A field of stress-release bedrock structural features occurs on the floor of western Lake Ontario south of Toronto, Canada. These features were investigated using side-scan and multibeam sonars, high-resolution seismic profiling, and submersible dive observations. The study region was mostly stripped of its glacial drift in late glacial time, and the region has since accumulated only a relatively thin, discontinuous cover (1–2 m) of lacustrine sediment. The stress-release features affect the flat to gently dipping interbedded shales and calcareous siltstones of the Upper Ordovician Georgian Bay Formation. The features consist of sub-lakefloor buckles, about 50–100 m wide with structural relief of 5+ m, and surface bedrock popups, 10–15 m wide with a general relief of 1–2 m. Deeper bedrock faults are possibly associated with some of the sub-lakefloor buckles. Trends of the popups and buckles can be grouped into six modes from 7.5° to 347.5°. Abutting and sediment onlap relationships suggest that the pop-ups formed throughout late and postglacial time following the Last Glacial Maximum ∼20,000 yr ago. The earliest set of popups is estimated to have formed before 9500 B.P.; they trend WNW, collinear with isobases of glacial rebound, and do not parallel major geophysical or structural linear zones in the region. These and other factors suggest that this set developed in response to glacial rebound-induced stress. Later popups form an irregular pattern with several orientations of axes, suggesting that the horizontal principal stress vectors were of similar magnitude. The decrease of rebound strain with time and clockwise rotation of modern contours of basin tilting relative to glacial lake isobases suggest that popups today are likely a response to reduced glacial stress combined with far-field tectonic stress.

Abstract Geomorphic systems are inherently complex and are the product of a unique integrative history of surface processes, making it difficult for engineers, scientists, and resource managers to regulate environmental change in order to attain a specific management goal. A powerful means for unpacking geomorphic complexity is hierarchical classification of the dominant process-response elements of a geomorphic system. We apply hierarchical analysis specifically to the evaluation of the probable environmental effects of erosion-abatement projects at drumlin bluffs along the southeastern shore of Lake Ontario, assessing their impact across a variety of spatial and temporal scales. Some of the conclusions about probable impacts of shore-stabilization structures on the Ontario lakeshore are not intuitive, but are logically derived from the systematic hierarchical analysis of the system’s geomorphic complexity.