Important Access Information
This cache can only be accessed when driving UP the hill towards Cascade Lookout. There is parking for 2 cars right across from the outcrop. There is no parking on the way down.
This cache is seasonal and should be accessible for 6 to 7 months of the year. The road is closed in the winter and gradually opens up in the spring as the snow melts.
General Geological History
The rocks underlying Manning Park form part of the Cascades Mountain Range, which extends from southern British Columbia to northern California. The Cascades are in turn, part of the Cordillera, which include the entire western mountain ranges of North America, from Mexico to Alaska. Most of the rocks are considered to be in the Paleozoic Era (650-225 million years ago). During the Paleozoic and Mesozoic (225-65 million years ago), a large submarine trough was situated off the current coast of North America. As the forces of wind and water eroded the North American land mass during the Paleozoic and Mesozoic Eras, rivers carried the resulting sediments westward into the ocean. These sediments were deposited into this trough and eventually Continental drift slammed the trough onto the western edge of North America and formed the many mountain ranges that exist today. Various mountain building episodes brought these rock units (sandstones, shale, conglomerates, and other sedimentary rocks), to their present location in Manning Park. (From Geological Survey of Canada (GSC) Bulletin 238 by J. A. Coates, 1974.)
Earthcache Local Geology
At the site of this earthcache, most of the rocks belong to the Jackass Mountain Group and are comprised chiefly of sandstone, shale, and conglomerate. All of these units are formed in water. Sandstone is just that, small sand grains that are deposited in layers that are ultimately compacted into solid rock. Shale is very fine-grained muds and silts, usually deposited in quiet waters and ocean basins. Conglomerate consists of varying proportions of coarser-grained pebbles (or clasts, or cobbles), often cemented together by finer-grained material called the matrix. Conglomerates form in fast-moving waters such as rivers, and the high energy of quickly moving water is needed to transport the larger rock pebbles.
There are two major classifications of conglomerate rocks: (i) clast supported, or (ii) matrix supported. Clast supported means the coarse-grained cobbles or clasts are basically what support and hold the unit together, often the larger clasts touch each other and the finer-grained matrix materials infill the pore space between the clasts. Matrix supported means there are generally fewer clasts and the fine-grained matrix holds or cements the unit together and supports the larger clasts within the fine-grained sands and silts.
A typical example of the conglomerate at GZ.
The rock exposure at GZ shows the contact between a dark black, fine-grained shale, and a lighter-coloured, coarse-grained conglomerate. These units were all waterlain, meaning they were deposited by water, either creeks or rivers, and can ultimately be deposited into lakes or oceans. The units represent different grain sizes ranging from fine- to coarse-grained. When answering some of the questions in this earthcache, think about the rate of water flow required to move these different materials. Generally a higher flow rate can move larger materials. The contact should be fairly obvious given the colour difference between the black shale and the whitish conglomerate. If you wander around, you can see pits in the conglomerate where the clasts have eroded out. (A bonus for the puzzle lover would be to fit the clasts back into their respective holes ;-))
To log this Earthcache, please answer the following questions:
- Approximately how wide is the bed of shale?
- Is the contact sharp or is it an intermixing of two different units?
- Is the fracturing in the shale parallel to the contact?
- What unit is more fractured, the shale or the conglomerate?
- What is the largest clast you can see in the conglomerate unit?
- Are the clasts all the same rock type or are they a mixture of different rock types (lithologies)?
- Do you think the conglomerate is clast or matrix supported?
- Which unit do you think was deposited in fast running water and which one in slow water?
- Optional. Feel free to post photos of your visit to this Earthcache.