Digital Varves

Varves of the Month for 3/1/2010 - 3/31/2010

Varves in the Don Valley, Toronto

Scale bar in cm.

Click on image to download original image file

This month's varves are from a brickyard exposure in the Don Valley in Toronto, Ontario, Canada. The Don Valley Brick Works (link1, link2) is no longer active and is presently a park. The varves were photographed in 1980 by Jack Ridge while on a field trip led by Robert Stewart, then a graduate student at the University of Western Ontario. The varves are probably part of the Scarborough Fm. That was deposited in an early Wisconsinan glacial lake in the Lake Ontario Basin. This lake would have formed as the eastern outlet of the Ontario Basin was blocked by advancing ice in the St. Lawrence Valley.

Winter layers (the upper portions) of each varve are the dark gray units that show up very well in this image because the sediment was partially dry at the outcrop. Some of the well displayed features of the varves are the gradational contacts between the lighter summer layers and overlying winter layers and the laminated character of the summer layers. Also conspicuous is the ice-rafted debris composed almost exclusively of dark gray diamicton that appears to be lined up along laminations throughout the summer layers. This debris probably represents the release of fragments of basal till that adhered to icebergs during calving. The ice-rafted debris rarely occurs in the winter layers. The large till fragment on the left side of the image appears to have sunk into the winter layer below, depressing a thin light silt layer into the underlying darker winter bed.

Past Varves of the Month...