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The
Erratic Walk (glacial landscapes and mapping at IslandWood)
| Overview:
Students will be able to recognize glacial erratics and to understand
the processes responsible for their development. |
Key goals
and objectives: A glacial erratic is a rock that was brought
from another location by glacial movement, and left in a random
location. In this lesson SWBAT learn how we know this, and to recognize
certain rocks as an erratic. SWBAT identify rocks around the IW
core campus area (and along the trails) that may or may not be glacial
erratics.
Introduction and Activity: The last ice age created
most of the surface features in the Puget Sound Lowland Basin, and
also brought many rocks from Canada that were not here originally.
These rocks are called glacial erratics and are generally
igneous and/or metamorphic. How do we know this? What kind of rock
is at our feet (igneous, metamorphic,
or sedimentary)?
The core
lesson: When a glacier covers the ground it scrapes and
scours the earth, picking up things — trees, plants, and rocks.
The pressure of the ice freezes the ground underneath, and ice wedges
between hairline cracks in the rocks. After some time, the pressure
increases and the ice expands, and the rock may crack and break
further. This is called ice wedging, and it does not have to happen
only with glaciers, but can happen anywhere that there are repeated
frosts in the winter.
When the glacier passes over some of these rocks that have been
loosened by ice wedging, the glacier plucks the rocks apart and
carries them downstream. This is called glacial
plucking (Hamblin and Christiansen 1995:368).
The rocks vary in sizes, from small handheld boulders to the size
of small cars and refrigerators.
When the glacier recedes, the rocks suspended in the ice are left
behind. Many of the rocks that you see around this campus are examples
of such movements, and some may have already been visible, while
others were discovered and excavated in the building process.
At the peak of the glacier
advance (Kruckenberg 1991: 19) Seattle would have been under
three thousand feet of ice (1991: 21), or about five or
six Space Needles tall.
Part 1: We are going to map the glacial erratics on the
IslandWood Core Campus, so grab a marker, map, and sharp observation
skills, and keep your eyes open for random rocks. First find where
you are on the map. How many rocks do you think are erratics between
here and the Learning Studios? Write down how many you expect to
find. Mark their approximate location on your map.
[Distribute laminated maps, markers, and use attached Map as a guide.
Give student groups 15 minutes to get as many as possible on the
page, or until arrival at destination whichever comes first. This
may be done on the way to venue of Part 2; for example, from the
garden or lodge to the LS.]
Part 2: Before the glaciers came, the rivers of Cascades
and Olympics flowed into the lowland area; those sedimentary rocks
comprise the bedrock. This
layer appears to be sorted [a good example in on the lower loop
trail, between the spine trail and the lower dam, just before the
bridge). We are going to watch a video about the glaciations of
the Puget Sound Lowland, which left behind newer sediments—
erratics, till (fine clay),
and unsorted moraines [vashon6.mov].
How fast does the glacier advance and retreat? Watch carefully how
quickly the fresh water changes to salt water; how would that affect
the ecosystem? What would happen to those animals? How would that
affect any people living in the area? How many times does it change?
[3: fresh to salt with the advance, salt to fresh with the advance;
and fresh to salt with the retreat].
[This can be done before mapping the erratics, or as follow-up activity.
This video is currently available on learn.IslandWood.org]
Conclusion:
Part 1: How many rocks did you find? Do any of them look
the same? What does that tell you about where they came from? Did
you find all of them? Did you find any that are not marked in the
master? What kinds of patterns do you notice about the placement
of the rocks? [On the side of the trails]
How would these random rocks help you if you chose to live here
(i.e. what are they, can they be used to make tools, etc.?)?
[Compare to attached map, with erratics marked.] Did you find all
of them? Did you find any that are not marked on the master?
Part 2: What would the landscape look like, before, during,
and after the glacial advance? How would these random rocks help
you if you chose to live here (i.e. what are they, can they be used
to make tools, etc.?)? How many years would it be, do you think,
after the glacier retreated before the forests returned?
Assessment:
Built-in to the experience or based on a follow-up. Every once and
a while stop at one of the other rocks in the central campus, and
ask, “What is this? How do we know that it got here? Where
did it come from? What kind of rock is it? Etc.”
Extension:
Students should write an “I am like…” poem to
tell the story of a rock, glacier, geologic feature. They should
include vocabulary herein — glacier, erratic, glacial plucking,
ice wedging, moraines, etc.
Safety Considerations: No immediate safety considerations,
except to send kids off in too many disparate directions in search
of erratics on their own.
Since all the rocks mapped are large boulders (or in clusters) there
is no danger of moving these rocks.
Alternative
Conceptions
Background
Information: see the glacier page.
References:
see the citations page.
Created by Matthew
John Brewer on November 23, 2003, modified on March 8, 2004.
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Theme:
glacial erratics, glacial landscapes
Concepts: relative time, quantification,
order, and scale
Skills: observation, communication, infer,
mapping
Age group: 12- 97
Venue/s:
Part 1: IW core campus at one of the glacial erratics (see
attached map).
Part 2: at any computer with access to learn.islandwood.org
Materials:
3-4 blank laminated core campus map; Erratics.pdf
Time:
45 minutes to complete both parts in either order
Set
up: none
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