Shoreline Outing Summary: Mission Road Pleistocene Deposits, Mar 13th, 2019
Roughly twenty shoreline members made it out on Wednesday to investigate 12,500 year old sand deposits laid down 10m below sea level back when the the waves would have been lapping at the spot where today you find the front entrance of the Home Depot at Lerwick & Ryan roads. We collected quite a few shells from a number of mollusc species (see annotated photograph below) and looked at a brief window of the local Pleistocene sediments. To follow up on some of the things we were curious about, there are some notes, diagrams and photographs below.
What did we find?
We found 12,500 shell deposits that showed a diversity of sizes and shapes indicative of a natural concentration of shell material driven by water currents. In the 250ml plastic container of shells, we had the following
Barnacles (Balanus sp.)
Tubeworms (Serpula vermicularis)
Scallops (Chlamys sp.)
Limpet (Acmaea sp.)– with bore hole from (?) gastropod radula
Mussels (Mytilus sp.)
Butter Clam (Saxidomus giganteus)
Echinoderm spines (see pictures– in the fine sediments)
Echinoderm spine (rod-shaped fragment among 1/2mm sand grains.
What is the sequence of unconsolidated sediments in our area and how old are they?
The shells, bones & wood excavated from the site give a corrected age of around 12,5000 years based on radiocarbon dating. We were digging through the “Capilano” and/or “Salish Sediments”, layers that were deposited along the sea floor (and shore) after the ice had retreated. Looked down an excavated face in the quarry and saw poorly-layered glacial till (“Vashon Drift”) beneath (material melted out of the ice sheet). This in turn would be underlain by stratified sand and clays of the Quadra Sands, which were deposited by waters running off the leading edge of the ice sheet as it retreated and advanced sometime between 20-30,000 years ago. The Quadra Sands are the sediments that make up most of the Willemar Bluffs in Cmox as well as the cliffs around Seal Bay, Denman Island and Quadra Island. I am pasting below a chart showing the ages, composition and sequence of major layers of sediment in our region from Jan Bednarski’s (Geological Survey of Canada) 2015 report on surficial deposits just south of Deep Bay.
John was remarking on how well-preserved the shells are given their age and how porous the sediments were. These shells would be subject to all the chemical weathering that the pore waters could bring about. Perhaps the density of shell material acted as a buffer to keep acids from leaching away too much of the material. As many of you saw, some of the mussels still had a hint of nacre left on them, meaning that the organic matter has not completely broken down, either.
These types of materials are broadly referred to as sub-fossils. True fossils are old and encased in consolidated sediments. Really it doesn’t matter– these are evidence of ancient life in our region just around the time that we start to see physical evidence for human habitation in our region. The assemblage of shells is interpreted to represent a cold water community akin to what would be seen around 60 degrees north today.