Beach-goers who take a stroll along the waterB次元官网网址檚 edge of Yellow Point, Gabriola Island and Sayshutsun are almost sure to encounter rock circles in the sandstone shorelines.
Spherical rock formations called concretions, sometimes referred to as cannonballs, are commonly found where wave and weather erosion has worn away debris and the sandstone itself, revealing the formations that rise above the layer of sandstone bedrock.
Concretions are similarly shaped, but each has its distinct cracks and other scars from weather wear and geologic processes, augmented by sea life and ocean debris that form around them, making each appear to have been uniquely crafted by nature as an abstract stone sculpture.
TheyB次元官网网址檙e one of the oldest geological features people come across in the region. Shoreline concretions in the Yellow Point area formed in the sandstone from 85 to 65 million years ago, according to Jerome-Etienne Lesemann, professor at Vancouver Island UniversityB次元官网网址檚 Earth science department.
B次元官网网址淭here are various units of sandstone in the region and theyB次元官网网址檙e all part of a broader package of sedimentary rock that we call the Nanaimo Group,B次元官网网址� Lesemann said. B次元官网网址淭hereB次元官网网址檚 many kilometres of sedimentary rock that are part of the Nanaimo Group and within that there are some distinct units of sandstone. You have some in the big tall cliffs on Gabriola. ThatB次元官网网址檚 one example. Yellow Point thereB次元官网网址檚 a whole bunch of sandstone. [Saysutshun] is another place where thereB次元官网网址檚 lots of sandstone. Those big concretions are preferentially formed within those sandstone beds.B次元官网网址�
Lesemann explained concretions form within and from the sandstone itself when sand grains cluster around something that creates a nucleus, such as a small fossil, piece of organic debris or even a clump of coarser sand grains.
B次元官网网址淚n the spaces between the sand grains there are fluids that can circulate,B次元官网网址� Lesemann said. B次元官网网址淭hereB次元官网网址檚 water and that water can contain all kinds of different dissolved chemical elements.B次元官网网址�
Minerals diffused in the water mix with the sand grains to effectively form a B次元官网网址榞lueB次元官网网址� or cement B次元官网网址� hence the name concretions B次元官网网址� in the spaces between the sand grains.
B次元官网网址淚tB次元官网网址檚 that cement thatB次元官网网址檚 going to create this localized hardening of the sandstone B次元官网网址� so early in the process of transforming the sand into sandstone, most likely, is when the beginning of the creation of these concretions happens,B次元官网网址� Lesemann said. B次元官网网址淚t starts fairly early in the process and then, over time B次元官网网址� weB次元官网网址檙e talking probably into the hundreds of thousands to millions of years B次元官网网址� we convert or transform the sand into sandstone and these concretions have already been initiated. TheyB次元官网网址檙e there.B次元官网网址�
The mineral acting as the binding agent is often calcite (calcium carbonate) or in some cases an iron oxide present in groundwater. The even diffusion of the mineral creates the concretionsB次元官网网址� spherical shapes. Erosion over thousands of years wears away the softer sandstone around the hard concretions more quickly, leaving them protruding above the sandstone layer. Concretions vary in colour due to the effects of weather exposure and of the type of binding mineral.
Lesemann said concretions are common in sandstone deposits with similar characteristics to the Nanaimo Group, but only become visible where geological forces and erosion reveal them.
Concretions also appear in cliffs around the region. One large specimen commuters see driving to work on the Island Highway to Nicol Street in south Nanaimo has a crack resembling a mouth, and an artist painted it black and white to create the face of an orca peering out of the cliff side.
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