Sword in the Stone EarthCache
-
Difficulty:
-
-
Terrain:
-
Size: 
(not chosen)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
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.
Additional Hints
(No hints available.)