Ze Cobbles on Sombrio EarthCache
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The gravel road leading down to the parking lot is home to many large pot holes (6 to 10 feet across, and any where from 4 to 10 inches deep) A vehicle with high clearance is highly recommended. If you wish to leave your car on the side of the gravel road, you can walk the extra 1 1/2 km or so to the parking area. From the parking area, the beach is an easy ten minute walk. Walk past the camping area and along the beach to get to the posted co-ordinates. Here you will be standing in the middle a beach made up of large cobbles. Cobbles are made up of many different kinds of rock like sandstone, granodiorite, and basalt. With a little bit of searching, the three basic types of rocks (sedimentary, igneous, and metamorphic) can be found here. Cobbles tend to be smaller at the waters edge and get larger as you go up to and past the high tide line. If you decide to visit Slot Canyon Hidden Waterfall GC695FW, you will see boulders along the shore line and farther out in the water. Hazards include rocks, waves, and locals.
Enjoy yourself and have fun!
The focus of this earthcache is the process that formed the cobbles we see on site today.
Before you log this earthcache as found, PLEASE copy and paste the questions, and email me (click message this owner and choose message center) your answers:
1) Listen, watch and describe, in a couple of sentences, what happens at the shoreline with each ocean wave? Things to note in your answer
- what is moving
- frequency of movement
- is there more movement with the in-coming or out-going wave?
- slope of the beach
- anything else you care to add
2) Cobbles found up the beach near GZ and beyond the high tide line line: what would you say is their average diameter?
3) What would you say is the average diameter of the cobbles at the waters edge and what explanation can you think of for this difference?
4) Please explain in a sentence or two, What action do you think has played the biggest part in rounding the cobbles:
- the glaciers bringing the till
or
- the waves hitting/ moving the till around?
5) Cobbles are made up of an assortment of rocks. Can you find and identify three different kinds of rock?
What is a Cobble?
The geological term cobble is used to describe a stone of a particular size, which is approximately two and a half to ten inches. Colors range from grey to black to purple, depending on the origin of the stone. The term boulder is used to describe a rock fragment that has a diameter larger than 10.1 inches.
How are Cobbles made?
The rocks are being pushed up the beach and rolled back down with each wave.The force of waves rolls the rocks on the shoreline around. Over thousands of years, this constant movement has rounded the jagged edges of the broken rock pieces and formed them into the cobbles you see today.
Boulder/Cobble Shoreline
Rocky coasts, where the local geology supplies a source of sediment, is where you can find boulder/cobble shorelines. It takes thousands of years for these shorelines to form. Boulders and cobbles begin as chunks of rock fragments in glacial till. Till is made up of rock fragments that were; ground up under the ice sheets, deposited on the surface of glaciers, pushed up in front of advancing glaciers, or carried along on the back of the glacier. The erosion of bluffs or cliffs composed of till can be a source of till. Till can also be made from sediment that was deposited by rivers which formed when the ice sheets melted which transported glacial sediment to new locations.
The dominant sediment on a boulder/cobble shoreline takes the form of boulders (rock greater that 25 cm in diameter) or cobbles (rounded rock 5 to 25 cm in diameter) that may overlie smaller sediment (sand/gravel) or a bedrock shelf. Boulder and cobble shorelines support a varied assortment of organisms that may be mobile or attached to rocks. Boulder/cobble shorelines can be found in a few locations near Victoria, for example on Mt. Douglas beach, Sombrio beach and some locations along the Victoria waterfront by Dallas Road.
Boulder/cobble shorelines are shallow sloping sediment beaches which are moderately resistant to erosion. The beach slope and form are determined by wave and tide action. On moderate to high wave energies, the smaller cobble material will be mobile, while in sheltered areas, the beach material will be stable. Boulder/cobble shorelines at moderate and high wave exposures may be subject to erosion during storms, however the finer sediments in the mixture are more easily transported and can accumulate in pocket beaches between more stable shore types if longshore transport is present. Boulder/cobble shorelines may be stable or dynamic and are likely to change on a human time scale.
Boulders and cobbles on the beach (very low tide)
Additional Hints
(Decrypt)
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