The freeze-thaw weathering regime at a section of the Niagara escarpment on the Bruce Peninsula, Southern Ontario, Canada

The bedrock freeze‐thaw and moisture regimes at an actively eroding site on the Niagara Escarpment, Bruce Peninsula, southern Ontario, were monitored between December 1983 and April 1984, and the results compared with amounts of debris collected in an adjacent rock trap. Frost wedging in pre‐existing rock fissures is the primary mechanism responsible for the observed rockfall events; hydration effects are negligible. Debris production was more closely related to the duration of the freezing leg of the freeze‐thaw cycle than to intensity or to cycle frequency. Release also coincided with periods of high pore saturation (> 60 per cent) and the seepage of water from cracks and fissures. Fewer freeze‐thaw cycles were recorded in the air than at 1 and 3.5 cm in the bedrock. The number of cycles that could be declared geomorphologically effective according to established temperature criteria was normally less than half the total number of freeze‐thaw cycles recorded in both air and bedrock. Under the current temperature regime at the field site, few effective cycles are capable of penetrating more than 5 cm into the free face.