Examination of a streamlined flow tract in southern Ontario, Canada using LiDAR‐based terrain mapping reveals a landscape that was extensively scoured by ice‐constrained subglacial meltwater. Multiple scours and channels without related sediment fans/deltas are eroded into carbonate uplands at the Shield‐Palaeozoic margin. Erosion extended across the highest uplands, flanks and lowlands with inset eskers, resulting in a streamlined landscape covered with rounded boulders. Palaeozoic carbonate escarpments reveal rock quarrying on stoss and lee sides, likely indicating ice plucking, load deformation and hydraulic fracture. Meltwater overtopping uplands implies ice lifting off the bed, then re‐grounding as flow waned. The widespread erosion, overtopping and presence of boulder lags are indicative of vigorous meltwater scouring of ~10 m of till from carbonate uplands, leaving smooth drumlinized surfaces, scoured by sheetflow and possible ice drag. Rough terrain is inferred to have been meltwater scoured into a series of remnant landforms: poorly defined irregular ridges, scour pits and hummocks, which alternatively possibly resulted from ice deformation and melt‐out. We speculate that rough terrain resulted from pressurized, turbulent flow during progressive, rapid ice sheet closure and ice contact. Erosional landscapes upflow near the Palaeozoic margin can be correlated downflow through the Peterborough drumlin field to well‐documented channel incision and infill sediments beneath Oak Ridges Moraine (ORM), linked by a regional unconformity. A plausible sequence of events includes erosion of a regional unconformity and a complex series of related events that include meltwater flow, ice floatation, then closure, with combined pressurized scour, deformation, ice/hydraulic erosion and later stagnation. Similar to Icelandic jökulhlaups, flooding likely lasted days/weeks such that eroded uplands formed rapidly (<1‐year) followed by matching <1 year deposition (~10–50 m gravel‐sand‐rhythmite fining upward sequence), documented downflow in subglacial channels beneath ORM. Our results suggest a model of sheet flow progressing to channelized flow for meltwater discharge could be considered elsewhere.
Sharpe et al. (Tue,) studied this question.