Geology of the terraces at Cochrane, Alberta

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The prominent terraces north of Bow River at Cochrane, Alberta, divide readily into an upper and a lower set. The three terraces of the upper set represent deltas built into lakes ponded in front of Laurentide ice. They were constructed between 19 000 and 15 000 years ago, when Classical Wisconsin glaciation was near its maximum. Due to the inhospitable conditions then prevailing, these upper terraces contain few or no fossils. During a warm interstade that followed, the glaciers shrank and Bow River deepened and enlarged its valley until ice readvance, about 12 000 years ago, stopped this phase of erosion. Before the glaciers in the Bow and its tributary valleys again retreated, more than 10 000 years ago, Bow River had deposited vast quantities of fill in its valley. Since then, the river has carved the five lower terraces from that fill and from the underlying bedrock.The valley fill is here named "Bighill Creek Formation". It is a valuable source of gravel and sand, and a prolific supplier of vertebrate fossils. Most of the latter come from a sand unit, here called "Clarke Pit Member", which extends throughout much of the formation. Radiocarbon dates indicate that the member is about 11 000 years old.