TEXTURE OF SURFICIAL MATERIALS
Textural Terms and Symbols
Definition of Textural Terms
Application of Textural Terms
General Use of Textural Terms
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Texture refers to the size, shape and sorting
of particles in unconsolidated clastic sediments, and the proportion and
degree of decomposition of plant fibre in unconsolidated organic sediments.
Particle size is considered to be the length of the intermediate ("b")
axis of a clast (Figure 1). For particles
2 mm in size or larger, this dimension can be measured by hand. Smaller particle sizes can be estimated in the field by hand texturing (see Appendix II) or measured by laboratory procedures. The proportion of sand particles may
be determined in the field by sieving. The Wentworth
Grade Scale (in Krumbein and Sloss, 1963, p. 96) is the basis of this particle
size classification, with the exception that 0.002 mm has been adopted as
the silt/clay boundary.
Figure 1. Model of the principal axes of a clast.
Particle shape refers to the degree of roundness of clasts
greater than 2 mm. Roundness is the degree of rounding or smoothing (c.f.
angularity) of the edges and corners (Figure 2).
Figure 2. Particle roundness: two general classes are used in terrain mapping (rounded and angular).
The degree of sorting or particle size
distribution refers to the range of particle sizes comprising a sediment.
A "well-sorted" sediment, such as eolian dune sand, consists of a narrow size range, whereas a "poorly-sorted"
sediment, such as ice-contact (glaciofluvial) material, encompasses a broad
size range.
| Specific Clastic Terms | Common
Clastic Terms | Organic Terms |
| Name |
Map Symbol | Name | Map Symbol | Name | Map
Symbol |
| blocks | a | mixed
fragments | d | fibric | e |
| boulders | b | angular
fragments | x | mesic | u |
| cobbles | k | gravel |
g | humic | h |
| pebbles | p | rubble |
r | - | - |
| sand | s | mud |
m | - | - |
| silt | z | shells |
y | - | - |
| clay | c | - |
- | - | - |
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The definition of the specific and common clastic, and organic texture terms
are provided below. Table 1 illustrates the relationship of size and roundness
in the textural classification of the clastic
sediments; example textures are presented in Figure 3.
Table 1. Relations of size and roundness, clastic textural terms.
Specific Clastic Terms
Specific clastic textures have a narrow size range and an implication of
clast shape (e.g., rounded versus angular)
for particles greater than 2 mm in size.
| Name | Definition |
| blocks | Angular particles greater than 256 mm
in size. |
| boulders | Rounded
particles greater than 256 mm in size. |
| cobbles |
Rounded particles between 64 and 256 mm in size. |
| pebbles | Rounded particles between 2 and 64 mm in
size. |
| sand | Particles between
.0625 and 2 mm in size. |
| silt | Particles
between 2µm and .0625 mm in size. |
| clay |
Particles less than 2µm in size. |
Common Clastic Terms
Common clastic terms refers to a grouping of specific clastic size ranges
and, in some cases, an indication of clast
shape.
| Name | Definition |
| mixed fragments | A mixture
of rounded and angular particles greater than 2 mm in size. |
| angular fragments | A mixture of angular
fragments greater than 2 mm in size (e.g., a mixture of blocks and rubble). |
| gravels | A mixture of two
or more size ranges of rounded particles greater than 2 mm in size (e.g.,
a mixture of boulders, cobbles and pebbles), and may include interstitial
sand. |
| rubble | Angular
particles between 2 and 256 mm (and may include interstitial sand). Note:
In general, little or no fine material will be visible on a rubble surface.
At depth, sand, silt and clay particles may occupy the interstices between
the coarser particles. |
| mud | A
mixture of silt and clay; may also contain a minor fraction of fine sand. |
| shells | A sediment comprised
dominantly of shells and/or shell fragments of variable sizes. |
Figure 3. Textural examples of various materials: (a) blocky talus (colluvium);(b) cobbles and boulders on a beach; (c) well-bedded glaciofluvial sand and gravel; (d) till made up of rounded and angular particles (mixed fragments) in a silty sand matrix; (e) silty glaciolacustrine sediments; and (f) bedded glaciofluvial sands.
Organic Terms*
Organic terms are based on the proportion and degree
of decomposition of the organic materials.
| Name | Definition |
| fibric | The least decomposed of
all organic materials. It contains amounts of well-preserved fibre (40%
or more) that can be identified as to botanical origin upon rubbing. |
| mesic | Organic material is at a
stage of decomposition intermediate between fibric and humic. |
| humic | Organic material is at an advanced
stage of decomposition; it has the lowest amount of fibre, the highest bulk
density, and the lowest saturated water-holding capacity of the organic
materials; fibres that remain after rubbing constitute less than 10% of
the volume of the material. |
| Note:
| Rubbed fiber is the fibre that remains after rubbing a sample
about 10 times between the thumb and forefinger. |
*
See Canada Soil Survey Committee (1978) for full definitions.
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Up to three textural terms can be used to describe the texture of a
surficial material; textural term symbols are placed before the surficial
material symbol; where two or three textural symbols are used they are listed
in reverse order of importance so that the symbology may be easily verbalized;
a textural component will not generally be shown if it constitutes less
than 20% of the total volume of the deposit. Table 2 shows the corresponding
range in percent amount of each texture class when 1 to 3 textural terms
are used.
Table 2. Percent composition for texture descriptions comprised of 1 to 3 symbols.
| Number of Symbols |
Example |
Subordinate 2 |
Subordinate 1 |
Dominant |
| 1 |
s
(sand) |
- |
- |
80-100
(sand) |
| 2 |
gs
(gravelly sand) |
- |
20-50
(gravel) |
50-80
(sand) |
| 3 |
bgs*
(bouldery
gravelly sand) |
<30 |
25-40
(gravel)* |
40-60
(sand) |
* Note: In this example the gravel term indicates pebbles and cobbles (see example below).
The three-fold textural description may be made up of any combination
of specific clastic, common clastic, and organic class textural terms. Note:
See organic surficial material definition for criteria used to distinguish
clastic and organic sediments (see Organic).
Using two or three textural terms together indicates that either the
various textures are intermixed or they are interstratified
(Example A). Discrete areas of different textured materials are indicated
by a composite unit symbol (Example B).
| Example A: | zsLb | indicates that the lacustrine material consists of a uniform silty sand, or inter layered beds of silt and sand; |
|---|
| Example B: | zLb/sLb | indicates the occurrence of discrete areas of silt and sand.
|
|---|
- Where specific and common clastic
textural terms are used together, the common term will denote two or more
size ranges not accommodated by the specific term.
| Example: | bgFt | indicates that the common texture class gravel (g) does not include the specific texture class boulder (b), but consists only of cobbles and pebbles. |
|---|
The omission of textural terms from a terrain unit description implies
that the texture of the surficial material lies somewhere within the range
of textures defined for the surficial material. Lack of a texture symbol
indicates the mapper had insufficient knowledge to be more specific.
Back to Texture of Surficial Materials
Mappers should be aware the texture of surficial materials can vary both
laterally and vertically within a terrain unit. Thus, a texture based on
a field observation or laboratory analysis from one site may not necessarily
represent the texture for the entire terrain unit.
Mappers can imply the degree of sorting
of a surficial material through the use of single or multiple specific or
common clastic texture terms. A well-sorted
material will generally be described by a single specific textural term.
For example, the texture of a well-sorted eolian sand dune would be indicated
by the specific texture symbol "s". Poorly sorted or less sorted materials,
on the other hand, will be described by the use of two or three specific
and/or common textural terms. For example, three textural terms such as
"zsd"could describe a poorly-sorted till comprised of mixed rounded
and angular fragments greater than 2mm with a lesser amount of sand and
silt. Occasionally, for specific purposes, mappers may find it necessary
to use a specialized texture classification such as the Unified texture
classification (The Asphalt Institute, 1978) or some other system. Some
existing textural classification schemes are shown in Figure 4. Any such
modifications should be fully documented in the attached map legend.
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Figure 4. Various classification systems for particle size.
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