Drift dispersal in central Southern Ontario /
Abstract
Two tills are readily identi-f i able in central Southern
Ontario, a very stony, loose deposit o-f variable matrix
(Dummer till) and a moderately stony, fissile and compact
deposit that is more homogeneous (drumlinized till). The
quantity o-f Precambr i an, Paleozoic and Shadow Lake Formation
(Paleozoic) rock types were determined and corresponding
isopleth maps drawn. The changes in lithology content occurred
in the direction o-f transport, there-fore, compositional
isopleths o-f till may be considered equipotential lines for
the reconstruction of glacier flow paths.
Areal gradations of drift lithology indicated that the
prime agents of dispersal were ice and glacial meltwaters.
The down-ice abundance trend of till components indicated a
dispersal pattern showing the concentration of a given
lithology type peaking within a few kilometres of the source
followed by a rapid decline and thereafter, a more gradual
decrease with increasing distance. Within the esker
deposits, igneous rocks may form the major component and can
extend further onto the limestone plain than in the adjacent
till.
Evidence is presented that indicates the "style" of
dispersal was one in which glacial ice may have been strongly
influenced by local bedrock topography and the regional
structural trends. The ice tended to follow pre-existing
valleys and lows, depositing till composed mainly of local
bedrock. Gradations in Paleozoic clast content showed that the local bedrock lithology became the primary till component
within 3 km of down-ice transport.
Evidence is presented that indicated the last glaciation
may have occurred as a relatively thin ice mass, followed
by stagnation and recession. No evidence of a lateglacial
re-advance was found within the study area. Because
of the lack of a contact between the Dummer and drumlinized
till, and because of results showing gradation of the Dummer
till into the drumlinized till (as indicated by lithology
content and grain size), it is suggested that no re-advance
occurred.