Geomorphic analysis and optically stimulated luminescence (OSL) ages from undated Lake Agassiz beaches and adjacent fluvial sediments on Riding Mountain in Manitoba provide insight into their early history. New OSL ages of 14.5±2.4 and 13.4±0.7 ka on the oldest (Herman to Norcross) beaches of Lake Agassiz near the Canada-U.S. border indicate that the Laurentide Ice Sheet (LIS) retreated from that part of the Agassiz basin by ~14.5 ka. To the north along Riding Mountain, the Herman strandlines are absent, and OSL ages on the oldest beach there average 12.9 ka, which links it to the younger Norcross-Tintah strandlines. In adjacent Riding Mountain, OSL ages and geomorphological relationships of a large abandoned glacial spillway >200 m above the oldest beaches of Lake Agassiz indicate that this channel predates retreat of the LIS and formation of beaches in this part of the Agassiz basin, with ice remaining in this area until after 14.5 ka. OSL ages on the Gimli beach 170 km to the east are >3000 yr older than conventional assignments, suggesting that it formed during the Moorhead low-water phase 12.8–10.6 ka. Luminescence ages support the conclusion that the Campbell beach formed ~10.9 ka near the end of the Moorhead low-water phase.

The late Wisconsinan deglaciation of the Riding Mountain Uplands was associated with the Lockhart Phase of glacial Lake Agassiz (11,600-10,800 B.P.). During the waning stages of the Falconer ice advance (post 11,400 B.P.) a large area of glacial ice stagnated on the Riding Mountain Uplands. Subsequent downwasting generated a drainage network consisting of several superglacial lakes, spillways and meltwater channels. Many of these glacial rivers eroded their ice beds and incised into the substratum. Glaciofluvial sediments were deposited as sandurs, eskers and kames. Subaqueous fans were deposited in the superglacial lakes and a major delta was built into the north end of glacial Lake Hind. During this time an advance of the Valley River Sublobe created the Mears kame moraine. Following this advance. Northeastern ice (the Assiniboine Sublobe) retreated from the Rivers - Rapid City Endmoraine to the Brookdale position. As downwasting of the stagnant ice on the uplands continued, the superglacial lakes drained and an entrenched drainage system developed on the stagnant ice moraine complex. RÉSUMÉ La déglaciation du Wisconsinien tardif du plateau de Riding Mountain était associée à la phase Lockhart du lac glaciaire Agassiz (11,600-10,800 B.P.). À la fin de l'avancée glaciaire Falconer (après 11.400B.P.), une vaste étendue de glace est resiée stagnante sur le plateau de Riding Mountain. L'ablation subséquente a laissé un réseau de drainage composé de plusieurs lacs supraglaciaires ainsi que des canaux de débordement et de fonte. Plusieurs de ces rivières glaciaires ont érodé leur lit de glace et le substrat. Les sédiments fluvio-glaciaires ont été déposés sous formé de sandurs, d'eskers et de kames. Des cônes subaqueux ont été déposés dans les lacs supraglaciaires et un delta important a été formd à l'extremité nord du lac glaciaire Hind. Pendant ce temps, une avancde du lobe secondaire de la rivière Valley a cté é la moraine à kames Mears. Après cette avancée, la partie nord-est du glacier (le lobe secondaire Assiniboine) à reculé, dépuis la moraine frontale de Rivers - Rapid City jusqu'à la position de Brookdale. Pendant l'ablation du glacier stagnant sur le plateau, les lacs supraglaciaires se sont drained et un réseau de drainage encaissé s'est développé sur le complexe de moraine associé au glacier stagnant. [Traduit par le journal]

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