Glacial sediments are found across formerly glaciated regions across the world and host a variety of important resources, ranging from groundwater to hydrocarbons, aggregate material, and mineral deposits. In southern Ontario, Canada, thick successions (up to 200 m) of Quaternary glacial sediments are truncated by large valleys (>30km long, 2 to >8.5 km wide, and up to 200m deep) that formed subglacially and have characteristic morphology and infill stratigraphy. These valleys are interpreted as (a new class of) tunnel valleys and strongly affect groundwater resources and flow systems at local and regional scales. The overall context of the valleys is evaluated through an introduction to the study area, objectives, and background information on subglacial systems and geologic history of south-central Ontario (Chapter 1). Interpretation of valley genesis in Simcoe County is provided through an integrated, multi-faceted approach, involving: description of the morphology and sediment infill succession within the valleys from surficial mapping, sedimentological logging of continuously-cored boreholes, and geophysical surveys (Chapter 2); delineation and characterization of seismic architecture from high-resolution lake-based sub-bottom profiles in one of the valleys (Chapter 3); detailed site-scale field description of the internal characteristics of the regional Late Wisconsin till sheet in various subglacial settings (Niagara Escarpment, uplands, lowlands; Chapter 4); comparison of the characteristics of the subglacial bed within the study area to adjacent regions in southern Ontario (Chapter 5); and a synthesis of the major findings from all the different components of this investigation and suggestions for future work to shed further light on several questions that arise from this study (Chapter 6). Together, key data from these studies of tunnel valleys and related deposits – a near-continuous till sheet on the surface of uplands and along the flanks and floors of the tunnel valleys, multi-stage drumlinization of the till sheet following development of the tunnel valleys, variations in internal facies and physical properties within the till sheet in different subglacial settings, localized distribution of coarse-grained tunnel valley in-fill sediments, and gradational upward transitions from tunnel valley in-fills to fossiliferous proglacial lacustrine sediments – indicate multiple phases of subglacial meltwater, and direct subglacial, erosion and deformation contributed to the development of the valleys over a protracted time period during the Late Wisconsin. Landform and sediment associations within the valleys in Simcoe County and surrounding parts of the bed of the former Laurentide ice sheet in south-central Ontario, are inconsistent with previous conceptualizations involving the presence of large (>1000 km2) subglacial lakes and the storage and discharge of regional-scale subglacial meltwater sheetfloods followed by ice stagnation. This study provides new data and insight to help refine reconstructions and better understand the evolution of past ice dynamics and subglacial processes, evaluate competing theories of regional landscape evolution, and provide new conceptual and (hydro)stratigraphic frameworks for future hydrogeological investigations related to groundwater exploration and use. / Thesis / Doctor of Philosophy (PhD)