Government-administered earth-science activities in Canada

Abstract

A corollary of J. Chlachula's interpretation that chipped quartzite cobbles at the contact of Lake Calgary sediments and underlying till represent an “early Late Wisconsinan” lithics industry is the conclusion that Laurentide and Cordilleran ice did not coalesce in Late Wisconsinan time. Contrary to that conclusion are (1) NW–SE-oriented, streamlined landforms in the foothills area which could have been created only by coalescent ice flow, along with numerous radiocarbon dates which indicate that Laurentide ice did not advance to central and western Alberta before Late Wisconsinan time, (2) cosmogenic 36Cl dates of 12–17 ka on the Foothills Erratics Train, indicating Late Wisconsinan coalescence, (3) an estimated thickness of 1000 m or more of Late Wisconsinan coalesced ice in western Calgary based on mapped Laurentide limits 100 km south of Calgary. Furthermore, recently discovered rare Shield clasts in the till at the Varsity Estates and Silver Springs sites suggest that the till was deposited by ice at least partly of Laurentide origin; as Laurentide ice retreated Lake Calgary would have immediately inundated the site, allowing no time or space for human cobble-chipping. Paleomagnetism indicates that the lake sediments at the Varsity Estates site rotated 25 degrees to the NW after deposition, presumably by rotational slumping. Slumping may have been syndepositional, or due to channel cutting during periods of lake drainage and refilling. Neither scenario either supports or contradicts the archeological interpretation. It is possible that the broken and chipped clasts interpreted to be artifacts were created by natural processes.