Take a look at the rock face at this location. There are several distinct layers of rock making up this wall, and they each tell a story. See if you can identify them and figure out how they might have been formed. Follow the layers around the bend to the east of the coordinates and observe whether they remain on the same plane or whether the angle at which they lie changes.
Sedimentary Rocks
When large rocks are exposed to wind, water, ice, or the movement of other rocks, they break down into smaller particles called sediments. The wind, water, or ice carries these sediments along, sometimes for great distances. As they are carried along, these particles are bumped, scraped, scratched, and jostled which causes them to change in size and shape. They can end up as rounded boulders, gravel, tiny pebbles, or even grains of sand or particles of silt.
When the wind, water, or ice subside the sediments are left behind. These deposits sit for long periods of time and undergo changes. They get compacted, buried, and subjected to changes in temperature-- all of which leads to lithification (the process that turns sediment into rock). Water that flows between the particles holds minerals that crystallize and cement the particles together.
Sediment particles that are formed from broken bits of other rock are also known as clasts. The size and shape of these clasts determines how the rocks are classified and named.
Shale is a sedimentary rock whose individual grains are less than 1/256 mm in diameter. The individual sediment grains cannot be seen without a microscope. In order for such tiny particles to settle out of the water that carries it, the water must be still or very slow moving, so shale is often formed at the bottom of lakes or deep oceans. Shale splits into very thin layers and it breaks into flakes when struck.
Shale
Sandstone is another type of sedimentary rock, and its particles are considerably larger than that of shale (1/16mm- 2mm.) The particles can be seen with a magnifying glass or often with the naked eye. The mineral composition of the sand as well as the environment in which it is deposited help to determine the characteristics of the resulting sandstone. This type of rock is formed in thin layers and sometimes splits or crumbles.
Sandstone
Conglomerate is a third type of sedimentary rock consisting of rounded clasts held together by a cemented mix of smaller particles. The clasts can sometimes be very large. The rounded shape of the clasts is usually the result of fast moving water or strong waves that tumble the rocks. They can also be the result of glacial till (a mixture of rock, sand, gravel, etc. that is gathered up by a moving glacier and carried along with it or pushed in front of it.) Large pebbles settle first and the spaces between them are filled in with smaller particles and minerals that cement the entire structure. The type of clasts that are found in the rock identify the source of the sediment.
Conglomerate
Burnaby Mountain is located in an area known as the Georgia Depression-- a low-lying area between the mountains of Vancouver Island to the west and the Coast and Cascade Mountains to the east. This area experienced a number of ice ages, the latest as recently as 15,000 years ago. Glaciers moved from higher to lower areas and brought sediments and till to the area. When the ice retreated, melting water from higher elevations brought sediment as well. With each new ice age, existing sediments were compacted and new deposits were layered on top.
Because it is a relatively young mountain (in geological terms) the surface rocks of Burnaby Mountain have not been exposed to as much pressure and heat as the underlying bedrock, and can be somewhat soft and uncemented. Rocks of varying levels of hardness are evident at this site.
To claim a find for this Earthcache, please e-mail the cache owner the answers to the following questions: (Please do not post the answers in your logs.)
1) How many distinct layers of sedimentary rock do you see at this location?
2) In what order do you see the three types of rock, starting at the bottom?
3) Approximately how thick is the layer of shale?
4) How big is the largest clast that you can observe in one of the layers of conglomerate?
5) What do these layers tell you about the formation of Burnaby Mountain? Think about the depth of each layer and how that particular rock layer might be formed.
Thanks for visiting this Earthcache
references: http://geology.com/rocks/sandstone.shtml
http://geology.com/rocks/conglomerate.shtml
http://geology.com/rocks/shale.shtml
Roadside Geology of Southern British Columbia, by Bill Mathews and Jim Monger; Heritage House Publishing 2010
Geology for Dummies, by Alecia M. Spooner; Wiley & Sons 2011
Vancouver Geology, by Dr. John E. Armstrong http://www.gac-cs.ca/publications/VancouverGeology.pdf
Burnaby Mountain Field Trip Geog. 317 2006 Lab 4 http://www.landfood.ubc.ca/biomet/geog317/Lab4-BBYM.pdf