Abstracts

e-entrant bedrock valleys on the Niagara Escarpment. Spencer’s pioneer work in the late
19th century on buried bedrock valleys has had profound influence on the subsequent
studies on these geologic features as to their origins and age. A commonly held view is that
these bedrock valleys and depressions were carved by the Tertiary rivers and subject only to
limited glacial modification in the Pleistocene. Many adopted this concept in subsurface
mapping and their interpretation of such features in southern Ontario and elsewhere in
the Great Lakes region. Consequently, the bedrock valleys and depressions are often interpreted
as relic river channels with a dendritic planview and any deep bedrock depressions
encountered locally in geotechnic and groundwater drilling are thought to be segments
of such fluvial features. With the availability of large datasets of borehole records, advent
of computer-aided contouring technology, and the advances in our understanding of the
subglacial processes and hydrology, the regional buried bedrock valleys and depressions
can be mapped and studied in a much detailed way. Recent bedrock topography mapping
has shed new light onto these geologic features as to their orientation, planviews, spatial
relationship, and internal geometry, e.g., longitudinal profiles. This work has also revealed
for the first time, below Lake Erie, a glacially-scoured large bedrock valley named Long
Point trench along the lake basin. The newly acquired information does not confirm the
previously assumed relic river channels and drainage connections. Instead, it suggests that
the bedrock valleys can best be explained in the context of glacial and subglacial meltwater
erosion. To be specific, the deep bedrock gorges are likely of subglacial meltwater tunnel
valley origin and those broad ones such as Laurentian and Ipperwash troughs are created
through erosion by multiple glaciers over the Pleistocene. The glacial and subglacial meltwater
processes have generated extensive clayey to gravelly deposits that fill the bedrock
valleys and depressions with great thickness. Many of the gorge-like bedrock valleys likely
developed during the Late Wisconsinan and they contain concentration of glaciofluvial
sand and gravel deposits which are important regional aquifers.
230 - The pre-Late Wisconsin stratigraphy of Simcoe County,
southern Ontario
Riley P.M. Mulligan & Andy F. Bajc
Ontario Geological Survey, Sudbury Ontario, Canada
Sediment drilling investigations by the Ontario Geological Survey in Simcoe County have
identified thick sediment successions (>140 m) underlying Late Wisconsin glacial deposits.
These sediments record environmental conditions that existed prior to and during the
last build-up of the Laurentide Ice Sheet. Pre-Late Wisconsin sediments are subdivided
into 5 stratigraphic units (SU’s). A lowermost glacial package directly overlies Paleozoic
bedrock. It is composed of two distinct tills, a lower sandy till (SU1) and an upper silt-rich
till (SU3), rarely separated by fine-grained glaciolacustrine deposits (SU2). The upper surface
of the silt till is extensively weathered and, in places, leached of carbonate. This weathering
records base levels up to 30 m below modern Lake Huron and represents a regional
unconformity in the subsurface. Overlying the weathering surface are alluvial sand, silt and
gravel deposits (SU4) rich in organic material. Preliminary analysis of pollen and plant
macrofossils suggests a cold, dry, sub-arctic climate. A possible interglacial assemblage
64 IAH-CNC 2015 WATERLOO CONFERENCE