The Guelph Moraine, a previously undesignated end moraine between Hespeler and Rockwood, is described and shown to be the consequence of a limited readvance that interrupted the general recession from the Guelph area of ice that deposited the Port Stanley Till rather than of a precursive advance of younger ice that laid down the Wentworth Till and eventually produced the Paris Moraine.From Rockwood and following broadly the Eramosa–Speed river a gravelly outwash train is traced southwest toward Hespeler, where it appears to merge with sandier deposits of Grand River provenance and to pass into an extensive sand "plateau" south of the town. These pre-Guelph sands and gravels were overridden by ice at Rockwood and southwest of Guelph during the readvance. Meltwaters were forced at first to detour westward along the upper Speed River and Ellis Creek valleys but later found their way beneath ice at Rockwood to initiate subglacial erosion of the gorges and adjacent dry-valley systems.The Eramosa–Speed "spillway" was therefore established during the Guelph stage, and the Eramosa–Speed river adopted its present course during recession from the Guelph Moraine. Paris stage meltwaters deposited extensive outwash plains along the southeast side of the Guelph Moraine and had to break through this moraine to reach the Eramosa–Speed system. All events took place during the Port Bruce Stade.