Mount Mazama (located some 1100 km southwest of Calgary) was formed starting about 400,000 years ago, from a cluster of overlapping volcanoes. For most of its history, Mazama’s eruptions were not especially violent. But around 6800 years ago, an explosive eruption sent ash high into the stratosphere with a force 100 times greater than the Mount St. Helens eruption in 1980. The ash blew northeastwards, blanketing almost 1.3 million square kilometers, including the southern half of Alberta.
The eruption drained the magma chamber which fed the volcano, and the mountain collapsed inwards, creating a huge crater, called a caldera. The crater was over 1200 meters deep and 9 km wide and today the site is known as Crater Lake, the second deepest lake in North America.
Mazama Ash is often found in exposures along river valleys in southern Alberta. Like all volcanic ash deposits, the Mazama Ash has its own distinctive mineral composition, which makes it useful for correlating and dating deposits across large areas. It is a particularly important tool for archeologists studying Alberta’s past because they use it to date remains of older cultures. Any artifact found below the ash is considered quite rare.
When you visit the posted coordinates in Fish Creek Park, you should notice how less than a meter of younger silts (light) and soil (dark) has been deposited on top of this ash in the intervening years. Note also how the ash was deposited on an older layer of dark soil, and that the older soil is on top of still older river gravels. These gravels indicate an early higher level of Fish Creek. Also at this location you can see blocks of the much older Porcupine Hills sandstone.
If there are no leaves on the trees and there is no snow on the ground, you can see the ash band from the deck of bridge 6.
PLEASE NOTE: while you may be tempted to get an up close view of the ash layer by scrambling up the embankment along the narrow dirt trail, please DON’T. That is not an official trail and the Fish Creek Park personnel have specifically requested that any geocachers visiting this location stay on official paved or granular trails. Thanks for your cooperation in this aspect of park preservation. You can get an up close view of the ash layer by visiting the Mazama Ash – Edmonton earthcache location (GCZDAF).
To log this cache, please complete the following:
1. Carefully study the embankment and email the cache owner (do not post) your estimate of the length of the exposed ash layer. Also estimate the average height of the exposed blocks of Porcupine Hills Sandstone.
2. A photo of someone in your group holding a GPSr, with the embankment in the background, would be much appreciated.
References:
Mussieux, R. and Nelson, M., 2005 reprint. A Traveller's Guide to Geological Wonders in Alberta. The Provincial Museum of Alberta. pp. 74
Foley, J., 2006. Calgary’s Natural Parks – Yours to Explore. Calgary Field Naturalists’ Society, pp. 106