Several stratigraphic sections of the Rideau beds (basal Black River group) near Sydenham, Ontario, were described and sampled. From these samples a set of thin sections was made and the mineralogies and textural features displayed of each were studied in detail. One of many interesting textural features found was the replacement of terrigenous particles by interstitial calcite. The green pigment of the Rideau beds is probably a mixture of chlorite and illite.

Four mechanical analyses of the basal conglomeratic sandstones were made and the size distributions statistically analysed. The separated sands were examined by microscope in detail in order to make determinations of mineralogy, roundness, and sphericity.

The environment of deposition of the Rideau beds appears to have been the shallow neritic zone of a transgressing epicontinental sea. Terrigenous material was locally derived from the underlying Precambrian rocks and the Potsdam sandstone. The modal sand sizes common to each sandstone analysed reflect the size distribution of source materials, not selective sorting during deposition. Fluctuating currents passed over the base level of deposition during most of Rideau sedimentation.

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