upperrnost Ordovician strata in the Toten and Nes-Hamar ares of the Oslo Region, southern Norway, are represented by a ca. 100 m thick, predominantly carbonate sequence, named the Mjøsa Limestone. Five subdivisions of the formation are recognizable: the lower Fumesfjorden Member; the middle, laterally equivalent Eina, Gålås, and Bergevika Members; and the upper Holtjern Member. Further subdivision into formal and informal units illustrates the lateral and tempora! facies diversity, which is characteristic of the formation. Three shallowing-upward sequences are distinguished and these permit a chronostratigraphic correlation to be effected.