| Summary: | During a 1995 aerial video survey of the coastlineof Johnstone Strait, an unusual shoreline featurewas noted and termed “clam terraces” (inset)because of the terrace-type morphology and theapparent association with high clam productivityon the sandflats. Typical alongshore lengths of theterrace ridges are 20-50m, and across-shore widthsare typically 20-40m.An area with an especially high density of clamterraces was noted in the Broughton Archipelago,between Broughton and Gilford Islands ofsoutheastern Queen Charlotte Strait. Clam terracesin this area were inventoried from the aerial videoimagery to quantify their distribution. The terraces accounted for over 14 km of shoreline and365 clam terraces were documented.A three-day field survey by a coastal geomorphologist, archeologist and marine biologist wasconducted to document the features and determine their origin. Nine clam terraces weresurveyed. The field observations confirmed that: the ridges are comprised of boulder/cobblesizedmaterial, ridge crests are typically in the range of 1-1.5m above chart datum, sandflats arecomprised almost entirely of shell fragments (barnacles and clams) and sandflats have very highshellfish production. There are an abundance of shell middens in the area (over 175) suggestingthat the shellfish associated with the terraces were an important food source of aboriginalpeoples.The origin of the ridges is unknown; they appear to be a relict feature in that they are not activelybeing modified by present-day processes. The ridges may be a relict sea-ice feature, although themechanics of ridge formation is uncertain. Sand accumulates behind the ridge because the supplyrate of the shell fragments exceeds the dispersal rate in these low energy environments.The high density areas of clam terraces correspond to high density areas of shell middens, and itis probable that the clam terraces were subjected to some degree of modification by aboriginalshellfish gatherers over the thousands of years of occupation in the region. (Document contains ...
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