Chapter

2

Landscapes, Geomorphology, 

and Site Description

By Philip Schoeneberger, Douglas Wysocki, Craig Busskohl, and 

Zamir Libohova, USDA-NRCS.

Introduction

This chapter describes information that is recorded about the overall 

setting and site features for a soil. This information includes the 

physiographic and landscape setting, geomorphological characteristics, and other information specific to the area where the soil is 

described. The setting and site often include information on drainage 

pattern, parent material, bedrock, erosion, land cover, and relationships 

to vegetation communities. Detailed information about describing soil 

profiles is provided in chapter 3.

A core mission of the National Cooperative Soil Survey (NCSS) 

of the United States is to reliably identify, inventory, and communicate 

information about soils and the earth systems of which they are a part. The 

Earth’s surface, including the soils upon it, forms an ordered but complex 

mosaic, consisting of many pieces of different sizes, shapes, origins, and 

composition. It is a very human thing to try to make sense of this mosaic 

by identifying recurring patterns and to separate pieces into groups 

with similar form, content, and function. Depending upon a person’s 

perspective and goals, there are infinite ways to partition the Earth’s 

surface into meaningful subsets. A person interested in agriculture will 

have different management goals and examine different site criteria than 

someone interested in construction. The initial perspective will determine 

which variables are important to highlight, which can be grouped 

together, and which need to be separated. The NCSS has traditionally 

been based on multipurpose perspectives and goals that integrate aspects 

of agriculture, forestry, engineering applications, animal husbandry, and, 

in recent decades, ecosystem function and environmental sustainability. 

These multiple perspectives led to an assemblage of descriptive criteria 
22 Chapter 2

and protocols rooted in earth science that were later expanded to address 

ecosystems and human-altered settings and features.

Geology

Among the many perspectives that can be applied, geology provides 

the most reliable and robust context for understanding natural earth 

systems, including soils, across the widest range of environments. It is 

a perspective that recognizes and details the primary framework upon 

which natural processes and humans operate. Geology largely defines 

the material architecture of the soil, i.e., the composition, general 

arrangement, and lateral extent of these materials. It helps to explain 

the configuration and distribution of the materials of which the soil is 

composed. 

Geomorphology

Geomorphology is the study of landforms, the materials of which 

they are made, and the dynamics by which they are made and function. 

It is at the center of understanding what earth materials are, how they 

interact, how they originated, and how far they extend and where 

similar conditions and materials are likely to occur. It focuses on the 

combinations of composition, stratigraphy, shape, and topography of the 

materials and the geologic processes that give rise to and modify them. 

Soil Geomorphology

Soil geomorphology addresses geomorphic details and dynamics at 

and near the Earth’s surface that affect, or are affected by, soil processes 

and products. It specifically addresses the distribution, properties, 

and dynamic behavior of soils—issues that traditional geology and 

geomorphology do not emphasize because of scale or minimize because 

of perspective. These soil issues are particularly environmentally and 

economically meaningful because they occur at the “human-operative 

scale,” i.e., the scale at which most land use decisions are made and 

applied and their consequences felt.

Soil science, particularly soil geomorphology, is based upon a robust 

relationship between lithology, hydrology, stratigraphy, geomorphology, 

and, to a slightly lesser degree, biota and climate (Wysocki et al., 2012; 

Schaetzl and Thompson, 2014; Buol et al., 2011). In many settings, 

hydrology and hydropedology dominate the physical redistribution 

of materials and catalyze the chemical reactions that transform earth 

materials (Simonson, 1959; Lin, 2012).
Soil Survey Manual 23

Boundaries and Transitions

Some parts of the landscape and the soils on them are separated 

from their neighbors by distinct, sharp boundaries over a lateral distance 

of just meters. For example, a stream terrace may be sharply separated 

from adjacent cliffs and talus cones by an abrupt, easily observed scarp 

(fig. 2-1). Other parts of the landscape and the soils on them have lateral 

boundaries that are very gradual and indistinct, transitioning over tens of 

meters or kilometers. For example, a loess mantle thins gradually with 

increasing distance from the source of the loess. 

Figure 2-1

Distinct, sharp breaks between landforms are evident over short lateral distances, as 

shown by the talus cone in this canyon along the Palouse River in Washington.

Scale

Another major determinant in conveying soil and geomorphic 

information is the scale of interest (e.g., local, regional, global), which 

is established in initial survey perspectives and goals. Scale partly 

predetermines what is “relevant” and what is not, what must be “shown” 

and what cannot. A person managing a 1-hectare homesite has different 

information needs and relevancies than a regional planner who evaluates 

and manages a city or State. The scale of interest can be quite different for 
24 Chapter 2

different users, and the appropriate level of information to be collected 

and delivered differs with the scale. At regional scales, it is typically 

appropriate to evaluate and emphasize landscapes and very large, 

constituent landforms and to minimize smaller landforms and microfeatures. For example, in an order 3 reconnaissance survey, dune fields, 

mountain ranges, or bolsons may be evaluated. Conversely, for localized 

surveys, it is typically necessary to focus primarily on smaller landforms, 

microfeatures, or pieces of landforms and to give only nominal attention 

to landscapes. For example, in an order 1 survey, barchan dunes, slip 

faces, or head slopes may be evaluated. The provisional soil survey of 

the Outer Banks of North Carolina, at a scale of 1:12,000 (USDA-SCS, 

1977) is an example of a localized survey that presents the setting in 

considerable detail.

Digital Soil Mapping and Scale

Widely available GIS tools and digital soil mapping methods have 

made it much easier to move between scales. For example, geospatial 

information that was previously constrained by scale may be combined 

or split apart (see chapter 5). It is now possible to produce: (a) primary 

surveys capable of spanning a much wider range of scale, and (b) 

derivative or second-generation resource maps from a primary spatial 

database that span wider ranges of scale than the initial survey. The 

capability to move between scales must coincide with primary data 

that accommodates such changes in scale. For example, traditional 

soil inventories were made based upon stated, relatively narrow spatial 

scales (e.g., order 2 or 3). The initial field data collected, assigned 

classes, spatial separations (e.g., polygons), interpretations, and other 

information remain constrained by the rules and decisions for the scale 

at which they were originally developed. It is typically possible to 

aggregate data upwards in scale with a minimal impact on information 

integrity. It is much more challenging to do the opposite and legitimately 

disaggregate primary data to create finer resolution information. With 

new tools, software, and statistical methods, it is possible to produce 

information with finer resolution using disaggregation techniques. 

However, the original data, metadata, and inherent decisions made at the 

original inventory scale remain determinate; those original biases will 

persist across scales. Some morphometric parameters, such as slope, 

can be readily replaced or enhanced with higher resolution information, 

such as LiDAR. In some cases, such as the separation of soil bodies 

at finer resolution, previous information can be more problematic and 

new supporting data appropriate to the new scale may be needed. 
Soil Survey Manual 25

Capturing Soil-Landscape Relationships at 

Various Scales

It is typically impractical or financially prohibitive to make an ideal 

number of field observations. Consequently, representative observation 

sites need to be chosen wisely and used to extrapolate to areas that cannot 

be visited. Observation efficiency can be greatly enhanced by developing 

soil-landscape models that capture recurring spatial relationships. Part 

of such model development includes selecting an appropriate scale. As 

discussed previously, scale determines what can and cannot be shown. 

Robust soil-landscape models are subject to incremental changes and 

refinements that reflect the accumulation of knowledge and data during 

the progression of the survey. 

A site may have one of three common scales of models: a large area 

or catchment (landscape) scale, a hill (landform) scale, and a hillslope 

position or pedon (microfeature) scale. The appropriateness of each scale 

depends upon the perspectives and objectives of the survey. Each scale 

can be applied, but each conveys somewhat different information and 

has different strengths and limitations. For example, for a setting along 

the border of the Gypsum Plains State Physiographic Area in Culberson 

County, Texas, a series of hierarchical (nested) landscape models can be 

developed and applied. At a landscape scale, the area presents eroded 

structural hills and alluvial plains. The dominant geomorphic processes 

are tectonic and erosional (and, to a minor extent, fluvial) and are 

regional in scope. At a landform scale (fig. 2-2, top image), which is finer 

than landscape, the same area presents an eroded, structural hill (questa) 

and valley floor (alluvial flat). The dominant geomorphic processes 

are erosional and fluvial and are local in scope. At a microfeature scale 

(fig. 2-2, bottom image), the area presents discrete subsets within 

landforms. The dominant geomorphic processes are hillslope erosion 

and deposition (slope wash processes) and fluvial modification (alluvial 

deposition processes) and are very localized in scope. Different scales 

yield different information.

Hillslope-scale processes commonly express themselves differently 

at different positions (i.e., summit, shoulder, upper slope or backslope, 

footslope, and toeslope). Each hillslope position represents a processdominated area, as demonstrated by a progressive reduction of rock 

fragment size across the ground surface from the summit to the toeslope 

position (fig. 2-3). 
26 Chapter 2

Figure 2-2

Dominant 

Geomorphic 

Processes:

Landform Scale 

(Order 2): 

hillslope erosion cuesta (scarp slope)

alluvial deposition

flood plain (alluvial flat)

hillslope deposition

Local Scale (Order 1): Pieces of Landforms

Hillslope Profile Positions

summit (not visible)

shoulder

upper backslope

lower backslope

footslope

toeslope

Geomorphic Component - Terraces

tread

A scarp slope of a cuesta above an alluvial flat. Scale determines which geomorphic 

descriptors can be effectively used and presented. Geomorphic evaluations of 

regional scope (landscape scale) can separate tectonic hills from areas dominated by 

fluvial processes (not shown). The more localized, landform scale can differentiate 

dominant landforms (cuesta and alluvial flat) and the dominant geomorphic processes 

that control them within the landscape (top image). More detailed erosional and 

depositional surfaces derived from dominant hillslope processes need to be represented 

at a finer scale. These include microfeatures (e.g., ribs and groves), hillslope profile 

positions (e.g., summits and shoulders), and geomorphic components (e.g., components 

of terraces or hills, such as nose slopes, head slopes, and side slopes; not shown).
Soil Survey Manual 27

Figure 2-3

FLAGSTONES (95%)

CN (95%)

CN, CO GR (90%)

CB, M and CO GR (85%)

F and M GR (70%)

F and M GR (5%)

0

20

40

60

80

100

0

10

20

30

40

50

60

 Upper Middle Lower Upper Lower

Backslope Footslope Toeslope

Elevation (m)

Slope (%)

Hillslope Position

F: Fine

M: Medium

CO: Coarse

CN: Channers 

CB: Cobbles 

GR: Gravel

Dominant Surface Fragment 

Size and (%)

Diagram of the change in surface fragments along a transect of a scarp slope on a 

cuesta in Culbertson County, Texas. The progressive reduction in dominant size and 

percentage as one moves down slope demonstrates the impact of hillslope processes 

at a local level. On the upper backslope, dominant erosional hillslope processes are 

evidenced by the presence of flagstones on the surface. On the middle and lower 

backslopes, the percentages and sizes of surface fragments (channers and others) are 

indicative of lateral transport. On the lower footslope and toeslope, rock fragments are 

dominantly medium and fine gravel with decreasing percentages. They are indicative of 

deposition processes.

Figure 2-4

A comparison of digital maps with 60 x 60 m (left) and 10 x 10 m (right) grid sizes 

demonstrates the detail at different resolutions. The 60 x 60 m grid visually captures the 

three major landform units but not the geomorphic components of hills, namely nose 

slope, side slope, and head slope. An even finer resolution, such as LiDAR elevation 

data, is needed to capture the physical expressions of very localized hillslope processes.
28 Chapter 2

In the digital environment, grid scales can be used to determine which 

resolution best captures important attributes for landscape modeling. 

What is apparent at one scale may not be apparent at another (fig. 2-4). 

Digital smoothing techniques, while appropriate for some analyses, can 

adversely affect the ability to identify features at a given scale (fig. 2-5).

Figure 2-5

a b

Side Slope

Nose Slope

Head Slope

Side Slope

Nose Slope

Head Slope

Images showing changes in slope class interpretation as affected by digital elevation 

model (DEM) resolution from LiDAR. Image a—Grid size (1 x 1 m) captures surface 

features nested within hillslope position. Image b—As resolution decreases (30 x 30 

m) and surface becomes digitally smoother, the geomorphic components of hills (nose, 

side, and head slopes) are no longer visible. 

Placing Soil-Landscape Relationships in Their 

Proper Context

Soil geomorphology can be used effectively to evaluate, recognize, 

and communicate the context (setting) of natural systems. Context is key 

to: 

1. Recognizing and understanding the materials and processes 

occurring at a given location or area.

2. Effectively predicting the distribution of materials that dominate 

the area. Commonly, the first and most useful question is “Where 
Soil Survey Manual 29

are you on the planet?” (i.e., the geomorphic context rather than 

the geographic coordinates). 

3. Recognizing the dynamic processes and relationships active 

between and within soils and land areas. 

Development Stages for Soil-Landscape Models:

Expectations vs. Reality

Developing accurate landscape models begins with using expert 

knowledge and experience to anticipate and portray relationships 

between landscapes, geomorphic systems, and soils. In whatever form, 

an initial model must be rigorously tested (“ground-truthed”) and revised 

as needed to accurately reflect actual, observed relationships of soils and 

landscapes.

Developing Initial Soil-Landscape Models

It is helpful for scientists to have a general expectation of what they 

will encounter in an area or at a given site. For example, if the site is in 

a river valley, it is reasonable to expect that the site will reflect fluvial 

dynamics and features in the landforms, sediments, and soils of the area. 

The location of the site within the river valley (e.g., headwaters vs. delta) 

can further refine preliminary expectations. Headwater fluvial sediments, 

and the soils that formed in them, may be expected to contain more, 

larger, and less rounded coarse fragments, be somewhat less well sorted, 

and have less contrasting sediment strata than fluvial sediments in a delta. 

In addition, general conditions and dynamics for a water table might be 

anticipated (e.g., gaining stream vs. losing stream) based on the prevailing 

climate. A preliminary model can provide a tentative framework in 

which scientists can begin to efficiently investigate, understand, and 

give order to a diverse natural world. In the digital world, this now 

includes considerable pre-mapping, based primarily upon improved, 

detailed digital elevation models and existing information (see chapter 

5). Preliminary soil-landscape models are helpful. The adage “If I hadn’t 

known about it, I wouldn’t have recognized it” is a relevant and practical 

truth. The more one understands a natural system, the easier it becomes 

to recognize the physical expressions of those systems.

Describing and Recording What is Actually There

Although soil-landscape models can be very useful at any stage of 

soil inventory, it is critical that a scientist maintain an open mind and 

adjust models as new information becomes available. A common error by 

field workers is to continue to defer to a preconceived expectation, such 
30 Chapter 2

as a preliminary landscape model, rather than to adapt their ideas based 

on what is actually encountered. If a landscape model fails to match the 

natural features and sediments actually found, it must be modified or 

abandoned. A useful model should fit the facts, not the reverse.

Consistently Describing Landscapes, 

Landforms, and Geomorphology

There are various major kinds of information typically gathered to 

identify, evaluate, or communicate geomorphic information. A descriptive 

system and set of terms that can be used consistently are important for 

conveying the information accurately between individuals and making 

valid comparisons from place to place. The NCSS generally uses the 

Geomorphic Description System, or GDS (Schoeneberger and Wysocki, 

2012). The system consists of three main sections: 

Physiographic location.—A named geographic area is specified with 

a defined location. 

Geomorphic description.—A discrete land surface feature (a separate 

entity) or an assemblage of features is identified. Features are 

categorized by dominant process of origin or geomorphic setting.

Surface morphometry.—Land surface shape or geometry is described. 

A discrete portion of a geomorphically defined land feature, area, 

or slope segment is identified. Microrelief, drainage patterns, 

and other surface features are also described.

Geographic and Physiographic Information

Geographic and physiographic information primarily addresses 

the question “Where is it?” It identifies the specific location of an area 

on the planet (e.g., the Appalachian Mountains). This information has 

powerful communication value to a wide range of people. Most people can 

relate a geographic name to some understanding of an area. Fewer users 

appreciate explicit technical descriptions (e.g., an eroded, folded, paleocontinental margin mountain system). The Appalachian Mountains and 

the Rocky Mountains are both mountain systems in the geomorphic sense, 

yet they differ in important geographic locations as well as geomorphic 

details. Their names emphasize geographic differences, not the geologic 

composition or type of mountains. Geographic and physiographic names 

may or may not accurately reflect the actual geomorphology. Confusion can 

arise if geographic names include geomorphic terms that do not adhere to 
Soil Survey Manual 31

their technical geomorphic meaning. For example, an area geographically 

identified as Thompson’s Bench may geomorphically be something other 

than a true structural bench, such as a stream terrace or a horst. 

Physiographic information combines geographic information with 

limited geomorphic information to describe location. In physiography, 

distinctions in topography, bedrock or parent material, watersheds, and 

other attributes are used to group geomorphically similar or related areas. 

Experience has consistently demonstrated that physiographic information 

is the most robust, versatile, and least ephemeral basis for describing and 

partitioning the landscape mosaic. Other location approaches emphasize 

spatial distinctions based on something other than physiography. For 

example, some approaches emphasize natural ecological environments or 

land management systems. The most appropriate descriptive framework 

for location is determined by the survey perspectives and goals. 

Physiographic Location

Because there are numerous and diverse users of soil survey 

information, there are various perspectives and objectives. A single scale 

of information cannot best serve all of them. A homeowner has different 

needs and interests than a regional planner or a resource management 

government agency with national scope. Information on physiographic 

location can be usefully partitioned within a generalized hierarchy of 

scale (table 2-1). In the United States, the highest three levels are based on 

work by Fenneman (1957) for the conterminous U.S. and by Wahrhaftig 

(1965) for Alaska. Additional lower levels describe physiographic areas 

within States and more localized areas. 

Table 2-1

Physiographic Location, Relative Scale (in Descending 

Order) and Examples in the U.S.

Physiographic 

location level Relative scale Example*

Physiographic division Continental scale Interior Plains

Physiographic province Regional scale Central Lowland

Physiographic section Sub-regional scale Wisconsin Driftless 

Section

State physiographic area State scale Wisconsin Dells

Local physiographic 

name

Local scale Blackhawk Island

* Progressive levels of detail.
32 Chapter 2

Maps are available for the three upper physiographic levels from 

Fenneman (1957). They are reproduced in the Geomorphic Description 

System (Schoeneberger and Wysocki, 2012). Maps of State physiographic 

areas are generally available through State Geological Survey offices 

or from the local university NCSS cooperators. Information on local 

physiographic names is primarily obtained from U.S. Geological Survey 

topographic quadrangle maps, where available.

Geomorphic Description

Geomorphic description attempts to answer the questions “What is 

it?” and “How did it occur?” for natural features and sediments. Earth 

surface features can be effectively arrayed in various ways for particular 

needs. Two ways that complement one another are: (1) master lists 

loosely stratified by scale into landscapes, landforms, and microfeatures, 

and (2) lists arrayed by geomorphic environment (fluvial, eolian, etc.). 

Scale

Earth surface features can be partitioned into any number of scale 

ranges, but three general levels have proven consistently effective: 

landscapes, landforms, and microfeatures. 

Landscapes, features at the coarsest scale, are collective groups or 

families of related landforms and typically cover large areas. Examples 

are a mountain range and canyonlands (fig. 2-6). They are most important 

to general or reconnaissance surveys (order 3 or 4, see chapter 4). 

Figure 2-6

A canyonlands landscape in the San Rafael Swell, Utah.
Soil Survey Manual 33

Landforms are discrete, individual features that are related to one 

another within the context of the larger landscape and can be mapped at 

conventional mapping scales, such as order 2 (fig. 2-7). They are typically 

local in size, but some can be quite large. It is helpful to remember that 

natural and anthropogenic landforms and microfeatures can be expressed 

as the result primarily of removal, transport, or deposition. For example, 

blowouts and borrow pits are the result of removal, longshore bars and 

active dunes are the result of transport, and alluvial fans and dredge 

spoils are the result of deposition.

Figure 2-7

Loess hill and river valley landforms in western Iowa, along the Missouri River.

Microfeatures are discrete, individual, earth surface features that are 

readily identifiable on the ground but are too small or intricate to display 

or capture at conventional mapping scales. Examples are vernal pools 

and turf hummocks (fig. 2-8). Where present, these mini-landforms can 

have substantial impact on internal water flow, soil development, natural 

ecosystems, and land management. 

There are many choices within each of these major categories, 

particularly landforms. The choices within each category are commonly 

arrayed as alphabetized master lists, which are particularly appropriate 

for databases. Because of the huge number of choices, it is helpful 

to use subsets for the three main categories, arranged by geomorphic 

environment or other groupings of commonality. 
34 Chapter 2

Geomorphic Environment

A geomorphic environment is a natural setting dominated by a 

geomorphic process of formation and modification and the resultant 

behavioral dynamics. For example, a fluvial geomorphic environment 

consists of landforms and associated sediments created directly by, 

or in response to, channel water flow (fluvial processes). In such a 

setting, present-day environmental dynamics, such as ground-water and 

water table dynamics, are likely to be largely controlled by the fluvial 

system that formed the area’s landscape. Table 2-2 lists the prominent 

geomorphic environments that are most relevant to soils in the United 

States and extensive elsewhere in the world. 

Multiple Geomorphic Processes

Sites may have evidence of more than one geomorphic process. 

Typically, these processes are not equal. One process tends to dominate 

a given land area, and other processes, if present, have a minor presence 

and influence. It is important to recognize the most pervasive process and 

its products because they establish the primary landscape configuration 

and sediment composition. It is also important to recognize secondary 

processes and their sediments where they substantively affect soils. For 

Figure 2-8

Turf hummock microfeatures in a wet meadow in Oregon.
Soil Survey Manual 35

Table 2-2

Prominent Geomorphic Environments and Processes in the 

U.S. and Examples

Geomorphic 

environment or 

other setting

Dominant process 

or attributes

Examples *

LS = Landscape

LF = Landform

Micro = Microfeature

Coastal marine & 

estuarine

Wave or tidal control; 

areas near shore, 

shallow submarine 

areas

LS: coastal plain

LF: nearshore zone

Micro: shoreline

Lacustrine Related to inland 

water bodies

LS: lake plain

LF: lakebed

Micro: strandline

Fluvial Concentrated channel 

flow

LS: river valley

LF: stream terrace

Micro: bar

Solution Dissolution and 

subsurface drainage

LS: cockpit karst

LF: sinkhole

Micro: solution corridor

Eolian Wind related 

(erosional and 

depositional)

LS: dune field

LF: barchan dune

Micro: slip face

Glacial Directly related 

to glaciers 

(glaciofluvial, etc.)

LS: till plain

LF: ground moraine

Micro: tarn

Periglacial Non-glacial, cold 

climate (modern or 

relict)

LS: thermokarst

LF: patterned ground

Micro: stripe

Mass movement Gravity LS: breaklands

LF: landslide

Micro: sag 

Volcanic & 

hydrothermal

Volcanic and/or 

hydrothermal 

processes

LS: volcanic field 

LF: lava flow

Micro: tumulus

Tectonic & 

structural

Regional and local 

tectonic processes or 

crustal movement

LS: mountain range

LF: graben

Micro: sand boil
36 Chapter 2

example, loess commonly mantles till plains in continental glaciated 

environments. If the loess cap is relatively thin, the dominant and 

most important geomorphic context is glacial because it explains the 

prevailing land features, topography, and unconsolidated materials. The 

loess should be recognized for what it is, and not some other geologic 

deposit, and correctly identified and described in the soil stratigraphy. 

If the loess cap is thick, it can supersede underlying glacial materials 

and function as the determinant geomorphic setting itself, such as loess 

hills. Although very thin (e.g., ≤ 25 cm) surficial sediments are prone to 

mixing by normal pedologic processes, to the point that they lose their 

identifying depositional morphology, they can still have an important 

influence on the soil. In this case, these materials may be identified 

even though their geomorphic process is not recognized overtly. Loessinfluenced colluvium is an example.

Some geomorphic descriptive systems attempt to capture both 

primary and secondary geomorphic processes at a site. This approach, 

which may be appropriate for mapping geomorphology, can become 

Geomorphic 

environment or 

other setting

Dominant process 

or attributes

Examples *

LS = Landscape

LF = Landform

Micro = Microfeature

Erosional Dominated by 

hillslope and sheetwash processes 

(non-concentrated 

channel flow)

LS: breaklands

LF: pediment

Micro: gully

Wetlands Vegetated and/or 

shallow wet areas 

and wet soils

LS: Everglades

LF: mangrove swamp

Micro: vernal pool 

Water bodies Surface water features; 

primarily open water

LS: ocean

LF: oxbow lake

Micro: pond

Subaqueous 

features

Permanently 

submerged features 

that support plant 

life or adjacent areas

LS: lagoon

LF: lagoon bottom

Micro: shoal

* For complete choice lists, see Geomorphic Description System (Schoeneberger and 

Wysocki, 2012).

Table 2-2.—continued
Soil Survey Manual 37

complex and confusing when addressing soil geomorphology in the 

context of soil inventory. The main objective of soil inventory is to address 

the geomorphic processes and products that directly or substantively 

influence soils and soil behavior. Larger scale or deep-seated geomorphic 

processes that do not directly or substantively affect soils, such as deepseated tectonic or structural phenomena, may be beyond the scope of 

soil inventory. This information can be included with the physiographic 

information or in general discussions.

Generic vs. Specific Geomorphic Terms

There is considerable range in the specificity of geomorphic terms. 

Some terms are very generic (e.g., uplands) and can be useful especially 

at the coarsest scales. While technically correct, generic terms convey 

relatively little information. Other terms are more specific (e.g., faultblock mountains). As a general rule, it is better to be more explicit than 

less. A specific term is more informative than an equally correct but more 

generic term. For example, “loess hill” provides more information than 

“hill.” 

Nested Features

The focus in geomorphic description is commonly the land feature 

(i.e., a single landform or a dominant landscape) that is most critical 

to soils and that conveys the most relevant context of that site or area. 

An example is “Alpha soil occurs on a dune.” The foremost feature 

should be the one that most directly impacts or defines the soil. In some 

environments, there are multiple landforms of different scales that are 

each relevant to soil behavior and important in documenting an area. For 

these, multiple landforms can be used in sequence (nested) to convey 

important setting information. An example is “Alpha soil occurs on a 

dune on a stream terrace.” If nested terms are used, typically no more 

than two or three are necessary.

Anthropogenic Features

Historically, soil survey in the United States has focused on natural 

processes, associated sediments, and resulting surface features, such as 

landscapes, landforms, and microfeatures. Human-altered surface features 

and materials were traditionally excluded or minimally recognized. They 

were considered to be artificial phenomena, unpredictable in composition 

and occurrence, and largely outside the scope of natural process-based 

soil survey. In recent years, however, there has been a growing awareness 

and acceptance of the impact of humans upon natural systems and 

associated features and materials. Anthropogenic features and materials 
38 Chapter 2

differ from natural phenomena in their origin and processes of formation 

but can be surveyed in ways similar to those used for conventional 

geomorphic entities. For example, they can be identified by recurring 

surface expression (form and arrangement), range of composition and 

internal arrangement, and lateral extent. As with natural landforms and 

materials, the ability to consistently partition anthropogenic features 

into meaningful subsets facilitates recognition of anthropogenic soil 

geography and greatly assists in land management decisions that concern 

them.

Terms describing anthropogenic features were adopted by the NCSS 

in 1993. A new geomorphic category was established to accommodate 

human-altered or -created features of all scales and to elevate their 

recognition to the same stature as natural features (Schoeneberger and 

Wysocki, 2012). Since then, appreciation of the extent of human alterations 

of the Earth’s surface has continued to evolve. As the number and variety 

of recognized anthropogenic features increased, proposals were made 

to divide anthropogenic features into three subsets loosely analogous 

to partitions of naturally derived earth surface features (Schoeneberger 

and Scheyer, 2005; Schoeneberger et al., 2012). Furthermore, the 

International Committee on Anthropogenic Soils (ICOMANTH, 2012) 

defined the phrase “anthropogenic feature” in a non-geomorphic context 

(any artificial artifact, mark, mold, impression, etc.). Subsequently, 

“anthropogenic features” (geomorphic context) has been replaced 

with three new, loosely scale-dependent categories: anthroscapes, 

anthropogenic landforms, and anthropogenic microfeatures.

An anthroscape is important both for its evocative simplicity as a 

term and its explicit recognition of human-modified lands as legitimate 

and significant areas. These lands are substantially different from 

natural systems because they have different sediments, arrangements 

of sediments, and water dynamics and subsequently require different 

management practices. An anthroscape is a human-modified “landscape” 

of substantial and permanent alterations formed by the removal, addition, 

or reorganization of the physical shape and/or internal stratigraphy of 

the land. It is associated with management for habitation, commerce, 

food or fiber production, recreation, and other human activities that have 

substantively altered water flow and sediment transport across or within 

the regolith. Types of anthroscapes include urban, suburban, reclaimed 

land, and agricultural. 

An anthropogenic landform is a discrete, human-made “landform” 

on the Earth’s surface or in shallow water that has an internal composition 

of unconsolidated earthy, organic, human-transported materials, or rock. 

It typically has straight line boundaries or geometric shape. It is the 
Soil Survey Manual 39

direct result of human manipulation or activities. It can be mapped at 

common soil survey scales, such as order 2 (> 1:10,000 to < 1:24,000). 

Anthropogenic landforms can originate from deposition (e.g., an artificial 

levee) or removal (e.g., a quarry; fig. 2-9). 

Figure 2-9

Quarries are an example of an anthropogenic landform.

An anthropogenic microfeature is a discrete, individual, humanderived form on the Earth’s surface or in shallow water that has a range 

in composition of unconsolidated earthy, organic, human-transported 

materials, or rock. It typically has a recognizable human-imposed 

shape. It is the direct result of human manipulation or activities. It 

typically cannot be mapped at common soil survey scales, such as order 

1 (< 1:10,000) but can be observed locally. Anthropogenic microfeatures 

can originate from deposition (e.g., a conservation terrace; fig. 2-10) or 

removal (e.g., a ditch).

Ideas of anthropogenic features will continue to evolve and grow 

in coming years. The proposal to recognize a new geologic age—the 

Anthropocene—continues to gain support and reflects this overall trend. 

Information on composition, occurrence, and behavior of anthropogenic 

features and materials, despite their unique differences from “natural” 

geomorphic phenomena and processes, can assist in wise land 

management.
40 Chapter 2

Surface Morphometry

Surface morphometry uses various terms to describe land surface 

shape or geometry, discrete portions of a geomorphic entity or slope 

segment, and miscellaneous features that are fundamental to soil and 

natural resource inventory. Several terms are discussed in the following 

paragraphs.

Elevation is the height of a point on the Earth’s surface, relative to 

mean sea level. This information is widely available from common GIS 

databases and historically from topographic maps. Elevation conveys the 

important climatic context and reflects the relative potential and kinetic 

energy available at a location. 

Soil slope has a scale connotation. It refers to the ground surface 

configuration for scales that exceed about 10 meters and range up to the 

landscape as a whole. It has gradient, complexity, length, and aspect. The 

scale of reference commonly exceeds that of the pedon and should be 

indicated. It may include an entire map unit delineation, a soil component 

within the map unit delineation, or an arbitrary area. Most commonly, 

slope is recorded in pedon descriptions for the segment of the landscape 

extending a few tens of meters above and below the site of the soil profile 

described and is representative for the landscape segment occupied by 

the soil component at that site. 

Slope aspect is the compass bearing that a slope faces looking down 

slope. It is recorded either in degrees, accounting for declination, or as 

a general compass orientation. The direction is expressed as an angle 

between 0 and 360 degrees (measured clockwise from true north) or as a 

compass point, such as east or north-northwest. 

Figure 2-10

Soil conservation terraces are an example of an anthropogenic microfeature.
Soil Survey Manual 41

Aspect can substantially impact local ecosystems. The impact generally 

increases as slope gradient and latitude increase. In the mid latitudes of the 

conterminous United States, this effect becomes particularly important on 

slopes of approximately 6 to 8 percent or greater. Increased or decreased 

solar radiation on slopes due to aspect can affect water dynamics across a 

site (fig. 2-11). In the northern hemisphere, north-northeast aspects reduce 

evapotranspiration and result in greater soil moisture levels, improved 

plant growth and biomass production, higher carbon levels, and improved 

drought survival rates for plants. Increased solar radiation on southsouthwest aspects increases evapotranspiration and decreases biomass 

production, seedling survival rates, and drought survival rates for plants.

Figure 2-11

Effect of slope aspect on vegetation and tree seedling survival. (Photo courtesy of Kerry 

Arroues)

Slope gradient is the inclination of the land surface with respect to 

the horizontal plane. It is also commonly referred to as “slope percent” 

or simply “slope.” It is calculated as the vertical distance divided by the 

horizontal distance (“rise over run”), multiplied by 100, determined at 

a point along a line oriented up and down slope. It directly controls the 

kinetic energy, erosive power, and sediment carrying capacity of running 

water (as overland flow or channel flow), all of which increase with 

increasing gradient. It inversely affects the amount of time that internal soil 

water is present. Many soil conservation practices, such as conservation 
42 Chapter 2

terraces, are designed primarily to reduce slope gradient to minimize soil 

erosion and increase infiltration. Slope gradient also directly affects land 

management practices by limiting ranges of operation for various types 

of equipment, such as tractors and log skidders. 

Slope complexity is the relative linearity or smoothness (simple) or 

irregularity (complex) of the ground surface leading down slope and 

through the point or map unit of interest (fig. 2-12). Simple slopes allow 

the maximum slope length with comparatively unimpeded slope wash 

processes. In contrast, complex slopes are composed of a series of steps 

commonly associated with bedrock-controlled benches or other stepped 

surfaces (fig. 2-13). These localized breaks in slope reduce slope length, 

alter slope wash processes, and commonly correspond to changes in soil 

types.

Figure 2-12

backslope

footslope

toeslope

shoulder summit

SU SH

BS FS SH

BS

SH

FS SH

BS

SH

FS

TS TS

FS

BS

SH SU

FS

BS

SH

SU

General

Scale

Detailed

Scale 

Simple Slopes Complex Slopes

(adapted from Wysocki et al., 2000)

Simple versus complex slopes and slope positions. Note the choice of a detailed or 

general scale approach to slope position for complex slopes (right side).

In many places, internal soil properties are more closely related 

to the slope complexity than to the gradient. Slope complexity has an 

important influence on the amount and rate of runoff and on sedimentation 

associated with runoff. It can also affect soil temperature through local 

variation in soil aspect. 

Traditionally, slope (gradient) classes are assigned to soil map units 

to convey the dominant range of slope gradients occurring within it. The 

numerical slope class limits of map units are not always consistent within 

or between survey areas. They can vary from one survey area to another, 

to better capture the local survey slope conditions, as long as they 
Soil Survey Manual 43

generally remain within the maximum (upper) and minimum (lower) 

class limits (table 2-3). Descriptive adjectives corresponding to specified 

slope ranges can be used in text. Such adjectives are slightly different 

for the mid-range slope classes, depending upon whether the dominant 

slopes are simple or complex (table 2-3). Gently sloping or undulating 

soil map units, for example, can be defined with slope class ranges as 

broad as 1 to 8 percent or as narrow as 3 to 5 percent. Classes may exceed 

the broadest range indicated in table 2-3 by one or two percentage points 

where the range is narrow and by as much as 5 percent or more where the 

range is broad. The slope class terms can also be used in naming slope 

phases of map units, as discussed in chapter 4. 

If the detail of mapping requires slope classes that are more detailed 

than those in table 2-3, some or all of the slope classes can be subdivided 

as follows: 

Nearly level.—Level, nearly level

Gently sloping.—Very gently sloping, gently sloping

Strongly sloping.—Sloping, strongly sloping, moderately sloping

Undulating.—Gently undulating, undulating

Rolling.—Rolling, strongly rolling

Figure 2-13

Complex slopes on a hillslope of interbedded sedimentary rocks in Wildcat Hills, 

Nebraska.
44 Chapter 2

In a highly detailed survey, for example, slope classes of 0 to 1 

percent and 1 to 3 percent would be named “level” and “nearly level,” 

respectively. 

Slope length is rarely used directly in soil mapping because its range 

across all the polygons of a soil map unit is highly variable. Furthermore, 

natural slope lengths are commonly interrupted and artificially shortened 

by human-made features such as ditches, roads, or field boundaries. 

Slope length does have important uses in key soil erosion programs and 

models, including the Revised Universal Soil Loss Equation, version 2 

(USDA-NRCS, 2016d). It has considerable control over surface water 

runoff and potential for accelerated water erosion. Generic terms such as 

“long” or “short” can be used to describe slope lengths that are typical 

of certain kinds of soils. These terms are typically relative within a 

physiographic region. A “long” slope in one place might be considered 

“short” in another. If such terms are used, they are defined locally. For 

observations at a particular point, it may be useful to record the length 

of the slope that contributes water to that point (called point runoff slope 

length) as well as the total length of the slope. The sediment transport 

slope length is the distance from the expected or observed initiation 

point upslope of runoff to the highest local elevation where deposition of 

sediment is expected to occur. This distance is not necessarily the same 

as the point runoff slope length. 

Relative slope segment position indicates vertical subdivisions of long 

slopes. It can be useful, especially in areas of substantial slope length, to 

identify general “slope segment” positions, such as lower third, middle 

Table 2-3

Definitions of Slope Classes

Classes for— Recommended slope 

(gradient) class limits

Simple slopes Complex 

slopes

Lower

(percent)

Upper

(percent)

Nearly level Nearly level 0 3

Gently sloping Undulating 1 8

Strongly sloping Rolling 4 16

Moderately steep Hilly 10 30

Steep Steep 20 60

Very steep Very steep > 45
Soil Survey Manual 45

third, and upper third. For example, the long slopes of mountainflanks 

commonly exhibit changes in bedrock stratigraphy somewhere along 

the slope that correspond to soil types that differ in parent material 

composition and type and amount of rock fragments. 

Slope shape is the dominant form of the ground surface curvature. 

It is expressed in two directions, which are paired (fig. 2-14): up and 

down slope (vertical, or perpendicular or normal to the slope contour) 

and across slope (horizontal, or along the slope contour). When used in 

tandem, the slope directions describe the configuration of the surface 

of a portion of the slope and the soil upon it. Both slope directions 

can be described by one of three curvature shapes: convex, linear, or 

concave. In the up and down direction, the surface of a linear slope is 

substantially a straight line when seen in profile at right angles to the 

contours. The gradient neither increases nor decreases significantly with 

distance (fig. 2-14, top row). An example is the dip slope of a cuesta. 

On a concave slope (fig. 2-14, bottom row), gradient decreases down 

the slope. An example is a footslope. Where the slope decreases, runoff 

water decelerates and tends to deposit sediments, as on the lower parts of 

the hillslope. Simultaneously, as surface water flow slows, it has greater 

opportunity to infiltrate into the soil. On a convex slope, such as the 

shoulder of a hill or ridge, gradient increases down the slope and runoff 

tends to accelerate as it flows down (fig. 2-15, middle row). If contours are 

substantially straight lines (parallel), as on the side of a lateral moraine, 

the across slope shape is linear. An alluvial fan has a convex contour 

that bows outward. A cirque has concave contours. In figure 2-15, nine 

possible combinations of linear (L), concave (C), and convex (V) slopes 

are shown. For both the up and down orientation and the across slope 

orientation, where the slope is convex (fig. 2-14, middle column), surface 

runoff water tends to diverge (spread apart) as it moves down the slope. 

As a result, overland flow is dissipated and both the erosive power and the 

amount of water available for infiltration are reduced. Where the slope 

is concave, surface runoff water tends to converge, or concentrate (fig. 

2-14, right column). The most intense concentration of running water 

occurs where both orientations are concave, as in a swale on a hillside or 

in a head slope at the head of a drainageway (fig. 2-15). The most intense 

divergence of running water occurs where both orientations are convex, 

as on a nose slope at the end of a ridge (fig. 2-15).

Hillslope profile positions (also called hillslope positions) refer to 

two-dimensional segments of a line used to describe slope position along 

a transect oriented up and down slope (normal to the slope contour). 

They do not address lateral dimensions. These line segments, progressing 

from the top of the slope to the bottom, are: summit, shoulder, backslope, 
46 Chapter 2

footslope, and toeslope (see fig. 2-12). These terms have proven useful 

for many decades because they can describe areas on slopes where soil 

bodies are consistent and breaks in slope curvature where soils typically 

change. They can be used alone or in a combination to verbally capture 

where soils recur up and down slopes. 

Geomorphic components are similar to hillslope profile positions 

(up and down slope) but include an additional lateral dimension (across 

slope) that enables distinctions to be made between the slope curvatures 

of land areas in three dimensions. They indicate patterns of surface water 

flow, such as concentration, dispersion, or parallel (lateral) flow. Not 

all settings, however, share the same, recurrent configurations. For this 

reason, geomorphic component descriptors have been developed for four 

different settings: hills, terraces and stepped landforms, mountains, and 

flat plains (tables 2-4 to 2-7 and figs. 2-15 to 2-18). 

As with hillslope profile positions, geomorphic components for 

hills have been widely used in one form or another. These terms and 

concepts work very well for partitioning and describing hilly terrain 

as functionally distinct members. However, these same concepts work 

poorly when applied to very gentle terrain. The kinetic energy of running 

water is dramatically and functionally less at low gradients. The erosive 

Figure 2-14

CV

L = Linear; V = Convex; C = Concave

CC

VV VC

LV LC

CL

VL

LL

(adapted from Wysocki et al., 2000)

Surface flow pathway

Slope shape based on combinations (up and down slope and across slope) of surface 

curvature.
Soil Survey Manual 47

Table 2-4

Geomorphic Component Terms for Hills

Geomorphic 

component term Typical attributes

Interfluve High, relatively level area that generally does 

not receive run-on surface flow; residuum, 

short-transport colluvium

Crest High, narrow area; converging backwearing 

slopes that form a lowered ridge

Head slope Convergent overland water flow; thickened 

colluvium, slope alluvium 

Side slope Parallel overland waterflow; colluvium, slope 

alluvium, pedisediment, residuum

Nose slope Divergent overland water flow; colluvium, slope 

alluvium, pedisediment

Free face Rock outcrop

Base slope Concave surface; colluvium, slope alluvium

Figure 2-15

Lower

order str eam

base slope

side

slope

nos

sl

e

ope

head

slope interfluve

base slope

Alluvial

fill

(Modified from Ruhe, 1969)

Colluvium and

s ol pe

al ul

v ui m

crest

free face (i.e., rock outcrop)

Higher order stream

Three-dimensional depiction of geomorphic components of hills.
48 Chapter 2

power of the water is reduced as well as its sediment carrying capacity, 

which determines what sediments are removed and which are left behind. 

Additionally, there is a general increase, compared to higher gradient 

systems, in the residence time of water, particularly internal soil water, 

which alters the biogeochemical dynamics and products. Therefore, 

new concepts and associated terms were developed for geomorphic 

components of flat plains. In a similar way, hillslope components were 

found to be inadequate when applied to high-gradient terrain. Very 

Table 2-5

Geomorphic Component Terms for Terraces and Stepped 

Landforms

Geomorphic 

component term Typical attributes

Tread Relatively level, broad surface

Riser Vertical or steep side slope separating treads

Figure 2-16

TREAD

RISER

100 yr. Flood Zone

Annual

Flood

Plain

Uplands Terraces Flood-Plain Steps

(adapted from Wysocki et al., 2000)

Three-dimensional depiction of geomorphic components of terraces and stepped 

landforms.
Soil Survey Manual 49

long and commonly complex slopes and much greater kinetic energy 

dramatically increase erosion potential and the sediment carrying capacity, can change sediment winnowing effects, can decrease soil water 

residence time, and can otherwise alter system dynamics and resulting 

sediments and soils. Therefore, new concepts and associated terms were 

developed for geomorphic components of mountains. Stream terrace 

and stepped landforms are also sufficiently unique to warrant separate 

geomorphic component descriptors. 

Table 2-6

Geomorphic Component Terms for Mountains

Geomorphic 

component term Typical attributes

Mountaintop High area (crest, summit); residuum or short 

transport colluvium, solifluction deposits; 

generally does not receive run-on surface flow

Mountainflank Complex slopes, long slopes, substantial 

gradients, colluvium, mass wasting deposits, 

talus

Free face Rock outcrop

Mountainbase Concave surface; thick colluvium, slope 

alluvium, mass wasting deposits 

Figure 2-17

Three-dimensional depiction of geomorphic components of mountains.
50 Chapter 2

Table 2-7

Geomorphic Component Terms for Flat Plains

Geomorphic 

component term Typical attributes

Rise Slightly elevated area (1-3% slopes)

Talf Very low slope gradients (0-1%); deranged 

or incipient drainage network; lacustrine 

deposits, alluvium, till, marine deposits, 

eolian deposits, and other flat-lying deposits

Dip Depressions; backswamp deposits, marl, 

organic deposits, and other deposits in lowlying areas

Figure 2-18

Three-dimensional depiction of geomorphic components of flat plains.

Microrelief refers generically to small, relative elevational differences between adjacent areas on the earth surface. In subaerial settings, 

minor elevational differences can profoundly influence plant growth 

above ground and, subsequently, water conditions below ground. The 

lateral scale across which the elevational differences occur is generally 

on the order of about 3 to 10 meters but can be smaller. A gilgai, which 

has micro site differences in patterned ground, is an example (fig. 2-19). 

Terms used to describe microrelief positions are microhigh, microslope, 

and microlow.
Soil Survey Manual 51

Figure 2-19

Example of Wedge Structure, Gilgai Microfeature,

and Microrelief

Master

slickensides

CaCO3

microhigh

microlow microslope

microslope

3-10 m

Half-wedge

(merging

slickensides)

2m

Parallelepiped

Wedge-shaped ped

Bowl

(modified from Lynn and Williams, 1992)

Water ponds in the microlows in this area of Vertisols that exhibits gilgai microfeatures 

(mounds and bowls).

 Drainage patterns (also called a drainage network) describe the 

recurring arrangement of interconnected drainage channels across 

the surface of a land area. They provide substantial insight into the 

underground, controlling bedrock or regolith (see Way, 1973) as well as 

the locations where these materials and the overlying soils change. Figure 

2-20 presents the more general patterns. Drainage patterns can be best 

observed and evaluated on aerial photographs, topographic quadrangle 

maps, or GIS spatial layers that present drainageway patterns in detail.

Figure 2-20

Annular Artificial

Continued on next page.
52 Chapter 2

Figure 2-20.—continued

Radial Rectangular

Parallel Pinnate

Deranged

(nonintegrated)

local

depression

Centripetal Dendritic

basin,

lakebed

Karst

Thermokarst Trellis

Illustrations and descriptive terms for drainage patterns.
Soil Survey Manual 53

Parent Material

Soil parent material refers to the unconsolidated, organic and mineral 

materials in which soils form. The unconsolidated material, or regolith, 

in which a soil develops exerts tremendous influence upon what that 

soil is and is not and how it behaves. Determining the parent material is 

therefore important in accurately identifying the composition of the soil. 

Parent material is more than just soil texture. Other attributes, such as 

mineralogy, stratigraphy, and the degree of sorting and particle rounding, 

can substantially affect soil behavior. Eolian sand, such as dunes, can 

behave hydrologically different than beach sand deposits, even though 

both are made of sand, due to differences in the internal arrangement and 

lateral continuity of primary particles. Accurate identification conveys 

direct and implicit information about the soil itself, the environment in 

which it formed, and its current environment. Soils provide a record of 

prevailing and past environments, climates, human activities, and much 

more. 

Importance of Parent Material in Understanding the Soil 

Soil formation involves alterations, such as additions, losses, 

transformations, and translocations and including weathering, of unconsolidated earthy or organic materials (Simonson, 1959). The parent 

material of a genetic soil horizon cannot be observed in its original 

state as it has undergone soil formation. Rather, the original state must 

be inferred from the properties that the horizon has inherited and from 

other evidence, such as the geomorphic context. In some soils, the parent 

material has changed little and what it was originally can be deduced with 

confidence. In other soils, such as some very old, highly altered soils of 

the Tropics, the specific kind of parent material or its mode of deposition 

is less clear and more speculative. Regardless, the influence that parent 

material exerts on the inherent properties and subsequent behavior of 

soil is substantial. Parent material determines the broad characteristics of 

what is geochemically present or absent. It directly affects the physical 

architecture that makes up a soil.

Much of the mineral matter in which soils form is derived from hard 

bedrock in some way. Glaciers may grind the bedrock into fragments 

and smaller particles and deposit the unsorted mixture as till. Wind and 

running water can abrade and entrain small particles that accumulate 

elsewhere as eolian or fluvial deposits. Bedrock may be weathered and 

significantly changed chemically and physically but not be moved from 
54 Chapter 2

its place of origin. Little may be gained from attempting to differentiate between geologic weathering and soil formation because both are 

weathering processes. It may be possible to infer that a material was 

weathered before soil formation. The weathering process causes some 

bedrock constituents to be lost, some to be transformed, and others to be 

concentrated.

Soil parent material is not always residuum weathered directly from 

underlying bedrock. The material that developed into the modern soil 

may not be related to the underlying bedrock at all. In fact, most soils did 

not form in place but were subject to transport and deposition by wind, 

water, gravity, or human activities. 

Seldom is there absolute certainty that a highly weathered material 

actually weathered in place. The term “residuum” is used if the properties 

of the soil indicate that it has been derived from rock similar to that 

which underlies it and if there is no overt evidence that it has been 

modified by movement. A decrease in the amount of rock fragments as 

depth increases, especially over saprolite, indicates that soil material 

probably has been transported down slope. Stone lines, especially if 

the stones have a different lithology than the underlying bedrock, are 

evidence that the soil did not form entirely in residuum. In some soils, 

transported material overlies residuum and illuvial organic matter and 

clay are superimposed across the discontinuity between the contrasting 

materials. A certain degree of landscape stability is inferred for soils that 

formed in residuum. A lesser degree is inferred for soils that developed 

in transported material.

Standard terms are used to describe both consolidated and unconsolidated materials beneath the solum that influence the genesis and 

behavior of the soil. Besides primary observations, the scientist uses his 

own judgement to infer the origin of the parent material from which the 

solum developed. Primary observations must precede, and be clearly 

separated from, inferences.

The lithologic composition, structure, and consistence of the material 

directly beneath the solum are important. Evidence of stratification of 

the material should be noted. It includes textural differences, stone lines, 

and changes in kind and amount of coarse fragments. Commonly, the 

upper layers of outwash deposits settled out of more slowly moving 

water and are finer in texture than the lower layers. Windblown material 

and volcanic ash are laid down at different rates in blankets of varying 

thickness. Examples of such complexities are nearly endless.

Where alluvium, eolian sands, volcanic ash, or colluvium is rapidly 

deposited on old soils, buried soils may be well preserved. In other places, 

the accumulation is so slow that the thicknes of the solum increases only 
Soil Survey Manual 55

gradually. In these places, the material beneath the solum that was once 

near the surface may now be buried below the zone of active change.

Where hard rocks or other strongly contrasting materials lie close 

enough to the surface to affect soil behavior, their properties and the 

depth to contact need to be measured accurately. The depth of soil over 

such nonconforming materials is an important criterion for distinguishing 

different kinds of soil.

General Kinds of Parent Materials

Broad groupings of parent material are discussed in the following paragraphs. Consistent use of terminology to describe parent 

materials in pedon descriptions and databases enhances the usefulness 

of the information and allows easier and more reliable comparison of 

soils that formed in the same kind of parent material. The NCSS has 

adopted standard terms for many kinds of parent material. These terms 

are presented in the Field Book for Describing and Sampling Soils

(Schoeneberger et al., 2012). The terms are fully defined in the Glossary 

of Landforms and Geologic Terms (USDA-NRCS, 2016b). 

Material Produced by Weathering of Bedrock

The nature of the original rock affects the residual material produced by 

weathering. Bedrock undergoes various changes as it weathers, beginning 

with the progressive removal of readily weatherable minerals, such as 

plagioclase feldspar and biotite mica. The relative ease of weathering of 

major minerals was described by Goldich (1938) and refined for some 

soil clay minerals by McClelland (1950). This weathering sequence 

indicates which minerals weather most readily and the relative order in 

which weathering progresses. Evaluating which minerals are present and 

which have been removed can indicate the degree of weathering that rock 

has undergone (Coleman and Dethier, 1986). 

In-place deposits.—Saprolite is soft, friable material produced by 

bedrock that has been highly weathered in place (in situ). The weathering 

process has removed mineral constituents but left the fabric and structure 

of the original rock without significant loss in volume (Pavitch, 1986). 

If the altered material has lost most or all rock fabric and structure 

and its original volume has been reduced (e.g., by void collapse), the 

unconsolidated, in-place earthy material is called residuum. Such 

distinctions are useful in recognizing close geochemical and physical 

relationships to the bedrock of origin. It is assumed that residuum is in 

situ and has not undergone substantive lateral displacement or transport. 
56 Chapter 2

Residuum is a major kind of parent material, particularly on older, stable 

landscapes and in warm and humid climates. 

If the soil is derived directly from underlying bedrock and exhibits 

little or no evidence of lateral transport, the parent material should be 

identified (e.g., residuum, grus, saprolite, bauxite) and then paired with the 

kind of bedrock from which it was derived (see “Bedrock” section). The 

point where rock weathering ends and soil formation begins is not always 

clear. The processes may be consecutive or overlapping. Quite different 

soils may form from similar or identical rocks under different weathering 

conditions. Texture, color, consistence, and other characteristics of the 

parent material should be included in the description of soils, as well as 

important remnant bedrock features, such as quartz dikes. Information 

about the mineralogical composition, consistence, and structure of the 

parent rock is useful and should also be included. 

Transported Material

Most soil parent materials have been moved from their place of 

origin and deposited elsewhere. The principal subsets of transported 

materials are typically arranged according to the main geomorphic 

process responsible for their transport and deposition. In most places, 

there is sufficient evidence to make a clear determination.

In soil morphology and classification, it is very important to observe 

and describe the characteristics of the parent material. It is not enough 

simply to identify the material. Any doubt regarding the identification 

should be mentioned. For example, it can be difficult to determine if 

silty deposits are alluvium or loess or to distinguish silty colluvium from 

silty residuum. It can also be difficult to distinguish certain mud flow 

deposits from till or to distinguish some sandy tills from sandy outwash. 

Additional observations across large exposures or at multiple locations 

help in making such distinctions. These distinctions provide supporting 

information needed to accurately inventory soils and thereby improve the 

accurate prediction of soil behavior.

Water-laid or water-transported deposits.—Alluvium is a widely 

occurring parent material. It consists of unconsolidated, sorted, clastic 

sediment deposited by running water, particularly channel flow. It may 

occur on actively flooded portions of modern streams. Remnants of old 

stream terraces may occur in dissected areas far away from, or high 

above, a present stream or occur as paleoterraces that are unrelated to the 

modern stream. In larger streams and rivers, a series of alluvial deposits 

in the form of stream terraces may loosely parallel the modern stream. 

The youngest deposits occur in the stream; deposits increase in age as 

they progress to higher levels. In some areas, recent alluvium covers 
Soil Survey Manual 57

older terraces. For example, younger alluvial fan sediments onlap and 

bury older fan sediments. Alluvium is also the dominant parent material 

in large tectonic valleys, such as the bolsons and semi-bolsons of the 

Basin and Range Physiographic Province in the western United States. 

On these broad, sloping landscapes, alluvium occurs as thick deposits 

on active alluvial fans and fan remnants or as broad, relatively level 

alluvial flats on basin floors. The further down a river system alluvium 

occurs, the better sorted the sediments tend to be. Larger stream systems 

commonly have backswamp deposits along low-gradient stream reaches. 

These lower energy areas are set back from the main channel and are 

dominated by sediments that are laminated and finer (silts and clays) than 

alluvium closer to the stream channel. Slope alluvium refers to hillslope 

sediments transported primarily by slope wash processes (sheet flow) 

rather than by the channel flow of streams. Crude lateral particle sorting 

is evident on long slopes, but it is much less evident than the particle 

sorting in alluvium derived from channel flow. 

Lacustrine deposits consist of clastic sediments and chemical precipitates that settled out of bodies of still water, such as ponds and lakes. 

Deposits associated directly with glaciers and laid down in freshwater 

lakes (glaciolacustrine deposits) or in oceans (glaciomarine deposits) are 

included with other glacial deposits. Numerous basins in the western U.S. 

contained moderate to large pluvial lakes during the Pleistocene epoch. 

These lakes have either drastically shrunk or disappeared during the 

warmer and drier climates of the Holocene epoch. The now dry lakebeds 

are known as playas or salt flats and contain thick lacustrine deposits 

dominated by silt and clay with interbedded layers of volcanic ash. Some 

also contain substantial evaporate deposits. Soils in the narrow margins 

of these barren playas are generally saline, depending on climate and 

drainage, and are sparsely vegetated with salt-tolerant plants.

Marine deposits settled out of the sea, lagoons, or estuaries and 

commonly were reworked by currents and tides. Subaqueous soils 

include sediments that remain under water. Some marine deposits 

were later exposed either naturally by falling sea levels or following 

the construction of dikes and drainage canals. Many of the soils of the 

Atlantic and Gulf of Mexico Coastal Plains in the southeastern U.S. 

formed in marine sediments deposited during a time of higher sea level. 

These deposits vary widely in composition. In low-energy settings, such 

as lagoons, sediments tend to be finer textured and may have intermittent 

or substantial amounts of organic materials. Higher energy settings can 

have substantial amounts of sandy material (such as in areas of inlets 

and barrier islands) or coarser rock fragments (such as in areas of rocky 

coasts and headlands). 
58 Chapter 2

Beach deposits mark the present or former shorelines of lakes or 

oceans. They consist of low sheets or ridges of sorted material. They are 

commonly sandy or gravelly (along non-rocky coasts) or cobbly or stony 

(especially along rocky coasts). 

Eolian deposits.—Eolian deposits are very well sorted windblown 

material. They are broadly divided into groups based on dominant particle 

size or origin. Examples are aerosols, dust, loess, and eolian sands. All 

but the finest wind-driven sediments share some depositional traits. 

“Lateral fining” refers to the progressive reduction in average particle 

size and deposit thickness as distance increases along the prevailing wind 

direction and away from the source area. The closer to an eolian sediment 

source (e.g., a large barren flood plain), the coarser the average particle 

size and the thicker the eolian deposit. The dominant particle sizes of 

discrete eolian deposits range from silt and very fine sands (loess) and 

from fine to medium sands (eolian sands). 

Eolian sands are significant due to their physical prominence and the 

wide range of distinct landforms (especially dune types) they produce. 

Very fine and fine eolian sands commonly occur as dunes (Bagnold, 

1941), and medium sands tend to form sand sheets. Eolian sands are 

common in, but not limited to, warm, dry regions. They characteristically 

consist of sands with a high content of quartz and a low content of clayforming materials. Sand dunes may contain large amounts of calcium 

carbonate or gypsum, especially in deserts and semi-deserts.

During periods of drought and in deserts, local wind movements 

may mix and pile up soil materials of different grain sizes, including 

materials with a high content of clay. Sand-sized aggregates of clay (e.g., 

parna) can even form dunes (parna dunes). In areas where sand and finer 

eolian materials are intimately intermingled, the eolian materials may be 

identified generically as eolian deposits rather than as distinct loess or 

eolian sands.

Loess deposits are important because their physical and mineralogical 

properties make them highly suitable to food and fiber production 

worldwide. Their texture is typically very silty but may range from fine 

silt to very fine sand. Most loess is pale brown to brown, although gray 

and red colors are also common. Some colors are inherited from the 

source material (geogenic colors). Other colors, particularly gray colors, 

may be caused by post-deposition soil formation, such as redoximorphic 

alteration resulting in iron reduction. Although thick loess deposits appear 

to be relatively massive, they have some gross vertical cracking with 

coarse polygonal structure and can support nearly vertical walls (e.g., 

roadcut walls) for many years. Silty deposits that formed in other ways 

have some or all of these characteristics. Windblown silt that has been 
Soil Survey Manual 59

leached and strongly weathered can be acidic and rich in clay, whereas 

some young deposits of loess that are mainly silt and very fine sand have 

a low content of clay. 

Other, finer windborne particles also affect soils in unique ways 

but are not generally recognized as a kind of parent material. Dust is 

composed of clay or very fine silt-sized particles and can be deposited dry 

or in precipitation. It can travel great distances from its point of origin, 

even circle the Earth in the upper atmosphere and be deposited in small 

increments across the world. After dust settles, the very fine particles 

are readily mixed into pre-existing soils and may substantially affect 

soil properties. However, they typically do not form readily identifiable, 

discrete deposits by themselves. Aerosols are the finest of particulate 

materials, so small that they can stay suspended in air for extended 

periods. Wood ash is an example. These particles are typically too fine 

and too diffuse to accumulate as separate deposits. Consequently, they 

are not identified as discrete parent materials in soil survey. Nonetheless, 

they can contribute meaningful amounts of carbon ash, pollen, quartz, 

or other materials to soils. They typically settle out as raindrop nuclei 

and infiltrate soil in suspension or settle in water bodies. Other soil 

constituents accompany precipitation, such as atmospheric elements in 

solution (fixed nitrogen, sulfur, calcium, magnesium, sodium, potassium, 

etc.) but are not included within the concept of parent materials. 

A conventional practice in considering geomorphic processes is to 

include volcanic eolian deposits, such as ash and pumice, with other 

volcanic materials (see “Volcanic Deposits” below) because their origin, 

including mineralogical composition and depositional dynamics, is 

closely associated with volcanism.

Glacial and periglacial deposits.—Glacial and periglacial deposits 

are derived from material moved and deposited by glacial processes 

or associated with cold climates. However, the two types have two 

distinct geomorphic process systems. Their processes and sediments 

are commonly associated because they share very cold climatic settings 

and driving forces. They are considered together here for convenience. 

Glacial refers to materials that have been directly created, moved, and 

deposited by glacial ice (i.e., drift and till). A conventional practice in 

considering geomorphic processes is to include glaciofluvial, outwash, 

and glaciolacustrine deposits among other glacial materials because their 

origins, including depositional dynamics, resulting stratigraphy, and 

mineralogical composition, are closely associated. 

Drift is a general, inclusive term for all material picked up, mixed, 

disintegrated, transported, and deposited by glacial ice or glacial 

meltwaters. The term is so generic that it is principally used for very 
60 Chapter 2

coarse scales that prohibit details. In many places, drift is mantled by 

loess. Thick mantles of loess are typically easily recognized, but very 

thin mantles may be so mixed by soil-building processes that they can 

scarcely be differentiated from the underlying drift.

Till is a type of drift that was deposited directly by ice and had little or 

no transportation by water. It is generally an unstratified and heterogeneous 

(i.e., unsorted) mixture of clay, silt, sand, gravel, and boulders. Some of 

the ice-entrained mixture settled out as the ice melted and was subject to 

very little washing or reworking by water (ablation till), and some was 

overridden by the glacier and became compacted (lodgement till). Till 

occurs in various glacial landforms. Ground moraines and recessional 

moraines are examples. In many places, it is important to differentiate 

tills of several glaciations. Commonly, the tills underlie one another and 

may be separated by other deposits or old, weathered surfaces. In many 

cases, till was later eroded by the wave action in glacial lakes. The upper 

part of such wave-cut till may have a high percentage of rock fragments.

Till ranges widely in texture, chemical composition, and degree of 

weathering. It is principally affected by the composition of the bedrock 

it has overridden and whose materials it has entrained. Tills of the midcontinental U.S. are underlain by sedimentary rocks, such as limestone 

and shale, and typified by heavy textures (clay, clay loams). In contrast, 

tills of northern Minnesota, New England, and Canada underlain by 

crystalline bedrock, such as granite, are typified by coarser textures 

(gravelly sandy loam). Much till is calcareous, but a significant amount 

is noncalcareous because no carbonate rocks contributed to the till or 

because subsequent leaching and chemical weathering have removed the 

carbonates. The two most widely occurring and operationally important 

types of till are ablation till and lodgement till. Ablation till is characterized 

by a comparatively low bulk density (e.g., 1.4 g/cm3) and occurs at the 

top of till deposits. Lodgement till formed beneath a glacier and was 

over-compacted. As a result, it has a very high bulk density (e.g., 1.8 

g/cm3) that substantially restricts internal water flow and makes 

excavation difficult. Some tills are identified by position of formation 

relative to the glacial ice. Supraglacial till formed by the sediments on top 

of or entrained with the ice that settled out as the ice melted (ablation till 

or melt-out till) or moved as localized mud flows (flow till). Subglacial 

till, such as lodgement till, formed beneath glacial ice. 

Glaciofluvial deposits are materials moved by glaciers and 

subsequently carried, sorted, and deposited by meltwaters flowing 

from the ice. Outwash is a parent material term for the detritus (chiefly 

sand and gravel) removed or “washed out” from a glacier by meltwater 

streams and deposited beyond the ice front or end moraine. The coarsest 
Soil Survey Manual 61

material was deposited nearer the ice. This outwash commonly forms on 

plains, valley trains, outwash terraces, or deltas in drainageways or in 

relict glacial lakes. Some outwash terraces may extend far beyond the 

farthest advance of the ice. Near moraines or in disintegration moraine 

landscapes, sorted glaciofluvial material may form kames, eskers, and 

crevasse fills. 

Glacial beach deposits consist of rock fragments and sand. They 

mark the locations of relict shorelines (i.e., strandlines) of former glacial 

lakes. Depending on the character of the original drift, beach deposits 

may be sandy, gravelly, cobbly, or stony.

Glaciolacustrine deposits are also derived from glaciers but were 

reworked and laid down in glacial lakes. These deposits range from fine 

clay to sand. Many of them are stratified or varved. A varve is the pair of 

deposition laminae for a calendar year. The finer portion reflects lower 

energy deposition during the cold season, and the slightly coarser portion 

reflects higher energy deposition during the warmer season when runoff 

is greater and wave action occurs.

In many places, it is difficult to distinguish between the different kinds 

of glacial sediments. For example, pitted outwash plains can be difficult 

to distinguish from sandy till in recessional moraines and wave-cut till 

can be difficult to distinguish from lacustrine material. Typically, even 

the most subtle differences can be identified from multiple, well planned 

field observations. This information is used to accurately determine the 

geomorphic setting and its associated sediments. Careful observations 

and descriptions of parent material, stratification, coarse fragment 

distribution, and the surface forms in which they occur provide hard 

evidence needed for correct conclusions. However, some situations are 

not fully understood at present because of their complexity or incomplete 

scientific knowledge. 

Periglacial deposits have several major types. Cryoturbates are 

deposits of sediments that have been mixed or preferentially sorted by 

seasonal frost heave, partial melting and refreezing of permafrost, or other 

non-glacial ice displacement processes. These processes can organize 

sediments in several ways. Internally, the materials typically exhibit 

convolutions or low-grade internal sorting, unlike the more horizontal 

layering typical of mineral soils in warmer climates. Surficial sorting, 

particularly of coarse fragments, can take the form of polygons or stripes 

or other patterned ground. Solifluction deposits consist of heterogeneous 

mixtures of textures, including rock fragments. The orientation of the 

rock fragments indicates the slow downslope movement that resulted 

in surficial lobes, sheets, and terraces. Solifluction deposits form in 

response to seasonal or partial thawing of the near surface “active zone.” 
62 Chapter 2

Periglacial parent materials can have wide aerial extent. Active or recent 

periglacial deposits occur most extensively at high latitudes or at high 

elevations outside of, or otherwise unaffected by, glacial ice. Relict 

solifluction deposits also widely occur in the form of relict patterned 

ground in association with former continental glaciated areas in mid 

latitudes.

Mass wasting (mass movement) deposits.—Some materials are 

transported primarily or completely by gravity. Transport can occur 

extremely quickly or gradually. Landslide deposits is a generic term that 

includes all forms of landslide materials. These deposits can be more 

explicitly identified based on the main mode of movement (table 2-8). 

Table 2-8

Types of Landslide Deposits 

Movement types Deposit attributes

Fall deposits Free fall, bouncing or rolling

Topple deposits Forward rotation over a basal pivot point

Slide deposits:

Rotational landslide

Backward rotation around a pivot point above 

the ground surface

Slide deposits:

Translational slide

Mass lateral displacement along a planar slip 

face

Spread deposits Layers plastically extruded by liquefaction

Flow deposits Wet or dry mass flow that behaves as a viscous 

liquid

Each of these movement types can be further subdivided to indicate 

the dominant kind of material moved: rock (consolidated bedrock 

masses), debris (unconsolidated material rich in rock fragments), or 

earth (dominantly fine-earth material). (See Mass Movement (Wasting) 

Types table in Schoeneberger et al., 2012.) These terms are useful in 

specifying different levels of detail needed to identify areas according to 

their associated deposits. They are also used to convey the composition 

of the present materials, which impacts land management decisions. 

Other kinds of gravity-related deposits are widely recognized. 

Colluvium is poorly sorted slope sediments that have been transported 

and accumulated along or at the base of slopes, in depressions, or along 

small streams primarily due to gravity, soil creep, and slope wash 

processes. Accumulations of rock fragments at the base of rock outcrops 

are called talus. Rock fragments in colluvium are typically very angular 
Soil Survey Manual 63

to sub-rounded due to relatively short transport distances and the limited 

abrasion associated with the process. In contrast, rock fragments in 

alluvium and glacial outwash are rounded to well rounded and waterworn.

Organic deposits.—Organic deposits are material dominated by 

carbon-rich plant or organism detritus. The organic material accumulates 

more rapidly than it decomposes. This unconsolidated material is 

commonly associated with, but not restricted to, wet soil or subaqueous 

conditions. Organic deposits can persist in extremely dry settings or under 

other conditions that reduce or eliminate microbial decomposition, such 

as low oxygen or low pH (acidic). These latter conditions can produce 

various types of organic accumulations that may become the soil parent 

material generically called “organic materials.” Organic deposits can be 

further defined according to the dominant plant material present, such as 

woody, herbaceous, grassy, or mossy. Different terms are used to modify 

an associated soil texture (e.g., mucky, peaty). Terms used in lieu of 

texture for organic materials include muck, peat, and highly decomposed 

organic materials (see chapter 3).

Some organic materials occur as alternating layers of different kinds 

that reflect the dominant vegetative cover at the time of deposition. Others 

are combinations of peat and mineral materials. In some places, organic 

materials cap, are intimately mixed with, or are discretely interlayered 

with volcanic ash, marl, alluvium, or eolian sands. Descriptions of 

organic material (see chapter 3) should include labels (e.g., woody 

organic materials) or notations identifying the origin and dominant 

botanical composition, to the extent that they can be reasonably inferred. 

Volcanic deposits.—Volcanic eolian deposits, such as ash and 

pumice, are treated separately from other eolian parent materials because 

of their unique mineralogy and depositional dynamics. Tephra, volcanic 

ash, pumice, and cinders are unconsolidated igneous sediments that were 

ejected during volcanic eruptions and moved from their place of origin. 

Most have been reworked by wind and, in some places, by water. Tephra 

is a broad, generic term referring to any form of volcanic ejecta. Various 

subdivisions are recognized and should be used when possible. Ash

is volcanic ejecta smaller than 2 mm. It can be subdivided into fine ash 

(< 0.06 mm) and coarse ash (> 0.06 and < 2 mm). Pumice is volcanic 

ejecta larger than ash (> 2 mm) that has a low specific gravity (< 1.0). 

Cinders are volcanic ejecta larger (> 2 mm and < 64 mm) than ash and 

heavier (specific gravity > 1.0 and < 2.0) than pumice. (See Pyroclastic 

Terms table in Schoeneberger et al., 2012.)

Anthropogenic deposits.—Human-transported material is a general 

term for solid phase organic or mineral material that can function as soil 

or soil-like material. It has been mixed and moved from a source area 
64 Chapter 2

to a new location by purposeful human activity, usually with the aid of 

machinery or hand tools. There has been little or no subsequent reworking 

by wind, gravity, water, or ice. Human-transported materials are most 

commonly associated with building sites, mining or dredging operations, 

landfills, or other activities that result in the formation of a constructional 

anthropogenic landform. Anthropogenic material differs from natural 

deposits in that its internal composition and stratigraphic arrangements 

depend upon the emplacement methods, tools, and intentions of people. 

It is generally more variable and less predictable in its content and 

configuration than material emplaced by natural processes. Nonetheless, 

it can be described and broadly quantified in ways similar to how natural 

materials are evaluated.

In database management, it is helpful of have an alphabetical master 

list of the many kinds of parent materials. The diverse kinds of parent 

materials can also be constructively arrayed within subsets based upon 

the dominant geomorphic processes that erode, transport, or deposit them 

(see “Parent Material” section in Schoeneberger et al., 2012). Table 2-9 

lists parent material groups based on geomorphic process or setting.

Table 2-9

General Groups of Parent Materials Based on Geomorphic 

Process or Setting

General groups Specific examples

Anthropogenic deposits Dredge spoil, mine spoil, earthy fill

Eolian deposits (nonvolcanic) Eolian sands, loess

Glacial and periglacial deposits Till, solifluction deposit

In-place deposits 

(nontransported)

Residuum, saprolite

Mass wasting deposits Mudflow deposit, talus

Miscellaneous deposits Diamicton, gypsite

Organic deposits Diatomaceous earth, grassy organic 

materials

Volcanic deposits Andesitic ash, pumice

Water-laid or water-transported 

deposits

Alluvium, lacustrine deposit

These subsets compliment and loosely parallel the geomorphic 

environment categories presented in the Geomorphic Description System 

used by the NCSS (Schoeneberger et al., 2012). Soil parent materials 
Soil Survey Manual 65

should generally relect the dominant geomorphic environment and vice 

versa. 

Multiple Parent Materials

Soil is commonly composed of layers of several different types of 

parent materials (e.g., colluvium over residuum) that are identifiable in 

the soil’s stratigraphy. For example, till is covered by a mantle of loess 

in many places. Thick mantles of loess are easily recognized, but very 

thin (e.g.,< 25 cm) mantles may be so altered by soil-building processes, 

such as pedoturbation, that they can scarcely be differentiated from the 

underlying till. The contact between substantially different (contrasting) 

parent materials in a soil is called a lithologic discontinuity. It should be 

documented using horizon description nomenclature (see chapter 3) and 

other descriptive conventions. 

Unconsolidated contrasting soil material may differ in pore-size 

distribution, particle-size distribution, mineralogy, bulk density, or other 

properties. Some of the differences may not be readily observable in the 

field. Some deposits are clearly stratified, such as some lake sediments 

and glacial outwash, and the discontinuities are sharply defined.

The primary deposition differences of multiple, contrasting parent 

materials can be confused with the effects of soil formation. Silt content 

may decrease regularly with increasing depth in soils presumed to have 

formed in till. The higher silt content in the upper part of these soils 

can be explained by factors other than soil formation. In some of these 

soils, small amounts of eolian material may have been deposited on the 

surface over the centuries and mixed with the underlying till by insects 

and rodents or freeze-thaw action. In others, the silt distribution may 

reflect water sorting.

Inferences about contrasting properties inherited from differing 

layers of geologic material may be noted when the soil is described. 

Generally, each identifiable layer that differs clearly in properties from 

adjacent layers is recognized as a horizon or subhorizon. Whether it is 

recognized as a discontinuity or not depends upon its degree of contrast 

with overlying and underlying layers and its thickness. 

A pragmatic balance is needed between identifying the dominant 

parent material layer(s) in a soil and not becoming overwhelmed by 

excessive detail. While there are no rigid criteria, such as a thickness 

minimum, it is particularly important to identify layers that are 

physically contrasting enough and thick enough to substantively affect 

internal water flow. There are several widely recognized exceptions for 

which numerous sediment layers are not comprehensively described. For 

deposits that are intrinsically highly stratified, whose lateral continuity 
66 Chapter 2

is intermittent, it is impractical to identify or sample every thin layer 

(lamina). For finely laminated alluvium or tephra deposits, only the larger, 

aggregate layers are identified and sampled as composites (bulked). 

Minor layers (laminae) within larger layers are noted but typically are 

not comprehensively documented nor sampled individually.

Bedrock

The term “bedrock” as used in soil survey refers to continuous, 

coherent (consolidated) rock. It can be a physical barrier within the 

solum that limits rooting depth or the immediate parent material source 

for residual soils. Bedrock helps to determine local topography and the 

soils that form across it. It can also indirectly impact soils. If fairly close 

to the base of the solum, bedrock can affect the presence or absence 

of ground water and preferential flow direction, depending upon its 

porosity. It can be a determinant factor in slope stability (tendency for 

landslides) or impact excavation, such as soil suitability for basements 

and road construction. Identifying the bedrock, whether its influence 

is direct or indirect, is essential in understanding the intrinsic chemical 

and physical behavior of both the rock material itself and its soil or 

regolith derivatives. Bedrock also has a major impact on soil geography 

and the accurate prediction of it. Boundaries between types of bedrock 

commonly, but not always, coincide with changes in overlying soil 

types. Therefore, accurate recognition and documentation of bedrock 

is generally essential. In some natural settings, the documentation of 

bedrock may be problematic, impractical, or unnecessary. Bedrock is not 

recorded if it does not exert substantial influence on the soil. An example 

is bedrock that is deeply buried by regolith, such as till, basin fills, and 

coastal or lacustrine sediments. 

Geological materials need to be defined in accordance with the 

accepted standards and nomenclature of geology. The accepted, 

authoritative names of the geological formations are recorded in soil 

descriptions. As soil research progresses, there is an increasing number 

of correlations between particular geological formations and the mineral 

and nutrient content of parent materials and soils. Examples include: (1) 

certain terrace materials and deposits of volcanic ash that are different 

in age or source, but otherwise indistinguishable, may vary widely in 

content of cobalt; (2) the phosphorus content of otherwise similar soils 

may vary widely due to similar limestones that can be distinguished in 

the field only by specific fossils.
Soil Survey Manual 67

Igneous rocks formed by the solidification of magma that originated 

within Earth’s upper mantle. There are two main types based on their 

mode of formation—intrusive and extrusive. Intrusive (syn., plutonic) 

types form at considerable depth in the Earth’s crust and possess a coarse 

grain texture due to the slow cooling of magma. Examples of intrusive 

igneous rocks that weather to soil parent material are granite, diorite, and 

gabbro. Extrusive (syn., volcanic) types form on the Earth’s surface or 

at very shallow depth and possess a fine grain texture due to the rapid 

cooling of magma. Examples of common extrusive igneous rocks are 

rhyolite, andesite, and basalt.

Sedimentary rocks formed from sediments laid down in previous 

geological ages. The principal broad groups of sedimentary rocks are 

clastic, chemical, and organic. Examples of rock lithologies in the 

clastic group are shale, sandstone, and conglomerate; examples of those 

in the chemical group are limestone, gypsum rock, and travertine; and 

examples of those in the organic group are coal and diatomite. There are 

many varieties of these lithologies. For example, chalk is a soft variety of 

limestone. Many lithologies are intermediate between the broad groups. 

Examples are calcareous sandstone and arenaceous limestone. 

Metamorphic rocks resulted from profound alteration of igneous and 

sedimentary rocks by heat and pressure. General classes of metamorphic 

rocks important as parent material are gneiss, schist, slate, marble, 

quartzite, and phyllite.

Kinds of Bedrock

Kind is the most important bedrock feature to describe. It indicates 

the general composition of the rock and how the rock and its weathering 

products are likely to behave. Soil survey follows standard conventions 

for rock type compositions and names (Neuendorf et al., 2005). There is 

a large number of officially recognized rock types. They are very detailed 

and can be functionally cumbersome for soil survey. Subsequently, soil 

survey tends to focus on broader categories and common rock types, 

particularly those found in the near surface environment. More obscure 

or minor rock types can be recognized if they have important economic or 

environmental impact. Some databases maintain long alphabetical master 

lists of bedrock. A helpful way to arrange the large number and variety 

of bedrock kinds is to separate them into widely recognized subsets, 

such as igneous, metamorphic, and sedimentary (Schoeneberger et al., 

2012; USDA-NRCS, 2016b). In addition to bedrock kind, descriptions 

of bedrock should include information about the spacing of fractures, 

degree of weathering, and depth to contact (if within or near the solum). 
68 Chapter 2

The general groups of bedrock types described earlier can be 

subdivided or rearranged slightly to provide groups of bedrock types 

commonly pertinent to soils:

Igneous-intrusive.—Examples are anorthrosite, diabase, and granite.

Igneous-extrusive.—Examples are a’a lava, andesite, and basalt.

Igneous-pyroclastic.—Examples are pyroclastic flow, tuff, and volcanic breccia.

Metamorphic.—Examples are amphibolite, gneiss, and schist.

Sedimentary-clastics.—Examples are arenite, argillite, and mudstone.

Interbedded.—Examples are limestone-sandstone, sandstone-shale, 

and shale-siltstone.

Evaporites, organics, and precipitates.—Examples are tufa, coal, 

and limestone.

Depth to Bedrock

Depth to bedrock is a crucial feature because of its impact on plant 

growth, internal water dynamics and direction, and land management. 

This is particularly true for agricultural soils if hard bedrock is within 

2 meters of the surface. Bedrock can limit rooting depth, reduce the 

potential soil water supply, affect internal water flow, and impact various 

mechanical activities, such as deep ripping, foundation excavation, fence 

post placement, and suitability for basements. The depth from the ground 

surface to the contact with bedrock should be recorded.

Fracture Interval

Most bedrock contains a natural joint or crack network that 

functions as by-pass flow routes for internal water. These fractures can 

vary substantially in the amount of water they are able to transmit and 

can potentially affect pond integrity, internal pollutant movement, and 

water well yields. If observable, the average horizontal spacing between 

vertical rock joints in the bedrock layer is described. 

Weathering

Not all bedrock is chemically and/or physically altered from its 

pristine state to the same extent. Weathering generally increases porosity 

and the water-holding capacity and reduces bulk density and coherency. 

A weathering class (e.g., slight, moderate, strong) can be assigned to 

record the bedrock’s subjective extent of weathering as compared to its 

presumed unweathered state.
Soil Survey Manual 69

Lithostratigraphic Units

Lithostratigraphic units are mappable rock or sediment bodies. In 

geochronology, younger units overly older units (law of superposition). 

Regolith units, both unconsolidated material and bedrock units, 

are identified and named according to standard conventions of the 

International Stratigraphic Code (e.g., North American Commission 

on Stratigraphic Nomenclature, 2005). Table 2-10 lists these units in 

descending rank. This naming system provides a standard, shorthand 

method of identifying and concisely communicating information on 

strata and rock type. It aids recognition of differences in geology and 

the soils developed in or on them. Some soils, particularly residual soils, 

can be linked to specific bedrock units. Other soils, such as loess, can 

occur across multiple bedrock units if their lithostratigraphic unit is not 

bedrock constrained. If possible, the hierarchical lithostratigraphic units 

at a site should be recorded (Schoeneberger et al., 2012). 

Table 2-10

Lithostratigraphic Units and Their Hierarchical Rank and 

Definition

• Supergroup.—The broadest lithostratigraphic unit. A supergroup is an 

assemblage of related, superposed groups, or groups and formations. 

It is most useful for regional synthesis.

• Group.—The second ranking lithostratigraphic unit. A group is 

a named assemblage of superposed formations and may include 

unnamed formations. It is useful for small-scale (broad) mapping 

and regional stratigraphic analysis.

• Formation (or Geologic Formation).—The basic lithostratigraphic unit used to describe, delimit, and interpret sedimentary, extrusive igneous, metavolcanic, and metasedimentary 

rock bodies (excluding metamorphic and intrusive igneous 

rocks). It is based on lithic characteristics and stratigraphic 

position. A formation is commonly, but not necessarily, tabular and stratified and is of sufficient extent to be mappable 

at the Earth’s surface or traceable in the subsurface at conventional mapping scales. 

• Member.—The formal lithostratigraphic unit next in 

rank below a formation and always part of a formation. 

A formation need not be divided selectively or entirely 

into members. A member may extend laterally from one 

formation to another.
70 Chapter 2

• Lens (or Lentil).—A specific type of member. A lens is a 

geographically restricted member that terminates on all 

sides within a formation.

• Tongue.—A specific type of member. A tongue is a 

wedge-shaped member that extends beyond the main 

formation boundary or that wedges or pinches out within 

another formation.

• Bed.—The smallest lithostratigraphic unit of sedimentary rock. A bed is a subdivision of a member 

based upon distinctive characteristics or economic 

value (e.g., coal member). Members need not be 

divided selectively or entirely into beds.

• Flow.—The smallest lithostratigraphic unit of volcanic rock. A flow is a discrete, extrusive, volcanic 

body distinguishable by texture, composition, superposition, and other criteria.

Erosion

Erosion is the detachment and movement of soil material. The 

process may be natural or accelerated by human activity. Depending on 

the local landscape and weather conditions, erosion can range from very 

slow to very rapid. Loss of the soil surface layer has a direct detrimental 

impact on site productivity and on off-site sedimentation and nutrient 

inputs. It is especially important to evaluate for environmental and 

agronomic purposes. The dominant kind and degree (relative magnitude) 

of accelerated erosion at the site should be estimated. 

Natural Erosion

Naturally occurring erosion sculptured landforms on the uplands 

and built landforms on the lowlands. Its rate and distribution in time 

control the age of land surfaces and many of the internal properties of 

the soils on them. The formation of the Channel Scablands in the State 

of Washington is an example of extremely rapid natural, or geologic, 

erosion. The broad, nearly level interstream divides on the Coastal Plain 

of the southeastern United States are examples of areas with very slow 

or no natural erosion.

Landscapes and their soils are evaluated from the perspective of their 

natural erosional history. Evidence that material has been moved and 

redeposited, including buried soils, stone lines, and deposits of windblown 
Soil Survey Manual 71

material, is helpful in understanding natural erosion history. Thick 

weathered zones that developed under earlier climatic conditions may 

have been exposed and become the material in which new soils formed. 

In landscapes of the most recently glaciated areas, the consequences of 

natural erosion, or lack of it, are less obvious than where the surface and 

the landscape are early Pleistocene or even Tertiary in age. However, 

even on the landscapes of the most recent glaciation, postglacial natural 

erosion may have redistributed soil materials on the local landscape. 

Natural erosion is an important process that affects soil formation and, 

like human-induced erosion, can remove all or part of soils formed in the 

natural landscape.

Accelerated Erosion

Accelerated erosion is largely the consequence of human activities, 

primarily those that result in a loss of soil cover, such as tillage, grazing, 

and cutting of timber. Kinds are listed in table 2-11 and discussed below.

Table 2-11

Kinds of Accelerated Erosion

Erosion kind Criteria

Wind Deflation by wind

Water: Removal by running water

 Sheet Relatively uniform soil loss; no channels

 Rill Small channels (can be obliterated by conventional 

tillage)

 Gully Big channels (cannot be obliterated by 

conventional tillage)

 Tunnel Subsurface voids within soil that are enlarged by 

running water (i.e., piping)

The rate of erosion can be increased by events besides human 

activities. For example, fire that destroys vegetation can trigger erosion. 

Spectacular episodes of erosion, such as the soil blowing on the Great 

Plains of the central United States in the 1930s, have not all been due to 

human activities; frequent dust storms were recorded on the Great Plains 

before the region became a grain-producing area. 

Accelerated erosion may not be easy to distinguish from natural 

erosion on some soils. A distinction can be made by studying and 

understanding the sequence of sediments and surfaces on the local 
72 Chapter 2

landscape as well as by studying soil properties. For example, in some 

areas of the eastern United States, native forests were cut and burned 

to create cropland. In some places where the soils were particularly 

susceptible, this resulted in extensive soil erosion. The sediments from the 

uplands can be observed on adjacent flood plains as a sequence of layers 

that, in some places, are up to a few meters thick over a buried soil. The 

contact between the original soil surface and new sediments commonly 

is evidenced by numerous pieces of charcoal above the contact, which 

presumably originated from the burning of timber. 

Wind Erosion

The term “wind erosion,” as used in this manual, in soil science 

generally, and by many geologists, indicates the detachment, transportation, and deposition of soil particles by wind, not the sculpture of 

rocks by windblown particles. Wind erosion in regions of low rainfall 

can be widespread, especially during periods of drought. Unlike water 

erosion, wind erosion is generally not related to slope gradient. The 

hazard of wind erosion is increased by removing or reducing the amount 

of vegetation. When winds are strong, coarser particles are rolled, or 

swept along, on or near the soil surface and finer particles are forced into 

the air. The particles are deposited in places sheltered from the wind. 

When wind erosion is severe, the sand particles may drift back and forth 

locally with changes in wind direction while silt and clay are carried 

away. Small areas in which the surface layer has blown away may be 

associated with areas of deposition in such an intricate pattern that the 

two cannot be identified separately on soil maps.

Water Erosion

Water erosion results from the removal of soil material by flowing 

water, including the detachment of soil material by the impact of raindrops. 

The soil material is suspended in runoff water and carried away. Some 

sediment may be carried just a few meters before being deposited, while 

other sediment may be completely removed from the site. Four kinds of 

accelerated water erosion are commonly recognized: sheet, rill, gully, 

and tunnel (piping). 

Sheet erosion is the more or less uniform removal of soil from an area 

without the development of conspicuous water channels. The channels 

are tiny or tortuous, exceedingly numerous, and unstable. They enlarge 

and straighten as the volume of runoff increases. Sheet erosion is less 

apparent, particularly in its early stages, than other types of erosion. It can 
Soil Survey Manual 73

be a problem for soils that have a slope gradient of only 1 or 2 percent; 

however, it is generally more of an issue as slope gradient increases. 

Rill erosion is the removal of soil as concentrated runoff cuts many 

small, but conspicuous, channels. It is intermediate in degree between 

sheet and gully erosion. The channels are shallow enough that they are 

easily obliterated by tillage. After an eroded field has been cultivated, 

determining whether soil losses resulted from sheet or rill erosion is 

generally impossible.

Gully erosion is the removal of soil by water along the line of flow. 

Gullies form in exposed natural drainageways, in plow furrows, in 

animal trails, in vehicle ruts, between rows of crop plants, and below 

broken human-made terraces. Unlike rills, they cannot be obliterated by 

ordinary tillage. Deep gullies cannot be crossed with common types of 

farm equipment. 

Gullies and gully patterns vary widely. V-shaped gullies form in 

material that is equally or increasingly resistant to erosion with increasing 

depth. U-shaped gullies form in material that is equally or decreasingly 

resistant to erosion with depth. As the substratum is washed away, the 

overlying material loses its support, falls into the gully, and is also 

washed away. Most U-shaped gullies become modified toward a V shape 

once the channel stabilizes and the banks begin to crumble and slump. 

The maximum depth to which gullies are cut is governed by resistant 

layers in the soil, by bedrock, or by the local base level. Many gullies 

develop headward, i.e., they extend up the slope as the gully deepens in 

the lower part.

Tunnel erosion may occur in soils with subsurface horizons or layers 

that are more subject to entrainment than the surface horizon or layer. 

Through ponded infiltration, the free water enters into the soil’s surfaceconnected macropores. Desiccation cracks and rodent burrows are 

examples of macropores that may initiate the process. The soil material 

incorporated into the moving water travels downward within the soil 

profile, and if there is an outlet, may move out of it completely. As a 

result, tunnels (also referred to as pipes) form, enlarge, and coalesce. 

The portion of the tunnel near the inlet may enlarge disproportionately 

to form a funnel-shaped feature, commonly referred to as a “jug.” This 

phenomenon is called “piping” or “jugging” and occurs especially in 

areas with appreciable amounts of exchangeable sodium.

Sediment carried by water typically is deposited wherever the water’s 

velocity slows, such as at the mouth of gullies, at the base of slopes, 

along streambanks, on alluvial plains, in reservoirs, and at the mouth of 

streams. Water moving rapidly can deposit stones. As it slows, it deposits 

cobbles, followed by gravel, sand, and finally silt and clay. The slope 
74 Chapter 2

length for sediment transport is the distance from the highest point on the 

slope where runoff may start to where the sediment in the runoff would 

be deposited.

Estimating the Degree of Erosion

Soil examinations can estimate the degree to which accelerated 

erosion has modified the soil. However, estimating the amount of surface 

soil that is no longer present can be very difficult. This is generally most 

feasible if sufficient areas of the soil are known to be little affected 

by past accelerated erosion and can be used for comparison studies. 

The recognition of eroded and uneroded phases of a soil is useful if 

at least some soil properties making up the eroded phase are different 

enough from those of the uneroded phase to impact the soil’s use and 

management. The eroded soil is identified and classified on the basis of 

the properties of the soil that remains and not on what was presumed 

to have been present in the past. In some cases, the eroded soil may 

classify differently from the uneroded soil. An estimate of the soil lost 

is described. Eroded soils are defined so that the boundaries on the soil 

maps separate soil areas with different use suitabilities and different 

management needs.

The depth to a reference horizon or soil characteristic in areas under 

a use that has minimized erosion are compared to the same properties 

in areas under uses that have accelerated erosion. For example, a soil 

that supports native grass or large trees with no evidence of cultivation 

could be compared with the same or similar soil that has been cleared 

and cultivated for a relatively long time. The depth to reference layers is 

measured from the top of the mineral soil because cultivation destroys 

organic horizons at the surface.

The depth to a reference layer must be interpreted in terms of recent 

soil use or history. The upper parts of many forested soils have roots that 

make up as much as one-half of the soil volume. When these roots decay, 

the soil settles. Removal of rock fragments can also lower the surface. 

Cultivation may cause differences in thickness of layers. The thickness 

of surficial zones that have been bulked by tillage should be adjusted 

downward to what they would be under natural conditions.

The thickness of a plowed layer cannot be used as a standard for 

either losses or additions of material because, as a soil erodes, the plow 

cuts progressively deeper. Nor can the thickness of the uncultivated 

and uneroded A horizon be used as a standard for all cultivated soils, 

unless the A horizon is much thicker than the plow layer. If the horizon 
Soil Survey Manual 75

immediately below the plowed layer of an uneroded soil is distinctly 

higher in clay than the A horizon, the plow layer becomes progressively 

more clayey under continued cultivation as erosion progresses. In 

this case, the texture of the plow layer can be a criterion of erosion. 

Comparisons must be made on comparable slopes. Near the upper limit 

of a soil’s range of slope gradient, horizons may normally be thinner 

than near the lower limit.

Roadsides, cemeteries, fence rows, and similar uncultivated areas that 

make up a small part of the landscape or were subject to unusual cultural 

histories must be used cautiously for setting standards. In these areas, the 

reference standards for surface layer thickness are generally set too high. 

In naturally treeless areas or in areas cleared of trees, dust may collect 

in fence rows, along roadsides, and in other small uncultivated areas that 

are covered with grass or other stabilizing plants. This accumulated dust 

may cause the surface horizon to become several centimeters thicker in 

a short time.

For soils having clearly defined horizons, differences due to 

erosion can be accurately determined by comparison to undisturbed or 

uncultivated sites. Guidelines for estimating erosion for soils with a thin 

A horizon and little or no other horizon are more difficult to establish. 

After the thin surface layer is gone or has been mixed with underlying 

material, few clues remain for estimating the degree of erosion. One must 

rely on the physical conditions of the material in the plowed layer, the 

appearance and amount of rock fragments on the surface, the number and 

shape of gullies, and similar evidence. For many soils having almost no 

horizon expression, attempting to estimate the degree of erosion serves 

no useful purpose.

Precise estimates of the amount of soil lost from a site based on 

comparison studies with a similar uneroded site are complicated by 

several factors. The goal is to establish map unit concepts that reflect 

the relative degrees of soil loss between eroded phases of a soil and that 

result in some significant differences in the use and management of the 

soils based on their current properties.

Degree Classes for Accelerated Erosion

The degree classes for accelerated erosion discussed below and 

listed in table 2-12 apply to both water and wind erosion. They are 

not applicable to landslip or tunnel erosion. The classes pertain to the 

proportion of upper horizons that has been removed. These horizons may 

range widely in thickness; therefore, the absolute amount of erosion is 

not specified. 
76 Chapter 2

Table 2-12

Degree Classes for Accelerated Soil Erosion 

Degree class

Criteria: Estimated % loss of the original 

combined A + E horizons, or the estimated 

loss of the upper 20 cm (if original, combined 

A + E horizons are < 20 cm thick)

None 0 %

 1 > 0 to 25 %

 2 25 to 75 % 

 3 75 to 100 %

 4 > 75 % and total removal of the A horizon

Class 1.—This class consists of soils that have lost some, but on 

average less than 25 percent, of the original A and/or E horizons or of 

the uppermost 20 cm if the original A and/or E horizons were less than 

20 cm thick. Throughout most of the area, the thickness of the surface 

layer is within the normal range of variability of the uneroded soil. Less 

than 20 percent may consist of scattered small areas with a significantly 

modified surface layer.

Evidence for class 1 erosion includes: (1) a few rills, (2) accumulation 

of sediment at the base of slopes or in depressions, (3) scattered small 

areas where the plow layer contains material from below, and (4) 

evidence of the formation of widely spaced, deep rills or shallow gullies 

without consistently measurable reduction in thickness or other change 

in soil properties between the rills or gullies. 

Class 2.—This class consists of soils that have lost, on average, 25 

to 75 percent of the original A and/or E horizons or of the uppermost 

20 cm if the original A and/or E horizons were less than 20 cm thick. 

Throughout most cultivated areas of class 2 erosion, the surface layer 

consists of a mixture of the original A and/or E horizons and material from 

below. Some areas may have intricate patterns, ranging from somewhat 

over-thickened surface layers where sediment has accumulated locally to 

small areas of uneroded soils on gentle slopes or severely eroded soils on 

steeper, convex slopes. Where the original A and/or E horizons were very 

thick, little or no mixing of underlying material may have taken place. 

Class 3.—This class consists of soils that have lost, on average, 75 

percent or more of the original A and/or E horizons or of the uppermost 

20 cm if the original A and/or E horizons were less than 20 cm thick. In 

most areas, material below the original A and/or E horizons is exposed 
Soil Survey Manual 77

at the surface, especially in convex positions in cultivated areas; the 

plow layer consists entirely or largely of this material. Even where the 

original A and/or E horizons were very thick, at least some mixing with 

underlying material generally took place. Despite the generally universal 

loss of surface soil, some areas exhibit intricate patterns, ranging from 

somewhat over-thickened surface layers where sediment has accumulated 

locally to small areas of only slightly eroded soils, generally where slopes 

are relatively gentle. 

Class 4.—This class consists of soils that have lost all of the original 

A and/or E horizons or the uppermost 20 cm if the original A and/or E 

horizons were less than 20 cm thick. In most areas, some or all of the 

deeper horizons have been removed throughout the majority of the area. 

The original soil can be identified only in small areas. Some areas may 

be smooth, but most have an intricate pattern of gullies. 

Land Cover

The type of land cover around the site where a soil is described should 

be recorded. As with other descriptive terms, it is best to use standard terms 

consistently. The NCSS has adopted a set of general terms that includes 

land cover kinds, such as artificial cover, barren land, crop cover, shrub 

cover, grass/herbaceous cover, tree cover, and water. Subtypes within 

these general classes are also recorded. See Schoeneberger et al. (2012) 

for the land cover types and subtypes used. 

In addition to recording the overall land cover condition at the site, 

more detailed analysis can be done to provide quantitative estimates of 

the surface cover at the site. The ground surface of most soils is covered 

by vegetation to some extent at least part of the year. In addition, rock 

fragments form part of the mineral material at the surface of many soils. 

The vegetal material that is not part of the surface horizon and the rock 

fragments together form the ground surface cover. The proportion of 

cover, along with its characteristics, is very important in determining a 

soil’s thermal properties and resistance to erosion.

At one extreme, estimation of cover can be made visually without 

quantitative measurement. At the other extreme, transect techniques 

can be used to make an almost complete modal analyses of the ground 

surface. If the ground surface is relatively permanent, more effort in 

documentation is justified. In many cases, a combination of rapid visual 

estimates and transect techniques is appropriate.

The ground surface may be divided into fine earth and material other 

than fine earth. The latter consists of rock fragments and both live and dead 
78 Chapter 2

vegetation. Vegetation is separated into canopy and noncanopy (litter). 

A canopy component has a relatively large cross-sectional area capable 

of intercepting rainfall compared to the area near enough to the ground 

surface to affect overland water flow. When determining susceptibility to 

erosion, both canopy and noncanopy vegetation are considered.

The first step in evaluation is determining the components (typically 

one to three) of the ground surface cover. A common three-component 

land surface consists of trees, bushes, and areas between the two. The 

areal proportion of each component must be established, such as by 

transect. If a canopy component is present, the area within the tree 

drip line (edge of where water drips from trees and onto the ground) 

is determined as a percent of the ground surface. For each canopy 

component, the effectiveness must be established. Effectiveness is the 

percent of vertical raindrops that would be intercepted. The canopy 

effectiveness is typically estimated visually, but a spherical densitometer 

may be used. In addition to the canopy effectiveness, the mulch must be 

identified for each component.

Transect techniques may be used to determine the mulch percentage. 

The mulch can be subdivided into rock fragments and vegetation. 

From the areal proportions of the components and their respective 

canopy efficiencies and mulch percentages, the soil-loss ratio may be 

computed for the whole land surface (Wischmeier and Smith, 1978). 

Other observations may include the percent of kinds of plants, size of 

rock fragments, amount of green leaf area, and aspects of color of the 

immediate surface that may affect absorption of radiant energy in an area.

Vegetation

It is important to evaluate and record details about the vegetative 

community of a site, particularly in non-agricultural settings, such as 

rangeland, marshes, and forests. Plants reflect the integrated effects of 

controlling water dynamics, climate, native fertility, human intervention, 

and other factors upon the soil. Baseline vegetation information centers 

on the plant species present and the extent of the area that they cover.

Typically, the dominant plant species present are identified and 

documented in descending order of prominence. The scientific name is 

used along with, or in lieu of, the common name. Common plant names 

are not preferred as they may not be unique. A species may be known 

by multiple common names in a region, depending upon local cultures 

and languages spoken. The appropriate scientific plant symbol (USDA-
Soil Survey Manual 79

NRCS, 2016c) is also recorded, for example, ANGE (Andropogon 

gerardii, or big bluestem). The amount of ground covered by each plant 

species recorded at the site is also estimated or measured.

Ecological Sites

Soils and natural vegetative communities are generally closely 

related. For this reason, soil survey efforts commonly include the 

correlation of soil map unit components with ecological site information 

for an integrated natural resource inventory. In the United States, the 

soils are formally correlated to ecological sites (USDA-NRCS, 2016a).

An ecological site is a conceptual division of the landscape. It is 

defined as a distinctive kind of land based on recurring soil, landform, 

geological, and climate characteristics that differs from other kinds of 

land in its ability to produce distinctive kinds and amounts of vegetation 

and in its ability to respond similarly to management actions and 

natural disturbances. Ecological sites combine soils, climate, landform, 

vegetation, and hydrology into groupings subject to similar management 

and with similar response to disturbance. Ecological site descriptions 

provide characterization information for each site, state-and-transition 

models (USDA-NRCS, 2016a) that depict vegetation dynamics, and 

information on use and management. Appendix 4 discusses ecological 

site assessments.

In determining soil types and ecological sites, the vegetation that was 

on site during soil formation is very important. Use of soil information 

for descriptions of ecological dynamics, state-and-transition models, and 

management recommendations depends upon the best characterization 

of the vegetation community, including its history and current potential. 

Soil and vegetation (historic and potential) are the primary criteria for 

grouping ecosystems or ecosites at finer scales (USDA-FS, 2005). Existing 

vegetation does not always reflect historic or potential vegetation.

Soil-ecological site correlation establishes the relationship between 

soil components and ecological sites. Ecological sites are correlated on 

the basis of soils and the resulting differences in species composition, 

proportion of species, and total production of the historic climax 

plant community. In some cases, it is necessary to extrapolate data on 

the composition and production of a plant community for one soil to 

describe the plant community on a similar soil for which no data are 

available. The separation of two distinct soil taxonomic units does not 

necessarily delineate two ecological sites. Likewise, some soil taxonomic 
80 Chapter 2

units occur over broad environmental gradients and may support more 

than one distinctive historic climax plant community. Changes in plant 

communities may be due to other influences, such as an increase or 

decrease in average annual precipitation.

Integrated Natural Resource Inventories

Integrated natural resource inventories incorporate several data 

elements, commonly at a variety of scales and with varying objectives. 

They typically use soil information as a key data element. Soil data, 

including maps, commonly provide a basis for spatial identification of 

combinations of features to define sites. Soil properties derived from soil 

survey data are grouped spatially and conceptually in logical units based 

on similarities in vegetative communities and in response to use and 

management. The concept of “site” has been used for many decades and 

includes a combination of several biotic and abiotic attributes. Ecological 

sites provide a conceptual framework in which data can be integrated for 

use by various agencies of the U.S. Government and other entities.

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