EarthCache

Sword in the Stone

A cache by Narnian Rockhound Send Message to Owner Message this owner
Hidden : 10/13/2010
In Ontario, Canada
Difficulty:
2 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

The Goal:
The goal of this EarthCache is to expose you to a fundamental boundary between Precambrian metamorphic rocks and Paleozoic sedimentary rocks, and to see an example of a paleosol.

SAFETY FIRST: This stop is a roadcut of rock along a busy highway. While there is plenty of room to pull over and see the rocks, be careful of the potential dangers from rocks falling from above and of vehicles in the road. This cache also requires you to use a knife blade, so be careful that you do not cut yourself while investigating these rocks.

Also, as you are investigating the site, please keep the impact to the area at a minimum, as per the "Leave No Trace" outdoor ethics of EarthCaching.

Tools you will need:
1) your GPSr;
2) a pocket knife (preferably one with a locking blade); and
3) a camera (optional).

To log this EarthCache:
1) The rocks here include sedimentary rocks that sit on top of weathered metamorphic rocks. Use your knife to test the durability of the various rocks by prodding them with it to see which ones are strong, hard, solid rocks, and which are weak, friable, weathered rocks. Carefully thrust your knife into the highly weathered upper portion of the granitic gneiss (the lower rock) - your very own "sword in the stone". If you are in the right section of the rocks (the weathered metamorphics), you can slide the knife blade in vertically. See the picture below for an example. Some of the sedimentary rocks (the shales) will allow you to slide a blade in between the thin layers horizontally, but these are not what you are looking for - get lower in the section to find the weathered metamorphic rock. See below for more details.
2) Optional task: Take a picture of your own "sword in the stone", like the one shown below, & upload it with your log. Geologists always document their field work with good, careful photographs, so you can consider this task as part of your training as a field geologist (plus, it will be proof to your friends that you are the rightful heir to the throne). Uploading a picture isn't required to log this cache, but I love to see how it works out for people so please post one if you can.
3) In an email, send me the answers to the following three questions:
A) At this location, how thick are the Paleozoic sedimentary rocks that lie above the unconformity? Answer in feet or meters.
B) What is the layering like in the rocks above and below the unconformity (flat, horizontal, tilted, folded, curved, inclined, none, etc.)? Choose a word(s) to describe both the lower, Precambrian granitic gneiss, and another to describe the upper Paleozoic sedimentary rocks.
C) Based on the description of the rocks given below and the description of unconformities at the wikipedia site, what type of unconformity is this?

Understanding the Geology of this site:
When rocks are exposed at the surface of the Earth, the processes of weathering & erosion begin to tear them apart. These are the processes that turn hard, solid rocks into soft, crumbly sand & clay. When rocks are weathered in place, the residual material is referred to as soil. Below the upper zone where the rock is being weathered, the rock remains hard and coherent. But near the surface where weathering occurs, the rock is being broken down into sand, silt, & clay. However, although it is slowly breaking down into soil, it may not initially appear much different from the rocks below and some investigation beyond simply looking might be necessary. Some things that geologists call soil don't look like black dirt.

We might assume that soils are always at the immediate surface of the Earth. However, with millions of years of history, rocks have been weathered & eroded at the Earth's surface for a long time. At some points in Earth's history, weathered rock formed soil that was then later buried by younger sediments. The soil is now capped by younger rocks. Such old soils are called paleosols, and they indicate that a long period of weathering & erosion occurred before they were later buried. When this happens, the sequence of materials consists of a hard, old rock at the bottom that becomes weak and weathered in a zone at its top, which is then overlain by hard sedimentary rocks. A sort of weathered rock sandwich! The boundary between the two rock types is called an unconformity, because erosion occurred before the younger rocks were deposited.

At this site, the lower rock below the unconformity is a Precambrian granitic gneiss, a metamorphic rock. This rock is about a billion years old, and after it formed deep in the Earth's crust, it was later exposed at the surface where it was weathered & partly eroded. Much later, about 500 million years ago, the weathered rock was buried beneath new, younger sediments that eventually hardened into sedimentary rocks. These Paleozoic sedimentary rocks here consist of sandstone, red and greenish shale, and limestone. The sandstone & limestone are both hard, solid rock, but the shale is weak and splits apart easily into very thin layers.

When you arrive at the site, you need to test the rigidity of the rocks above and below the unconformity with your knife. Prod them with it to see if they are hard, solid rocks, or soft, weakened materials. The weathered materials might look to you as if they are normal rocks, but as you can see from the photographs below, a knife blade can be forced right into the weathered rock! That can't happen unless the rocks are strongly weathered.


Exposure of the Precambrian-Paleozoic contact; knife blade is stuck in highly weathered basement material. Gray & white rock is the weathered metamorphic rock. The brown rocks above the knife that stick out a bit more are sedimentary rocks.


More Information:
Easton, R.M., 1987, Paleozoic-Precambrian unconformity near Burleigh Falls, Ontario Highway 36, Ontario. Geol. Soc. Am. Centennial Field Guide - Northeastern Section.


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Last Updated: on 5/25/2017 9:26:37 PM Pacific Daylight Time (4:26 AM GMT)
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Coordinates are in the WGS84 datum