Angular Unconformity
Vancouver Island is made up of twelve different geological
groups, each having been formed in different eras. This location
takes you to a rock outcropping that juts into the ocean and will
display rock from two different eras that make up the island.
The bottom layer of rock is vertically stratified Sicker Group
argillite (a hard, dense type of shale). The Sicker Group was
formed during the Paleozic era, about 374-360 million years
ago.
The top layer is the boulder and cobble conglomerate of the
Nanaimo Group. The Nanaimo Group was formed during the late
Cretacious period, about 85-65 million years ago.
What makes this site particularly interesting is that it is an
excellent example of a geological unconformity. Geologists define
an unconformity as “a time gap in the geological record
represented by an absence of rock due to a period of erosion or
nondeposition” (Yorath 192). More specifically, this is an
angular unconformity - “one separating two groups of rocks
whose strata are not parallel.” (Yorath 192) The following
graphic illustrates angular unconformity:
"Angular Unconformity" graphic used with permission from the
WikiMedia
Commons.
As you can see on the geological time chart below, the Nanaimo
Group was not laid directly over the Sicker group. There were
several other groups that helped form Vancouver Island between the
time the Nanaimo and Sicker Groups originated. At this location,
the rock that is missing between the Sicker and Nanaimo rock
represents 280-300 million years of history.
Graphic based on pg. 23 of Yorath, Chris. 2005. The Geology
of Southern Vancouver Island. Madeira Park, BC: Harbour
Publishing.
Information for this cache was obtained from: Yorath, Chris.
2005. The Geology of Southern Vancouver Island. Madeira Park, BC:
Harbour Publishing.
Validating Your Find
To log this cache, please send me the answers to the following
questions:
1. Describe both the Nanaimo and Sicker Group rocks.
- Sicker Group: Colour? Texture? Size of rocks embedded
within?
- Nanaimo Group: Colour? Texture? Cracks running horizontally or
vertically?
2. Define the geological term “unconformity.”
3. Once at the coordinates, look down at the rock you’re
standing on (the one that gives the best view of the unconformity,
not the unconformity itself). There are number of cracks running
along a particular geographic axis. Along which axis do these
cracks run (i.e. north-south, east-west)?
4. Go to N49 18.878, W124 12.877 and put your GPS on the ground.
How many meters above sea level is the top of the rock?
5. Take a photo of you and/or your GPS at the location and post
it. (OPTIONAL)
Access and Site Info
This cache is tide-dependant; go at low tide! Parking is found
at N49 18.886, W124 12.802. There is a short public access stairway
onto the rocks. The rocks at this location are very slippery when
wet. Keep an eye out for seals and eagles. Every time I’ve
been to this spot I’ve seen cracked open sea urchins all over
the place. Watch out for the spines that cover the urchin shells.
While they’re brittle from being in the sun, they’re
still sharp!