The Middle Cambrian Kalby Member of the Exsulans Limestone Formation exposed at the Læså rivulet on the island of Bornholm, Denmark, consists of a lower marl bed (previously known as the Kalby clay) and an upper pyritic bed. The marl bed in its present development is an unconsolidated unit; it has abundant trilobite moulds, mainly phosphatic microfossils, and contains about 35 wt. % CaCO3in the form of finely dispersed, non-cementing calcite grains. The marl bed is interpreted as a weathering residue of a former limestone corresponding to the Exsulans Limestone. In a consolidated state and soon after its deposition it was subjected to tectonic movements taking place in the Middle Cambrian. Its present lithology represents local decay of the carbonate rocks, including the overlying Andrarum Limestone Formation, in the Læså area. The faunal analysis of both beds of the Kalby Member is much extended, the fauna showing great similarity to that of the Borregård Member of the Øleå section, which suggests contemporaneous deposition of both members of the Exsulans Limestone within the time span of theTriplagnostus gibbusandTomagnostus fissus-Ptychagnostus atavuszones. The present findings support early observations on the marl bed as a secondarily decomposed carbonate rock, and the evidence is directly against subsequent theories of the bed having remained unconsolidated since its deposition. There is no definite evidence to support theories of the bed containing Lower Cambrian fossils which have been reworked and redeposited several times.