Surface weathering forms on a rocky shore platform in South Africa

Studies of weathering patterns along rocky shorelines have mainly focused on the evolution of large-scale shore platforms rather than the smaller scale landforms that are found upon them. Here, we describe small-scale weathering forms that are developed on a granite bedrock shoreline at Port Edward along the southeast coast of South Africa. These landforms reflect the interplay between biological, chemical, geologic, and wave/tide processes operating within the intertidal zone. Field data collection in this study focused on characterizing the micromorphology and thus the likely processes of evolution of surface weathering forms using a combination of Structure from Motion (SfM) photogrammetry and rock surface hardness using a Schmidt hammer. Two locations within the intertidal zone of the shoreline were examined. At both locations, a ‘control’ site in the upper intertidal/splash zone of the platform, and a ‘test’ site seaward of the control site were observed and measured in detail using the SfM and Schmidt hammer methods. For uniformity, all sites were ~2.5 m2 in area. Morphologically the test sites show small-scale (10 cm diameter, < 15 cm deep) enclosed depressions that vary from symmetric to asymmetric in cross profile, or regularly-spaced ridge-like corrugations (12 cm apart, 8 cm of relief) that develop down the face of planar but steep rock surfaces. These can be effectively captured and quantified using the SfM method. Overall, there are complex relationships between the surface rebound values (and their variability) and the presence or morphology of different weathering forms. This is attributed to feedbacks that exist between different weathering and erosion processes that shape the small-scale properties of shore platforms, influenced by wave splash, lithological controls, and microclimate.