Coarse Fragments

Coarse fragments

Use the appropriate coarse fragment modifiers according to the following table when adequate volumes are observed. Abbreviations provided in Attachment 1 are acceptable substitutes for the proper modifier. Misspelled modifiers are not awarded points.

Table 5. The course fragment modifiers are applied using the following volumes. These are volumes of course fraction in the volume of soil.

Coarse fraction        Modifier

< 15%           none needed

15-34%              modifier

35-60%          very modifier

60%       extremely modifier

For this purpose the terms “gravel” and “cobble” are defined as below:

“Gravel” – a fragment 2-75 mm diameter; any lithology and shape; as a modifier – gravelly.

“Cobble” – a fragment 75 to 250 mm diameter; any lithology and shape; as a modifier – cobbly.

“Stone”  – a fragment 250 to 600 mm diameter;  any lithology and shape; as a modifier – stony.

If gravels and cobbles occur in the same horizon, the largest coarse fragment modifier is used unless the smaller size fragment exceeds 2 times the volume of the larger.

Coarse fragment modifiers should be recorded in the column labeled “CF Mod.”

Recording in the “Class” column for the soil textural class is incorrect and if not recorded in “CF Mod” will result in loss of points for both “Class” and “CF Mod.”


The following is a quote from:

Soil Survey Division Staff. 1993. Chapter 3 – Examination and Description of Soils. In Soil Survey Manual, United States Department of Agriculture Handbook No. 18.

Rock Fragments

Rock fragments are unattached pieces of rock 2 mm in diameter or larger that are strongly cemented or more resistant to rupture. Rock fragments include all sizes that have horizontal dimensions less than the size of a pedon.

Rock fragments are described by size, shape, and, for some, the kind of rock. The classes are pebbles, cobbles, channers, flagstones, stones, and boulders (table 3-11). If a size or range of sizes predominates, the class is modified, as for example: “fine pebbles,” “cobbles 100 to 150 mm in diameter,” “channers 25 to 50 mm in length.”

Gravel is a collection of pebbles that have diameters ranging from 2 to 75 mm. The term is applied to the collection of pebbles in a soil layer with no implication of geological formalization. The terms “pebble” and “cobble” are usually restricted to rounded or subrounded fragments; however, they can be used to describe angular fragments if they are not flat. Words like chert, limestone, and shale refer to a kind of rock, not a piece of rock. The composition of the fragments can be given: “chert pebbles,” “limestone channers.” The upper size of gravel is 3 inches (75 mm). This coincides with the upper limit used by many engineers for grain-size distribution computations. The 5-mm and 20-mm divisions for the separation of fine, medium, and coarse gravel coincide with the sizes of openings in the “number 4” screen (4.76 mm) and the “3/4 inch” screen (19.05 mm) used in engineering.

The 75 mm (3 inch) limit separates gravel from cobbles. The 250-mm (10-inch) limit separates cobbles from stones, and the 600-mm (24-inch) limit separates stones from boulders. The 150-mm (channers) and 380 mm (flagstones) limits for thin, flat fragments follow conventions used for many years to provide class limits for plate-shaped and crudely spherical rock fragments that have about the same soil use implications as the 250-mm limit for spherical shapes.

Rock Fragments in the Soil

Historically, the total volume of rock fragments of all sizes has been used to form classes. The interpretations program imposes requirements that cannot be met by grouping all sizes of rock fragments together. Furthermore, the interpretations program requires weight rather than volume estimates. For interpretations, the weight percent >250, 75-250, 5-75 and 2-5 mm are required; the first two are on a whole soil basis, and the latter two are on a <75 mm basis. For the >250 and 75-250 mm, weighing is generally impracticable. Volume percentage estimates would be made from areal percentage measurements by point-count or line-intersect methods. Length of the transect or area of the exposure should be 50 and preferably 100 times the area or dimensions of the rock fragment size that encompasses about 90 percent of the rock fragment volume. For the <75 mm weight, measurements are feasible but may require 50-60 kg of sample if appreciable rock fragments near 75 mm are present. An alternative is to obtain volume estimates for the 20-75 mm and weight estimates for the <20 mm. This is favored because of the difficulty in visual evaluation of the 2 to 5 mm size separations. The weight percentages of 5-20 and 2-5 mm may be converted to volume estimates and placed on a <75 mm base by computation. The adjectival form of a class name of rock fragments (table 3-11) is used as a modifier of the textural class name: “gravelly loam,” “stony loam.”

Table 3-11. Terms for rock fragments

Shape and size1 Noun Adjective
Spherical, cubelike, or equiaxial:    
2-75 mm diameter Pebbles Gravelly
2-5 mm diameter Fine Fine gravelly
5-20 mm diameter Medium Medium gravelly
20-75 mm diameter Coarse Coarse gravelly
75-250 mm diameter Cobbles Cobbly
250-600 mm diameter Stones Stony
>600 mm diameter Boulders Bouldery
Flat:    
2-150 mm long Channers Channery
150-380 mm long Flagstones Flaggy
380-600 mm long Stones Stony
>600 mm long Boulders Bouldery
1. The roundness of the fragments may be indicated as angular (strongly developed faces with sharp edges), irregular (prominent flat faces with incipient rounding or corners), subrounded (detectable flat faces with well-rounded corners), and rounded (flat faces absent or nearly absent with all corners.

The following classes, based on volume percentages, are used:

Less than 15 percent: No adjectival or modifying terms are used in writing for contrast with soils having less than 15 percent pebbles, cobbles, or flagstones. The adjective “slightly” may be used, however, to recognize those soils used for special purposes.

15 to 35 percent: The adjectival term of the dominant kind of rock fragment is used as a modifier of the textural term: “gravelly loam,” “channery loam,” “cobbly loam” (fig. 3-17).

fig3-17_large

35 to 60 percent: The adjectival term of the dominant kind of rock fragment is used with the word “very” as a modifier of the textural term: “very gravelly loam,” “very flaggy loam” (fig. 3-18).

fig3-18_large

More than 60 percent: If enough fine earth is present to determine the textural class (approximately 10 percent or more by volume) the adjectival term of the dominant kind of rock fragment is used with the word “extremely” as a modifier of the textural term: “extremely gravelly loam,” “extremely bouldery loam.” If there is too little fine earth to determine the textural class (less than about 10 percent by volume) the term “gravel,” “cobbles,” ‘stones,” or “boulders” is used as appropriate.


 

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