The Black River and Trenton Groups (Middle Ordovician) of southern Quebec contain several beds of swelling claystone which do not resemble the normal marine shales present in these units. The stratigraphic analysis of these units shows that these peculiar beds correspond to several well defined horizons which correlate over large distances, and are thus very good marker horizons. The clay mineralogic analysis of these beds shows that the main clay mineral is a mixed-layer of illite–smectite type having 20–40% of expandable layers. These claystones are identified as 'K-bentonites.' They are interpreted as being the result of the sedimentation and lithification of volcanic ash ejected during a series of eruptions contemporaneous with the deposition of the sediments.