AQUIFER OR DRAIN? THE SOUTHERN ONTARIO ROCKWOOD BURIED BEDROCK VALLEY

Past work identified an extensive network of buried bedrock valleys in southern Ontario, evident as a series of linear depressions on the regional bedrock surface. This network has been the focus of water-supply investigations, e.g. the Dundas buried bedrock valley study. A recent regional 3D surficial mapping project centred on the Orangeville Moraine has provided the impetus for investigating the Rockwood buried bedrock valley. The valley extends from Rockwood northeast, past the town of Erin, bisecting the Niagara Escarpment at the Credit River Valley. The valley is important as it represents both a potential water source and a conduit for draining ground-water over the escarpment.

The well records used to generate the bedrock surface have a sporadic distribution so that valley thalwegs are often missed unless the wells happen to be over the deepest part of the valley. The Dundas buried bedrock valley study has shown that ground-based gravity surveys can be an effective tool for predicting buried valley systems and a similar approach was adopted for the current study. Eight gravity lines were surveyed along roads selected to be perpendicular to the target thalweg. In profile the widths and amplitudes of the negative residual anomalies increase to the northeast, suggesting that the buried valley is widening and deepening in that direction as it approaches the escarpment.

Four continuously cored holes were drilled along the length of the valley to characterize the fill and determine whether aquifers or aquitards are present. The southern two cores reveal late glacial till at surface overlying 45-55 m of gravelly sand or sand fining upwards to fine sand and silt, representing a significant potential aquifer. A lower diamicton package overlies valley bottom gravel. The northern core, located near the escarpment, has a similar thick fining upwards sequence of gravel, sand and silt. At this location, however, the protective till cover has been eroded and the uppermost 25 m is characterised by outwash sand and gravel. The final core, surficial outwash gravels over thick till, presents an interpretative challenge as it bears little resemblance to the valley-fill encountered north and south. This presentation will present the latest results of the project and explore the challenges of characterising the surficial sequence.