Day 3. Terrace and Soils Lab - River Changes over Time
Description: Today, we will examine the terraces of the Huntington River at the
Audubon Center. These terraces record the 14,000 year history of river behavior
and incision as base level, climate, and sediment supply changed following
glacial retreat. On each terrace, a group will dig and describe a soil pit. We will
use the soils to help us learn about changes that occur over time. We will use
the layering exposed in the pits to tell us more about the environment in which
the river deposited the sediment we see.
Specific skills and general knowledge: By the end of the day, you should be
able to make a basic description of a soil pit including delineating horizons and
describing soil textures and colors. You should be able to understand and
describe soil-forming processes as well as understand what a soil
chronosequence is and why it is important. You should be able to recognize
river terraces in the field and be able to create a river landscape history based on
what you see in a series of soil pits dug on terraces.
Gear: Glass Bottles, detergent, water, shovels, tarps. Munsell color books,
yardsticks, nails and flags, tapes, gps units, proXh unit, trowels, laminated maps,
laminated diagrams
Procedure
We’ll park up high and begin by walking from the Audubon center parking lot
down the trails from the highest terrace remnants to the river. Along the way, we
will mark sites for digging. There will be 5 sites including one just above the
modern floodplain. Take your digging equipment with you! On the way down,
we will dig a demo pit, practice with the GPS, get to know the color chart, and do
one settling experiment.
At each site, three or four people will dig a soil pit that is at least a meter in all
three dimensions. The goal is to dig through the over bank sand deposits and
into the gravel below. We will employ low-impact soil pitting techniques which
means top soil and sod should be carefully removed and all the soil should be
placed on a tarp.
Once the pit is dug, you want to describe it carefully so that you can make a
detailed log of the pit tonight. The log is a graphical representation of what you
observed in the field.
At lunch, we will meet at the lowermost pit. We’ll hear a short (5 minute)
presentation from the group that dug the pit and then move up to the next terrace
and next pit. As we leave each site, the hole should be filled and the sod
replaced.
THINGS TO KEEP IN MIND WHEN DESCRIBING SOIL PROFILES….
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How many soil horizons are there?
Delineate and describe the O, A, E, B, and C horizons if they exist in your
pit.
Start with “0” depth at the surface and measure down in cm.
How deep does each horizon extend?
What is the thickness of each horizon?
Describe the color of each horizon using the Munsell system
Describe the texture of each horizon (sandy, silty, clay or approximate
percentage mixture)
How well sorted is the sediment in each horizon?
Is the horizon graded? (Does the grain size get smaller toward the top of
the horizon?)
Is there visible organic matter in the horizon? How much?
Are there any rust colored stains present?
What is the nature of contacts between the horizon (sharp or
gradational)?
What is the elevation of the terrace on which your pit is dug?
WRITE UP
In the evening, each group should prepare a well-drafted soil pit log that shows
all their observations and interpretations. This can be done by hand or on the
computer. Along with this, please prepare a one paragraph summary (reflection)
giving your interpretation of the landscape history that the soils record over the
past 14,000 years.
It is reasonable to assume that the uppermost terrace is 14,000 years old (Lake
Vermont), the terrace at the main road level is about 9000 years old (overlying
fan date), the terrace below the road is 2000 years old, and the lowest terrace is
historic (post colonial). The swamp soil has an unknown age!
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Identifying Soil Texture by Measurement
1. Spread soil on a newspaper to dry. Remove all rocks, trash, roots, etc. Crush
lumps and clods.
2. Finely pulverize the soil.
3. Fill a tall, slender jar (like a quart jar) ¼ full of soil.
4. Add water until the jar is ¾ full.
5. Add a teaspoon of powdered, non-foaming dishwasher detergent.
6. Put on a tight fitting lid and shake hard for 10 to 15 minutes. This shaking breaks
apart the soil aggregates and separates the soil into individual mineral particles.
7. Set the jar where it will not be disturbed for 2 to 3 days.
8. Soil particles will settle out according to size. After 1 minute, mark on the jar the
depth of the sand.
9. After 2 hours, mark on the jar the depth of the silt
10. When the water clears mark on the jar the clay level. This typically takes 1 to 3
days, but with some soils it may take weeks.
11. Measure the thickness of the sand, silt, and clay layers.
a. Thickness of sand
deposit _____.
b. Thickness of silt deposit _____.
c. Thickness of clay deposit
_____.
d. Thickness of total deposit _____.
12. Calculate the percentage of sand, silt, and clay.
[clay
thickness]
______________ = _____ percent clay
[total thickness]
[silt
thickness]
______________ = _____ percent silt
[total thickness]
[sand
thickness]
______________ = _____ percent sand
[total thickness]