Honors Geology Sediments Lab
Sediments differ greatly in size, mineral composition, and shape of grains. Each of these
differences provides clues about the history of the sediment and the environment in
which the sediment formed.
Size
Clast size Size Range (mm)
Boulder
>256
Cobble
64-256
Pebble
4-64
granule
2-4
* note- granule is rarely used for reference- it could be called coarse sand
sand
1/16 - 2 (separate grains are distinguishable)
silt
1/256-1/16 (feels gritty)
clay
<1/256 (feels smooth)
sorting- sediment that has been transported by water or wind will be well-sorted, that is
it will contain only one or two clast sizes.
rounding: A clast like gravel starts out as an angular piece of rock. As it is weathered, it
becomes more rounded. Sand will take thousands of years to be mature, while gravel will
be rounded in only a few miles of river transport. The degree of maturity of a clast will
indicate the degree of weathering and transport from its source.
Maturity The maturity of sand can be determined by the roundness of the grains, the
degree of sorting as well as the mineral composition. A mature sand will contain only
quartz, since all other common minerals weather faster than quartz. The clay mineral is a
good indication of a lot of weathering. Iron oxide also indicates a lot of weathering,
especially in an aerobic environment. An immature sand, one that has undergone little
chemical weathering, will contain feldspar, mica, and ferromagnesian minerals as well as
chunks of rocks like granite or basalt.
Mineral composition
To identify the minerals present using a dissection microscope, use the
following guidelines:
1. quartz: glassy, colorless and transparent
2. feldspar: light in color, white, pink, showing cleavage
3. mica: shiny, looks like glitter
4. ferromagnesian: dark, opaque (magnetite can be removed with a magnet)
5. obsidian: dark, glassy, translucent
6. olivine: green, glassy
7. calcium carbonate: pearly white, reacts with acid
The concept of textural maturity does not apply to carbonate sands, because carbonate
grains are not derived from the weathering of source rocks on land. Instead, they are
produced in the area of deposition. Most carbonate grains are bioclasts, or pieces of
animal shells or skeletons. To determine whether sand grains are carbonate, test a grain
with acid. Do not put a drop of acid directly on the container. Take a few grains of
sediment, place them on an empty Petri dish, and place a drop of acid. You can see the
reaction better under the microscope.
The environment in which the sediment was deposited can be deduced from the
maturity and degree of sorting of the sediment. A mature sediment has been carried long
distances by water, or has been weathered by waves for a long time. Well-sorted and
mature sand can be found on beaches. Poorly sorted and immature sediments can be
found along rivers, closer to their source. Finally, immature and unsorted sediments have
not been transported by water at all.
Beaches: Rounded and sorted sand, gravel and pebbles. The maturity of the sediment
depends upon whether the beach is close to its source or whether the sediment has
traveled far. Beach sand will often contain some bioclasts (seashells).
Rivers- rounded well sorted sand, granules, pebbles. The maturity depends upon how far
the sediment has traveled in the river
Base of mountains- angular, poorly sorted and immature sediment. An alluvial fan at
the base of a desert mountain may contain angular or sub-angular sediment that has
traveled a short distance in a flash flood. Glaciers will deposit angular poorly sorted
sediments in or at the base of mountains
Tropical Reef- source of carbonate sands
Shallow sea or lake- sand at the beach, silt and clay further out.
Lagoon, tidal flats, swamps- clay mixed with organics
Sand dunes- extremely well sorted fine sand.
For each sediment sample, determine the following
 size or sizes of clasts
 degree of rounding
 mineral composition. (this is not always possible- or it may include rock fragments)
 Maturity of sediment
From these observations, deduce the following:
 In what environment were the sediments deposited?
 were they transported far from their source?
 What is a likely environment of deposition?
Sample: A
Sizes of clasts
Rounded,
subangular or
angular?
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Mature or
immature?
Sample: B
Sizes of clasts
Rounded,
subangular or
angular?
Mature or
immature?
Sample: C
Sizes of clasts
Rounded,
subangular or
angular?
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Mature or
immature?
Sample: D
Sizes of clasts
Rounded,
subangular or
angular?
Mature or
immature?
Sample: E
Sizes of clasts
Rounded,
subangular or
angular?
Mature or
immature?
Sample: F
Sizes of clasts
Rounded,
subangular or
angular?
Mature or
immature?
Sample: G
Sizes of clasts
Rounded,
subangular or
angular?
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition
Mature or
immature?
Sample: H
Sizes of clasts
Rounded,
subangular or
angular?
Mature or
immature?
Sample: I
Sizes of clasts
Rounded,
subangular or
angular?
Mature or
immature?
Sample: J
Sizes of clasts
Rounded,
subangular or
angular?
Mature or
immature?
Sample: K
Sizes of clasts
Rounded,
subangular or
angular?
Mature or
immature?
Minerals
present (or
rocks)
Far from
source?
Possible
environment of
deposition