Retreat! EarthCache
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This earth-cache takes you to a unique glacial formation in the Chilliwack river valley. Please stay on the trail, and DO NOT enter any fenced or gated areas, as this site in nearby the Slesse DND! The listed coordinates take you to the earth cache site. Parking coordinates are included in the waypoints.
This site will take the geo-cacher to a beautiful example of the slopes which are a part of Larson's Bench, along Chilliwack Lake Rd. A short hike to the site will provide a unique view of the glacial drift which was deposited on the second retreat of Alpine glaciers which covered this area thousands of years ago.
Some definitions which are needed to understand this earth-cache are as follows:
· Abrasion – the process by which sediments are eroded by forces such as water, wind or ice. The sediment (rocks or soil) are worn down and may have striations left in the surfaces.
· Alluvial – carrying sediment.
· Batholith – a large area of ancient magma which has hardened into bedrock; usually granite.
· Cobbles – larger rocks which have been abraded and rounded by being tumbled around in water or ice (much like a blender). Rocks which have been carried by rivers are usually well-rounded.
· Delta – a large area of fine sediments, clays and sands which are deposited by a river as it empties into a larger body of still water. These areas will often look like a large fan shape.
· Deposition – sediment which is deposited, usually by rivers or glaciers.
· Erosion – sediment which is taken away from one area, usually by rivers or glaciers.
· Erratic – large rocks or boulders which have been left behind by a glacier after it retreats. These rocks can sometimes be a different type of rock than is found in the area.
· Fluvial outflow – sediment which is left behind by water systems such as rivers or streams.
· Glacial outwash – the sediment and debris which is left behind by a glacier as it “retreats” or “shrinks” away from the area it once covered.
· Glacial drift – sediments left behind by glacial meltwater. These sediments are often stratified.
· Sandur – an area of glacial drift which is deposited in a flat area or valley as the glacier melts.
· Stratified – soil or sediment which is deposited in noticeable layers; one on top of the other.
· Till – glacial deposits.
· Un-stratified – soil or sediment which has no noticeable layers.
As glacial ice melts, it leaves all of the sediment behind which it was carrying. The area at this earth cache is called a Sandur. As the sediments are left behind, they form layers of sorted sediment. These areas look very similar to a delta plain. The primary difference between a delta and a sandur is that a sandur includes different sizes of sediment, while a delta is mostly fine grained material. As glaciers cover the land, melt-water inside the glacier deposits till. This is left behind upon glacial retreat. An alluvial river, in contrast, will erode sediments and carry them further downriver.
Larson's Bench forms a delta fan shape through the valley that was left behind as the glaciers retreated behind Chilliwack Lake. The resulting land forms give witness to an unsorted sedimentary deposit of fine sands and clays, as well as large abraded rocks and cobbles of granite which were carried along with the glacier.
The stratified layers of soil which can be seen on top of the cliff faces indicate deposits laid down long after the glacial retreat. Alternating layers of orange and black indicate oxidation deposits (orange) as well as events seen in the black, carbonized areas. Since the glacial retreat, many historic events have affected this area, laying down further layers of sediment through floods, fires and storms. The larger rocks seen here were carried from the Chilliwack Granitic Batholith sites found NW of the area.
In order to claim this earth-cache, please email Family Extremes (via the profile link at the top of the page) with answers to the following questions. Although it is optional, we would love to have pictures of you and your group at the earth cache site posted on the cache page.
1.) What, in your opinion, is the cause of the blackened layers on the top of the cliff?
2.) What is the general shape of the rocks in the area, and what rock types do you see here?
3.) In your opinion, why could these rocks not have been left by river flooding, rather than glacial deposit?
Again, for you own safety, please remain on the obvious trail, and do not enter the gated access road or nearby fenced compound! We hope you enjoy this area, and have learned some of the incredible history of the valley!
Additional Hints
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