You must e-mail me some unique bits of information regarding the sites that you learnt while there. This can be done by answering the following questions:
How was Pow Wow Rock formed? What evidence leads you to this conclusion?
How were the erratics in Southmere Park formed? What leads you to this conclusion?
Please don't post photos of the erratics. I will monitor logs and check my e-mails for appropriate information. Unfortunately, any log that does not meet the requirements will be deleted. Thanks for visiting an Earthcache!
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Most of British Columbia was covered by an ice sheet during the Late Pleistocene period.
South Surrey has a ridge running east-west indicating this was a terminal moraine.
Dotted through this area are erratics, of which, Pow Wow Rock is one.
Erratics - substantial boulders that have been moved vast distances from their bedrock sources by enormous ice sheets that covered much of British Columbia - can be found across the Salish Sea lowlands in British Columbia and Washington State. As the ice sheets melted, these boulders would be deposited. Pow Wow Rock, a boulder the size of a small building, has been recognized as a heritage site by the City of Surrey. It provides evidence of how the Late Pleistocene ice sheets brought these large boulders to this area.
Erratics are formed by glacial ice erosion resulting from the movement of ice. Glaciers erode by multiple processes including:
- Abrasion/Scouring – debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood, producing smaller glacial till.
- Plucking – pieces of bedrock are cracked off by glaciers, producing larger erratics.
- Ice thrusting – the glacier freezes to its bed, moving large sheets of frozen sediment at its base along with it.
- Glacially induced spalling – layers of rock are spalled off the rocks below the glacier during ice lens formation. This provides smaller debris, which is ground into the glacial basal material, to become till.
Evidence supports another possibility for the creation of erratics as well: rock avalanches onto the upper surface of the glacier (supraglacial). Rock avalanche - suberglacial transport occurs when the glacier undercuts a rock face, which fails by avalanche onto the upper surface of the glacier. The characteristics of rock avalanche–supraglacial transport includes:
- Monolithologic composition – a cluster of boulders of similar composition are frequently found in close proximity. Commingling of the multiple lithologies normally present throughout the glaciated basin, has not occurred.
- Angularity – the supraglacially transported rocks tend to be rough and irregular, with no sign of subglacial abraision. The sides of boulders are roughly planar, suggesting that some surfaces may be original fracture planes.
- Great size – the size distribution of the boulders tends to be skewed toward larger boulders than those produced subglacially.
- Surficial positioning of the boulders – the boulders are positioned on the surface of glacial deposits, as opposed to partially or totally buried.
- Restricted areal extents – the boulder fields tend to have limited areal extent; the boulders cluster together, consistent with the boulders landing on the surface of the glacier and subsequently deposited on top of the glacial drift.
- Orientations – the boulders may be close enough that original fracture planes can be matched.
- Locations of the boulder trains – the boulders appear in rows, trains or clusters along the lateral moraines as opposed to being located on the terminal moraine or in the general glacial field.