We understand that clam garden walls are built at the lowest low tide mark to maximize the size of the clam garden terrace. However, several processes influence how tall the wall ends up being. For instance, newly constructed walls will often be short (1-2 courses of rock high) and only reach their full height after generations of people placing rocks on the wall. On Quadra Island, we interpret single row rock alignments located at the current low-low tide zone to be unfinished walls built during the relatively recent upheaval and unrest associated with the post-colonial period. In some instances, very old walls are short because people have used rocks from the older walls to construct newer ones.
In general, however, we can use the fact that rock walls increase in height and breadth over time as a way to determine when they were last used. That is, given that over time walls are generally built to maximize butter and littleneck clam habitat, Indigenous cultivators generally did not construct the finished wall to be above the zone where these clams thrive.
4000-year-old clam garden at a 1.0 m tide 4000 years ago showing the zone in which butter clams (yellow) and littleneck clams (green) could be harvested (A). Today, as a result of dropping sea levels, this same garden sits well above the zone in which clams grow (B). The height of the terrace wall above chart datum is a relative indicator of its antiquity. Diagram created by Dana Lepofsky
We use the fact that the top of garden walls will be a certain elevation relative to the then-current low-low tide (“tidal height”) to figure out wall age. That is, in areas where ancient sea level curves are known, current wall height can be used as a proxy measure for age. In areas where ancient sea level was dropping for the last 4000 years, if we find a wall and terrace that is above the current productive zone of clams, we know that that wall is old. In areas of rising sea level, those oldest walls will be below water.
Keith Holmes (Hakai Institute) setting up targets on Quadra Island to collect high precision elevation data with drones to determine clam garden wall heights. While wall tidal height can be measured on the ground, the process is time-consuming and limited by the tides. Using this method, we determined the tidal height and age of over 800 segments of clam garden walls, ranging in age from 4000 years ago to quite recent.