Page 196 - Laboratory Manual in Physical Geology

6. The sediment that you just made by wearing down the corner of a rock clast is called detrital sediment (from the Latin

detritus , participle of detero , meaning “to weaken, wear away, rub off”). The term is also used to refer to all sediment that
is terrigenous (from the land)—worn and transported away from landscapes (rock fragments, mineral grains, and rock
material that has been weakened and decomposed by chemical weathering).
The Mississippi River carries
detrital sediment that has been Louisiana Mississippi
Mississippi
Mississippi
Louisiana
Louisiana
weathered from bedrock and worn
away from the landscape of much
of the United States. The river
flows downhill under the influence
of gravity and eventually flows into
the Gulf of Mexico, where its load
of detrital sediment temporarily
accumulates at the mouth of the
river on the edge of the Mississippi Mississippi Delta
Mississippi Delta
Mississippi Delta
Delta. On this NASA satellite
image of the Mississippi Delta,
write a “D” to indicate where the
main load of terrigenous detrital
sediment is being deposited at the Gulf of Mexico
edge of the delta. How do you
think the roundness of sediment in
the river will change from a place 29 39 45N, 90 33 48W in Google Earth™ (© Google Earth)
upstream where it was broken from
bedrock to the location where you
placed your “D” on the image?

B. Sediment falls and slides (rockslides) downhill under the influence of gravity and is transported by flowing agents like water,
wind, and ice (glaciers). As grains are transported, they scrape, chip, brake, and generally increase in roundness.

1. Glacial ice holds detrital grains of sediment in its firm grip while
the weight of the glacier exerts tremendous downward force and
gravity pulls the glacier downhill. You can model this process and
see what it does to grains of sediment. Place a piece of sandpaper
flat on the table. Next, firmly grip (like glacial ice) a piece of shale
with a somewhat flat side pointing down. In one motion press the
shale firmly against the sandpaper and push it forward one time.
Then use a hand lens to observe the shale surface that you just
scraped over the sandpaper. To the right of this paragraph, draw
the pattern of scratches that you observe. What would happen
to the shale surface if you kept grinding it straight ahead on a
10-meter-long strip of sandpaper?

2. Grains of sediment carried by water and wind move generally in
one main direction but are free to quickly change direction and
roll about so that all of their sides scrape and impact other grains
often. Imagine that the piece of shale above has been dropped
from a melting glacier and is being transported by a melt water
stream. To model what might happen to the shale grain, place it
onto the sandpaper, grip it lightly, and move it about against the
sandpaper in multiple directions. Turn the shale to a different side
and repeat. Now observe the newly scraped surfaces with a hand
lens. To the right of this paragraph, draw the pattern of scratches
that you observe.

C. REFLECT & DISCUSS Based on your work above, how could you tell a grain of sediment that was abraded and shaped in

a glacial environment from one that was abraded and shaped while being transported by water or wind?

175