Valley terraces and Huron basin water levels, southwestern Ontario

Abstract

Many of the westward-draining valleys east of southern Lake Huron contain a prominent raised river terrace. At Lake Huron, shore erosion has left the truncated end of the terrace hanging. Upstream, the terrace merges into modern flood plain. Some of the larger valleys contain two or three terraces that successively merge with the flood plain upstream. Stream gradients are over-steepened, and nickpoints are retreating upstream. Terrace sediments commonly comprise 1 or 2 m of fining-upward alluvial gravel, sand, and silt. In some valleys, downstream portions of the raised terrace are underlain by 5 to 10 m of fining-upward sediment, which is mainly silt. Terrace sediments are in many cases fossiliferous (pollen, plant macrofossils and wood, ostracodes, molluscs, insects). Radiocarbon dates reveal that the main (highest) terrace has an age of 11300 to < 10,500 B.P. and is correlated with transgressing Lake Algonquin. A middle terrace, which has an age of ∼10,000, is related to a lower Algonquin level, and the lowest terraces date 5000 to 6000 B.P. and correlate with the Nipissing transgression.