Front Cover. Earth from space. Photographed by Apollo 16. 

Back cover. View of Earth across Moon. Photographed by Apollo 11 . Quotation from Jacks and Whyte, 1939. 

Frontispiece: Dust plume rises to an elevation of 1,500 m above the San Joaquin Valley southeast of 

Bakersfield, Calif, in December 1977 windstorm. Dust streams from canyons in the foothills of the Tehachapi 

Mountains (lower left). All anemometers in the southern San Joaquin Valley failed, but pre-failure records 

indicated wind velocities to 312 km per hour. Powerline towers were uprooted or toppled; orchards, 

vineyards, and vegetable crops were destroyed; fence posts were cut in two by wind-driven sand; and large 

areas of the valley-bounding hills were denuded, leading to massive water erosion and debris flows in 

subsequent rainstorms. Dust from the southern part of the valley, carrying the valley-fever fungus, was blown 

as much as 650 km to the north and caused an epidemic of valley fever. Photograph by Sam Chase. 
Geologic Processes at the Land Surface 

Geologic Processes 

at the Land Surface 

U.S. GEOLOGICAL SURVEY BULLETIN 2149 

By Howard G. Wilshire, Keith A. Howard, 

Carl M. Wentworth, and Helen Gibbons 
Conversion Factors 

For readers who wish to convert measurements from the metric system of 

units to the inch-pound system, conversion factors are listed below. 

Multiply By To obtain 

millimeters 0.039 inches 

centimeters 0.394 inches 

meters 3.281 feet 

kilometers 0.621 miles 

square meters 10.764 square feet 

square kilometers 0.386 square miles 

hectares 2.471 acres 

U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For sale by 

U.S. Geological Survey, Information Services 

Box 25286, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government 

Text and illustrations edited by Helen Gibbons 

Library of Congress Cataloging-in-Publication Data 

Geologic processes at the land surface I by Howard G. Wilshire ... [et al.]. 

p. cin.-(U.S. Geological Survey bulletin; 2149) 

Includes bibliographical references (p. - ). 

Supt. of Docs. no.: I 19.3:2149 

I. Geomorphology. I. Wilshire, Howard Gordon, 1926-

11. Series. 

QE75.B9 no. 2149 

[GB54.5] 

557.3 s-dc20 

[551.3] 95-41143 

CIP 
Preface 

S cientific discoveries in recent years have properly emphasized the fragility and global interconnectedness of our 

environment. There is growing awareness of the serious global consequences of environmental disturbances, 

both from natural causes such as El Nifio events, volcanic eruptions, hurricanes, catastrophic earthquakes, and meteorite impacts, and from human causes such as acid rain, greenhouse warming, depletion of atmospheric ozone, extinction of living species, and pollution of soil, water, and air. We urgently need to better understand the global environment in order to develop national and international policies that prevent or accommodate environmental changes. The 

scientific community elsewhere is responding to this need by organizing and expanding research efforts under the 

sponsorship of the International Council of Scientific Unions' International Geosphere-Biosphere Programme, the 

World Meteorological Organization's World Climate Research Programme, and the U.S. Global Change Research 

Program. 

Improving our knowledge of the land surface and how it responds to change is of great importance in these efforts. 

Despite our total dependence on the land, we are endangering its richness by practices that cause such harms as 

irretrievable soil loss, degradation of forest and arid-land ecosystems, and pollution of soil and water with contaminants that are entering and damaging the food chain. This report focuses attention on the societal benefits and scientific opport nities of comprehensive study by the U.S. Geological Survey of the geologic processes operating at the 

land surfac . It outlines directions for research that will provide the basis to help rectify a dangerous neglect of our 

land resour es. 
Geologic Processes at the Land Surface 

Introduction.................................................................................................... 1 

Land and People at Risk ................................................................................ 2 

Research Can Help ....................................................................................... 14 

Processes: Principles, Rates, and Sensitivity to Change .............................. 16 

Knowledge Gaps ..................................................................................... 17 

Thresholds and Irreversible Change ........................................................ 21 

Techniques ................................................................................................... 24 

Knowledge Gained from Extreme Events ............................................... 25 

Long-Term Monitoring ............................................................................ 27 

Large-Scale Experimentation .................................................................. 28 

Dating Geologically Young Events ......................................................... 28 

Cartographic and Satellite-Based Aids .................................................... 30 

Conclusion ................................................................................................... 32 

Acknowledgments ....................................................................................... 33 

References Cited .......................................................................................... 33 

Glossary ....................................................................................................... 40 

Special Topics 

Landslides-An Example in Southern California .......................................... 2 

Subsidence in Parts of California ................................................................... 5 

Oil Field Brines .............................................................................................. 7 

Selenium Pollution in the San Joaquin Valley, California .............................. 8 

Loess-A Disappearing Resource ............................................................... 15 

Rainfall Thresholds for Debris Flows .......................................................... 22 
INTRODUCTION 

Natural processes operating at the land surface both 

make human life possible and also pose threats to that 

life. Those processes include erosion, deposition, soil formation, and water flow. Some of the hazards they produce take place rapidly, such as landslides, dust storms, 

and floods; others may be gradual events, such as topsoil 

loss, changes in ecosystems, and contamination of water 

and soil. 

Land surface processes have long attracted scientific 

study, but only in this century have efforts begun to quantify and synthesize knowledge of these processes. Our 

understanding of the earth's land surface could ripen into 

the kind of scientific revolution that impelled new synthesis of biological observations toward the theory of evolution in the 19th century, and the synthesis of geological 

observations that led to the theory of plate tectonics in 

the middle of this century. The intellectual and technical 

base now exists to support real progress in comprehending the fundamental nature of the various surface processes, their rates and effects, and their responses to natural 

and human-caused changes. 

Society's need for improved knowledge of surface processes is great, for shortsighted human use of land resources is increasingly threatening the long-term welfare 

and even the survival of humans in many parts of the 

world. 

Geologic Processes at the Land Surface 1 
LAND AND PEOPLE AT RISK 

Land-surface hazards cause major losses of life and 

property. Landslides cause property damage of billions 

of dollars and loss of thousands of lives each year throughout the world. The United States alone has perhaps 20 

million landslide deposits formed in the recent geologic 

past (Brabb, 1991 ). As populations expand into vulnerable areas, more people become at risk (see box below). 

Many hazards are created directly by humans who either do not understand or choose to disregard surface processes. Catastrophic failure of the Baldwin Hills Reservoir in southern California is a case in point (California 

Department of Water Resources, 1964 ). Between the hours 

of 12 noon and 4 p.m. on December 14, 1963, the dam 

failed, causing the loss of four lives and millions of dolLANDSLIDES-An Example in Southern California 

2 

Landslides are major causes of property damage and loss 

of life throughout the world (Brabb, 1991 ). A graphic 

example of landslide risk occurred March 4, 1995, in the 

coastal community of La Conchita, California. 

The community of La Conchita ('The Little Seashell") lies 

on a narrow strip of low land along the Pacific coast of 

Ventura County, Calif., wedged between U.S. Highway 101 

and the Southern Pacific railroad on the ocean side, and the 

base of steep, wave-cut cliffs that form the base of Rincon 

Mountain on the landward side. The unique geologic and 

geographic settings of this area combine to make it picturesque and scientifically interesting-but also susceptible to 

several natural hazards. 

Geologic mapping of the area by U.S. Geological Survey, 

California Division of Mines and Geology, and oil industry 

34° 119°27'30" 

geologists over several decades has revealed the geologic 

history and identified the natural hazards to life and property in 

this area (Weber and others, 1973; Sarna-Wojcicki and others, 

1987a). The mapping provides a basis for understanding the 

relationship between geologic processes and natural hazards. 

The earth's crust in the western Transverse Ranges, where 

the community of La Conchita is located, is undergoing rapid 

deformation as the Pacific crustal plate moves northwest 

relative to the North American plate and meets the obstacle of a 

large, westerly bend in the San Andreas fault zone (the 

boundary between these plates; Wentworth and Yerkes, 1971; 

Morton and Yerkes, 1987). As a consequence, the earth's crust 

in this region is being compressed and thickened, and in places 

forced upward. Geologically young sediments once deposited 

on the deep ocean floor are now being squeezed like putty in a 

22'~~~~----~~------~--------~------------~~------------~ 

30 / ,./ 

Location of La Conchita 

in relation to landslides, 

active faults, and marine 

terraces. 

EXPLANATION 

D Landslide-Arrows show 

direction of movement 

D Holocene terrace 

(1 ,800-5,800 years 

before present) 

D Quaternary terrace-Green 

line marks sand at bottom 

of unit ( 40,000-60,000 

years before present) 

D Quaternary andTertiary 

bedrock, deformed 

- Main fau lt 

--- Subsidiary faults 

Land and People at Risk 

N 

r 

1 km 

, / ,',.-,,J, , .... -, 

I , 

I 

I 

, 

, 

I 

, 

I 

,./ 

1 

/ 

/ ,-

/ 

I 

I 

I 

Rincon 

Mountain £:::,. / 

/ 

,."' / _____ ..... 

/ __ .,... 

I 

':.-

1 

I 
lars in property damage; loss of life would undoubtedly 

have been much greater had the failure occurred during 

the night when the caretaker who observed the developing problem was off duty. The failure was caused by subsurface erosion along a fault that formed in response to 

land subsidence, itself caused by extraction of oil from 

the nearby Inglewood oil field (Castle and Yerkes, 1976). 

Other problems that may arise from withdrawal of subsurface fluids include collapse of pore spaces in aquifers 

and permanent loss of subsurface storage space (see box, 

vise and pushed up above sea level. In places north of the city 

of Ventura. the land is rising as much as 15 mm per year 

(Yerkes and others, 1987). At La Conchita, the rate is about 3 

mm per year. These young sediments and rocks are poorly 

consolidated and very susceptible to erosion and downslope 

movement in the form of landslides and soil flows from the 

cliffs and mountain slopes above La Conchita, and floods, 

mudflows, and debris flows in the gullies north and south of the 

community. 

Several large, old landslides were mapped in the 1970's in 

the cliffs directly behind La Conchita; some smaller, more 

page 5). The widespread subsidence problem in the United 

States (fig. 1, page 6) is but a small part of the global 

problem (Johnson, 1991). 

Less visible but equally hazardous is the pollution of 

soil and water that results from a broad range of human 

activities. Problems of toxic pollution have become widely 

acknowledged in the last decade and today are suspected 

to be causes of health and congenital disorders (Devinny 

and others, 1990). Health problems in children living near 

the Love Canal in New York have been related to wellrecent landslides are nested within the older (see map, p. 2; 

Weber and others, 1973; Sarna-Wojcicki and others, 1987a). 

As a result of heavy rains in January and March of 1995, 

some of these landslides were reactivated, destroying nine 

homes near the base of the cliff {see photographs). The new 

slides now threaten to destroy additional homes. Highway 

101 and the Southern Pacific railroad were closed down 

numerous times in this area by earlier landslides and debris 

flows before La Conchita was developed in the 1920's. 

During the last several hundred thousand years, as this area 

was being uplifted above the ocean, the ocean itself was 

undergoing cyclic rises and falls in level that were climatically controlled. The ocean level fell during glacial periods, 

when ice was stored on land in glaciers, and rose during 

warm periods, when the glaciers melted and their water 

returned to the oceans. Over time, the uplift of the earth's 

crust in this region has been more rapid and continuous than 

the sea-level oscillations. As the land rose, the ocean carved 

notches into the rising Rincon Mountain during sea~ level 

highstands. The notches show up as more-or-less flat 

topographic benches, called terraces, at different elevations 

on the mountain. The oldest terraces are high up on the 

mountain and have been much modified by erosion. The 

youngest, most recently emerged terrace is the low, flat bench 

on which La Conchita is built Although a thin veneer of 

young alluvium and debris flows have covered the flat, 

beveled platform cut by the surf, the general shape of this 

terrace is still easily seen. Shells of clams and snails found in 

uplifted beach sands on this terrace yield radiocarbon ages of 

between about 1,800 and 5,800 years before the present 

(Lajoie and others, 1982). 

(Continued on next page) 

Oblique aerial view of the main landslide, slwwing lwuses engulfed along the 

lf]e of the slide. light streaks fonning a partial "V" on the face of the slide are 

broken remnants .of the road that crosses the area diagonally from left to right. 

Small $/ide to left is one of four thai broke out from topographic benches on 

the cliff face. Plwtograph on next page is at the right side of tlie toe of the 

slide. Aerial plwtograph. March 6, 1995, by AP/Wide \-lbrld Plwtos. 

Geologic Processes at the Land Surface 3 
documented pollution of soil by hazardous chemical waste 

(Goldman and others, 1985). Examples of great concern 

are coming to light as the industrial centers of Eastern 

Europe open up to world view. Uranium mining in the 

former East Germany has left about 500 million tons of 

chemical and radioactive waste spread over more than 

1,200 km2 of land. A single lake in this area contains about 

22,500 tons of arsenic in solution in water and in sediments at the lake bottom (Kahn, 1993). 

4 

Solutions to problems of soil and water pollution arisLANDSLIDES, continued 

Much of the community of La Conchita lies at elevations 

of between 6 and 12 m above mean sea level, elevations 

usually above the reach of high tides and winter storms. 

However, the community is vulnerable to the surf from 

exceptionally large storms and tsunamis, and these processes 

carved the steep cliffs behind the community, oversteepening 

the slopes and making them prone to landsliding. 

Above the level of the cliffs, at an elevation of about 120 to 

150 m, is another, broader bench that represents an older 

marine terrace. The base of this marine terrace (shown as a 

green line in map on p. 2) is again recognized by the 

presence of a beveled, wave-cut platform that was cut into 

the deformed marine sediments. A beach sand lies on top of 

this platform and contains many fossil mollusk shells. Ages 

obtained on these shells by amino-acid racemization and 

radiocarbon analyses indicate that this terrace is between 

about 40,000 and 60,000 years old (Lajoie and others, 1982). 

This terrace is also backed by old seacliffs and covered by 

younger alluvium and mudflows. The old sea cliffs are less 

steep and are partly buried, having been modified by erosion. 

The elevation of this terrace increases to the southeast to 

levels as high as 210m, indicating that uplift is greater in that 

direction. 

Deformed, clayey, relatively impermeable Quaternary 

bedrock that underlies the beveled marine terrace is overlain 

by well-sorted marine sand, which is in tum overlain by 

alluvium and mudflow deposits. This sequence of layers 

makes for an unstable combination that is particularly 

susceptible to landsliding when the permeable sediments are 

water saturated. 

Faults just inland from La Conchita (see map, p. 2) appear 

to be active (Sarna-Wojcicki and others, 1979, 1987a; Yeats 

and others, 1987). Thus, there is a potential of ground 

shaking and ground rupture in the vicinity and at La 

Conchita. The faulted bedrock, marine terrace deposits, and 

old landslides above La Conchita represent a nexus of 

geologic circumstances that favor rapid downslope mass 

movements in the future. Ground rupture, ground shaking, 

Land and People at Risk 

ing from mineral- and energy-resource extraction are 

likely to be expensive. For example, cleanup of contamination caused by a single abandoned gold mine in Colorado (Summitville Mine) is estimated to cost $100 to $200 

million (King, 1995). The magnitude of the problem can 

be appreciated by the fact that there are between 100,000 

and 500,000 abandoned or inactive mine sites in the 

United States alone. 

Low-quality deep water extracted during oil and gas 

production can present a serious disposal problem. In 1993 

and landslides can not only affect the houses in the community, but can also block the highway and railroad, and sever 

offshore and onshore oil pipelines, causing oil spills. Pacific 

Operators Offshore, Inc., which operates a crude-oil pipeline 

along the road above La Conchita, was forewarned of 

problems by the appearance of cracks in the road in late 

August 1994. The company took an elaborate series of 

precautions that averted a spill when the slide ruptured the 

pipeline. 

The lessons of La Conchita underscore the importance of 

geologic studies as elements of land-use planning. 

House on left was pushed onto the 

house at right by the slide. 

Photograph by Howard Wilshire. 

Prepared by Andrei M. Sarna-Wojcicki 
SUBSIDENCE IN PARTS OF CALIFORNIA 

¢:= The magnitude of subsidence caused by groundwater withdrawal 

during the period 1925-1977 at bench mark S661, near the center of 

greatest subsidence in the San Joaquin Valley (see Johnson, 1991) is 

illustrated by J.F. Poland, U.S. Geological Survey, who stands alongside 

a powerline pole. The approximate position of the land surface in 1925, 

1955, and 1977 is shown. The land surface subsided about 9 mfrom 

1925 to 1977. Photograph courtesy of Ivan Johnson. 

Subsidence caused by withdrawal of underground fluids, 

such as water or oil, is a widespread problem (Poland and 

Davis, 1969; Poland, 1984; Johnson, 1991) that may affect 

fluid circulation and sediment distribution at and below the 

earth's surface. Extraction of fluids and concomitant 

depressurization of the underground reservoirs has produced 

significant surface effects in such places as Wtlmington-Long 

Beach Harbor, Calif.; Venice, Italy; and Tokyo, Japan. 

Subsidence produced by extraction of oil from the Wtlmington 

oil field caused encroachment of the sea in the Wilmington-Long 

Beach Harbor area. Protection of the utban infrastructure, including, ironically, the production facilities of the oil field itself, 

required construction of elaborate dikes and pumping facilities. 

The impact of subsidence over more than 14,000 km2 in 

the San Joaquin Valley of California (photograph, upper left) 

-- Displacement along surface ruptures 

caused by the subsidence illustrated in 

aerial photograph (below) led to 

subsurface erosion (piping) of the dam 

foundation and failure of the Baldwin 

Hills Reservoir dam. Photograph taken 

December 1963; from Los Angeles 

Department of Water and Power. 

An oblique, south-southeast aerial view 

across the Baldwin Hills, Calif., shows 

the approximate configuration of 

cumulative subsidence caused by oil 

withdrawal, 1911-1964 (Castle and 

Yerkes, 1976), centering on the 

Inglewood oilfield. Contours (light 

lines, dashed where approximate) show 

amount of subsidence in meters. Also 

shown are active surface ruptures 

(heavy lines) and the location of the 

Baldwin Hills Reservoir (crossed by 

lower set of surface ruptures). 

Photograph by Spence Air Photos, 

1952. 

has also been significant. The large drop (about 9 m) in 

ground level between 1925 and 1977 caused by pumping of 

groundwater for crop irrigation represents a largely permanent loss of storage capacity in the subsurface aquifers 

(Poland and others, 1975). In addition, millions of dollars 

have been spent on repair of water-well casings and water 

transportation systems ruptured by uneven subsidence. 

The area and amount of subsidence caused by extraction 

of oil in the Inglewood oil field, southern California, is 

shown in aerial photograph, above. Subsurface compaction 

that produced the subsidence led to the formation of several 

sets of surface ruptures along the east side of the zone of 

subsidence. One set of these fractures caused the failure of 

the Baldwin Hills Reservoir dam (photograph, upper right; 

Castle and Yerkes, 1976). 

Prepared by Robert Castle 

Geologic Processes at the Land Surface 5 
Area 

Savannah 

New Orleans 

Baton Rouge 

Houston-Galveston 

Luke 

Queen Creek 

Stanfield 

Eloy 

Las Vegas Valley 

Raft River 

Sacramento Valley 

Santa Clara Valley 

SUBSIDENCE, IN METERS 

0 2 4 6 8 10 

} Louisiana 

Texas 

1------'-------., 

t-------'-, 

t-------' 

Nevada 

Idaho 

t---'---------, 

Los Banos-Kettleman City t-------c____--6-,2-00------., 

Tulare-Wasco 1-----.-------' 

Arvin-Maricopa 1-------.------' Lancaster 

San JacintoValley 0~~~2 ____ _.4 _____ 6~--~8--~~10 

SUBSIDENCE, IN METERS 

Figure 1. The magnitude of land subsidence due to groundwater 

withdrawal in the United States. The numbers in or beside the shaded bar 

indicate the area in square kilometers; horizontal scale indicates average 

subsidence in meters. From Poland ( 1984). 

alone, an estimated 25 billion barrels of water were produced with 2.5 billion barrels of oil in the United States, 

or 10 barrels of water for each barrel of oil (Kharaka and 

Wanty, 1995). The water is commonly more saline than 

seawater and may contain toxic organic and inorganic 

(some radiogenic) contaminants, such as phenols, benzene, toluene, radium-226, radium-228, radon-222, and 

various metals including, in places, lead and zinc (Kharaka 

and Thordsen, 1992; Kharaka and Wanty, 1995). Release 

of these oil field brines on the ground surface has caused 

significant long-term damage to natural ecosystems (see 

box, right; Weathers and others, 1994; Kharaka and Wanty, 

1995). As conventional natural-gas supplies in the United 

States are depleted, pressure has increased to exploit vast 

disseminated-gas deposits in the United States-so-called 

continuous reservoirs (Scott, 1995). These deposits contain small quantities of gas dispersed through large volumes of rock, are more costly to exploit, and will require 

close-spaced drilling of many hundreds of thousands of 

wells, with a high potential for production of huge quantities of brines for which satisfactory disposal procedures 

must be found. 

Contamination of soil and water from nuclear sites is 

of particular public concern. Research on nuclear weapons at the Oak Ridge National Laboratory, Tennessee, resulted in massive chemical contamination of bottom sediments in a reservoir downstream. The problem was first 

6 Land and People at Risk 

identified by Oak Ridge National Laboratory and U.S. 

Geological Survey scientists who discovered very high 

levels of mercury and arsenic in samples of vegetation 

collected along streams downslope from the laboratory 

(Marshall, 1983). Subsequently, stream fish were discovered to have taken up the contaminants. Bottom sediments 

of the reservoir were found to h~ve high concentrations 

of such elements and compounds as mercury, arsenic, antimony, cesium, uranium, PCB 1254 and 1260, 

chlorodane, cobalt, and paladium (Hoffman and others, 

1990). 

The 1986 nuclear powerplant accident in Chernobyl, 

Ukraine, contaminated air, water, and soil, and subsequently plants and animals, over a huge area of central 

and northern Europe (Bowen, 1988). Enormous clouds 

of radioactive smoke drifted as far as the United Kingdom and Switzerland (Bowen, 1988). Dry fallout of radionuclides contaminated about 20,000 km2 around the 

site (Levi, 1991). About 135,000 people were evacuated 

from downwind localities in Ukraine, Belorus, and Russia. Compliance with western standards adopted for radiation exposure would not only prevent repopulation of 

the 30-km zone around the Chernobyl plant in the foreseeable future but would also require evacuation of another 20,000 to 30,000 people from a larger surrounding 

area of 10,000 km2 that remains hazardous (Levi, 1991). 

In western Europe rain washed the radionuclides out of 

the air, producing surface gamma-radiation levels as much 

as three times normal; the highest levels reported for rainwashout contamination were in Scotland (Bowen, 1988). 

Rain-contaminated soil was then eroded and further redistributed by river transport. Concentrations of cesium137 in the Severn River in England remained nearly an 

order of magnitude above background 2 years after the 

accident (Walling and others, 1992). 

The paths toxic substances take from source to food 

chain commonly are detected only after much damage 

has been done. The path that toxic selenium in California's 

San Joaquin Valley follows-from sedimentary rocks to 

alluvial soils, then into surface water and ultimately into 

algae, fish, waterfowl, and other parts of the food chain 

at the Kesterson Wildlife Refuge and other locationshas only recently been explored in detail (see box, page 

8) (Presser and Ohlendorf, 1987; Severson and others, 

1991; Presser, 1994). The selenium problem exemplifies 

how natural geologic processes and human actions combine to create hazards. The element selenium occurs naturally in marine sedimentary rocks in the California Coast 

Ranges and is naturally dissolved from these rocks and 

redeposited where water evaporates from the valley floor. · 

Irrigation and flushing of salts from irrigated land dis-
OIL FIELD BRINES 

TEXON 

SCAR..../r'• 

UNIVERSITY LANDS 

12 

I • 

• 

• ~--~PRODUCING 

ROAD 1675 

36 OIL WELL 

~ PRODUCING 

GAS WELL 

-<>- DRY HOLE 

(Top). Map of Big Lake oilfield, 

prepared by Michael L . 

Weathers. 

(Bottom). Salinity distribution 

of formation water with depth 

of reservoir rocks from several 

petroliferous basins in North 

America (from Kharaka and 

Wanty, 1995). 

• 

·"*" 0 MILES 1/2 

(/) 

a: 

w 

1-

~ 1 

g 

S2 

~ 

'.:£ 

(.) 

~ 2 

a: 

5 

> a: 

w 

(/) 

w 

a: 

u.. 3 

0 

I 

1-

a.. 

w 

0 

4 

0 100 200 300 400 500 

WATER SALINITY, IN GRAMS PER LITER 

The discovery of the Big Lake oil field in west Texas in 

1923 heralded the opening of the highly productive Pennian 

Basin (Fritz, 1994; Weathers and others, 1994). In the 

following decades, brines produced along with oil in the Big 

Lake field were released on the surface, a practice common 

until the early 1960's; the brines ultimately covered an area 

of about 28 km2 (see map). In all, about 200 million barrels 

of brine were discharged. Salts were concentrated by a 

shallow water table perched above a relatively impermeable 

carbonate-cemented layer (caliche) at a depth of about 2 112 

meters. In the dry climate of west Texas, capillary movement 

of water upward from the shallow water table deposited salts 

0 KILOMETERS 0.8 

in the near-surface sediments, creating conditions in which 

no plant life could exist. As the vegetation died and the soil 

was exposed, wind and water erosion removed all topsoil and 

subsoil sediment to depths as great as 1.2 m as shown by 

exposed mesquite stubs. Blinding dust storms from the 

barren surface created traffic hazards on U.S. Highway 67, 

which crosses the denuded area (see map). Earlier efforts to 

reclaim the area by application of fresh water and reconditioning and mulching the soils failed. In 1989, the University 

of Texas System, U.S. Soil Conservation Service, and 

Marathon Oil Co. began remediation efforts in a 1.3-km2 area 

along Highway 67 with a goal of restoring the land to 

rangeland productivity. Through 1993, these efforts involved 

draining of more than 13 million barrels of saltwater; 

installation of an underground drainage system consisting of 

38,000 m of conduit, sumps, and sump pumps; building of 

terraces; ripping of the land between terraces; and aerial 

seeding with barley. The seeding in 1991 was followed by 

above-average rainfall, yielding " ... a fairly good density of 

barley over about three-quarters of the site'' (Fritz, 1994). By 

1992, the native plant kochia had reestablished itself over 90 

percent of the site. 

Although the salinity of water associated with oil and gas 

fields is quite variable from basin to basin (see diagram), in 

most cases the water is too saline to release on the surface. 

The safe disposal of brines produced from oil and gas fields 

thus raises important environmental and economic questions. 

As we look to the future when many more wells will be 

required to produce disseminated natural gas than are required 

to produce the same amount of gas from today's conventional 

fields, the problem of brine disposal will increase in importance 

(Rice and others, 1995; Kharaka and Wanty, 1995). 

Geologic Processes at the Land Surface 7 
8 

SELENIUM POLLUTION IN THE SAN JOAQUIN VALLEY, CALIFORNIA 

THE SELENIUM CYCLE IN CALIFORNIA'S 

SAN JOAQUIN VALLEY 

AND THE 

KESTERSON NATIONAL WILDLIFE REFUGE 

California Aqueduct 

Evaporation and 

I Selenium in plants I 

~ ! ~ ~ Upper Aquifer ---11 Clay layer ---l~------------------------~~~Lo-- _A_ ui fu-r 

~~ 

Land and People at Risk 

Fresno 
The conditions favoring movement of the potentially 

toxic element selenium (Se) in the environment are 

especially well developed in California's San Joaquin 

Valley. Paths taken by Se are shown in the diagram at left 

(not to scale). An abundant supply of Se occurs naturally 

in marine sedimentary rocks of Cretaceous and Tertiary 

age exposed in the mountains bordering the valley on the 

west. Selenium is transported to the valley floor as 

dissolved selenate in saline streams and in particulate form 

by streams, landslides, and mud flows. Under natural 

conditions, theSe was evaporatively concentrated in 

saline zones in the valley where it remained because of 

poor drainage conditions. Subsequently the Se has been 

redissolved by irrigation water and flushed into artificial 

subsurface drainage systems. Until 1986, the San Luis 

Drain collected subsurface drainage and transported it to 

the Kesterson National Wildlife Refuge, where Se in the 

drainage water was taken up by organisms in the food 

chain. 

Mechanisms by which Se accumulates in plants and 

animals in the Kesterson refuge include chemical 

reduction and methylation reactions. Concentration of Se 

in organs of plants and animals higher in the food chain 

(biomagnification) leads to impaired reproduction, 

inhibition of growth, mutation, and suppression of the 

immune system (Presser and others, 1994; Presser, 1994). 

In the 10 years following discovery of the Kesterson 

problem, waterfowl deformities have been documented in 

Tulare Lake bed, California; Middle Green River Basin, 

Utah; Kendrick Reclamation Project Area, Wyoming; Sun 

River Basin, Montana; and Stillwater Wildlife Management Area, Nevada; and high selenium levels have been 

found in waterfowl in Malheur National Wildlife Refuge, 

Oregon; Salton Sea area, California; Riverton Reclamation 

Project area, Wyoming; Gunnison River Basin/Grand 

Valley Project, Colorado; Middle Arkansas River Basin, 

Colorado/Kansas; and Belle Fourche Reclamation Project 

area, South Dakota (Presser and others, 1994 ). 

Prepared by Theresa Presser, drawn by Bruce Rogers 

solves this selenium into groundwater, eventually leading to its concentrated introduction into the food chainselenium is taken up from the water in algae and causes 

gross deformities in the offspring of birds that feed on 

the algae. Were it not for agricultural irrigation, most of 

the selenium would remain in the saline zones where it is 

relatively harmless. Were it not for the selenium, irrigation would not have such a negative effect on the food 

chain. Despite our improved understanding of the interconnected processes, the increasing selenium contamination of the Kesterson Wildlife Refuge has remained a 

problem since it was discovered more than 10 years ago. 

In fact, a recent Federal court decision (U.S. District Court, 

1994) ruled that the U.S. Bureau of Reclamation must 

complete the San Luis Drain, which would carry the contaminated agricultural waters to the San Francisco Bay, 

or provide some other out-of-San Joaquin Valley solution. Disposal in the San Francisco Bay would add to 

existing problems of local industrial selenium contamination (Brown and Luoma, 1995). Selenium resulting 

from disposal of agricultural wastewater has been identified at levels known to cause deformities in waterfowl at 

five additional areas in the Western United States, and at 

levels near or exceeding thresholds for adverse reproductive effects at six more sites (Presser and others, 1994 ); 

none of these are yet being corrected. 

Diversion of water from one region to another provides 

further examples of land use with potentially adverse longterm consequences to public health and land resources. 

Since 1913 the Owens River in California has been diverted to supply water to the San Fernando Valley and 

Los Angeles far to the south. The diversion dried up 

Owens Lake, whose dry bed now yields as many as ten 

major dust storms per year (Saint-Amand and others, 

1986) and is the largest single source of dust in North 

America (Gill and Cahill, 1992a). The dust storms (fig. 2) 

can impact the health of about 50,000 people who live 

downwind in the frequently impacted areas (Gill and 

Cahill, 1992b ). Water diversion throughout five continents 

has had similar effects (Gill, 1994 ). The Aral Sea of central Asia has shrunk by more than 40 percent since 1960 

as a consequence of upstream water diversion. About 40 

million tons of alkaline dust is blown annually from the 

dry bed and deposited in the region from the Black Sea to 

the Arctic Ocean (Gill and Cahill, 1992b; Micklin, 1988; 

Ellis, 1990). 

Long-term changes in vegetation in arid and semiarid 

regions have been caused by agriculture, domestic-animal grazing, road-building, wood-gathering, and military 

and recreational activities in arid lands. These changes 

produce national and global desertification (fig. 3) when 

Geologic Processes at the Land Surface 9 
productivity of arid land declines or arid ecosystems show 

a long-term change such as the invasion of preexisting 

grasslands by shrubs (Schlesinger and others, 1990). Sixty 

percent of Australia's arid lands are in need of remedial 

actions to avoid or minimize irreversible damage from 

various human activities (Roberts, 1987). In North 

America, 28 percent of the arid land, nearly half of it in 

the United States, is severely degraded, and nearly 20 

percent of the earth's arid lands are similarly afflicted 

(Sheridan, 1981; Sabadell and others, 1982; Dregne, 1984; 

Mabbutt, 1984 ). 

One indicator of desertification is salinization, the 

buildup of salts in the soil caused by evaporation of 

irrigation water from poorly drained land. As the water evaporates, it leaves progressively more salts, derived both from the water and from fertilizers, at the 

surface. As salts accumulate in the soil, crops fail to 

-grow, and the lands, already cleared of their natural vegetation, are then abandoned to accelerated wind erosion. 

One solution to this problem has been over-irrigation to 

flush the salts off the land down drainage ditches; however, the drainage water is much saltier than the incoming irrigation water, causing downstream problems. Reduction of water quality in the Colorado River by flushing of salts from agricultural land in Arizona has required 

expensive remediation to satisfy treaty obligations of the 

United States to Mexico (Sheridan, 1981 ). Salinization 

is an important problem in irrigated lands in the southwestern United States (fig. 4) and elsewhere in the world 

(Barrow, 1991). 

Deterioration of forested lands is another long-term 

problem. Removal of timber and clearing of land for grazing and agricultural uses has long since stripped much of 

the world's easily accessible native forests in temperate 

climates. Now these enterprises have shifted to steeper 

terrain, which is highly sensitive to erosion, and to tropical rainforests, with negative consequences to streams and 

soils (fig. 5). In the 1980's, rainforests in ten undeveloped and developing nations were being stripped at an 

annual rate of more than 60,000 km2, an area nearly the 

size of West Virginia (Ryan, 1992). Between 1978 and 

1988, nearly 200,000 km2 of rainforest were stripped in 

the Amazon basin, causing reduction of biological diversity in an area greater than 580,000 km2 (Skole and Tucker, 

1993). In the U.S. Pacific Northwest, clearcutting of oldFigure 2. This sequence of photographs shows a dust storm as it 

developed on Owens dry lake, Calif., November 27, 1991 (Reid and 

others, 1994). The photos were taken looking westward from an 

automated station at Cerro Gordo in the 1nyo Mountains at an elevation 

of about 1,500 m above the dry lake floor. A, before the storm; 10:00 

a.m.; B, 11 :30 a.m.; C, 1:00 p.m.; D, 2:00p.m. Photographs by JeffS. 

Reid, Air Quality Group, University of California, Davis. 

10 Land and People at Risk 
150 

Desertification of 

Arid Lands 

(>?M Slight 

l::m:::rmii Moderate 

IIIII Severe 

- Very severe 

Figure 3. World map showing the degree of desertification-changes in productivity or vegetation communities due to human 

activities-in 1981. Note that extreme climatic deserts, such as the Sahara, are placed in category of "slight" desertification 

because of the sparseness of their human population and hence lightness of human impact (from Dregne, 1984 ). 

A 

Colorado River 

Basin 

0 500 KILOMETERS 

i 

N 

I 

Lower 

Rio Grande 

Figure 4. Salinization of irrigated farmland is one facet of the desertification problem. A. This map 

shows areas with major salinity problems in the Western United States (modified from EL-Ashry and 

others, 1985). B, Salt deposits form on agricultural Land in the southern San Joaquin Valley, Calif., 

where irrigation water with dissolved salts rises by capillary action to the ground surface and 

evaporates. Photograph by Howard Wilshire. 

Geologic Processes at the Land Surface 11 
Figure 5. Problems of deforestation. A, Deforestation and annual grass 

burnoffs in Madagascar s central plateau region result in massive erosion 

of the barren steep slopes; the scene here is north ofTananarive. The rivers 

are more prone to flooding because of accelerated runoff, and their heavy 

sediment loads cause siltation of coastal lowlands through which they pass 

(Jolly and others, 1984; Green and Sussman, 1990). Photograph by Frans 

Lanting!Minden Pictures. B, Pinyon-juniper forest destroyed by chaining 

for conversion to rangeland in Utah. In chaining, the tree cover is removed 

by stretching chain with hardened cutting edges between two bulldozers 

which traverse the forest. In steep terrain like this (east of Zion Canyon 

National Park, Utah), soil loss can be severe before grasses are established. 

Photograph by Howard Wilshire. C. Slash-and-burn deforestation in the 

upper Amazon basin in Ecuador. Stripping of vegetation for farming results 

in high erosion rates and rapid deterioration of the soil. Photograph by 

Howard Wilshire. 

growth forest in steep terrain results not only in rapid erosion but also in degradation of an additional land area, 

comparable to or larger than the clearcut area itself, from 

exposure of forest edges to wind, water pollution, downstream erosion and sedimentation, and microclimate 

changes (Janda, 1978; Council on Environmental Quality, 1981; Brown and others, 1991). New Zealand's lowland rainforests have been largely removed in the past 

100 years (Salmon, 1980), and the once prolific and varied forests have been replaced by rangeland and by monoculture farms of single imported tree species (fig. 6). 

Similar problems exist in the U.S. rainforests of Puerto 

Rico and Alaska, and in Canada, Australia, and other nations. 

Desertification, deforestation, and many agricultural 

practices contribute to serious national and global soil 

degradation. Nations have thrived or failed historically 

according to the ability of their land to sustain agricultural productivity (Carter and Dale, 1974; McCracken, 

1987). Moderate to extreme soil degradation globally affects nearly 3 billion acres, an area larger than China and 

India combined (fig. 7), and the global rate of topsoil loss 

is 7 percent per decade (Brown and Wolf, 1984 ). About 

30 million acres of arable land are destroyed and abandoned annually worldwide (Pimentel and others, 1995). 

Soil degradation and topsoil loss, in turn, exacerbate the 

global problem of food scarcity in the face of a growing 

population (Brown, 1981; Fyfe, 1989; Pimentel and others, 1995). In addition to loss of productivity, soil erosion 

also causes air and water pollution, with attendant health 

effects (Huszar and Piper, 1985). Pimentel and others 

( 1995) estimated the annual direct and indirect costs of 

soil erosion in the United States to be $44 billion, and the 

annual cost worldwide to be nearly $400 billion. 

Processes of land degradation are mostly cumulative. 

Because the rates of soil regeneration and natural cleansing of contaminated soil are slow, a comprehension of 

how to balance the rates of degradation with the rates of 

regeneration is crucial to human survival. We already have 

12 Land and People at Risk 
Figure 6. Clearcutting of lowland rainforest on North Island, New Zealand. A, Steep slopes and high rainfall have led to major 

problems of landsliding and soil erosion. B, Monoculture forest of imported pines replaces lowland rainforest. Trees are planted 

like row crops up-and-down the slopes. This configuration, combined with periodic harvesting, leads to heavy soil loss (average 

annual rainfall is about 250 em). Photographs by Howard Wilshire. 

EXPLANATION 

[:=J Areas of serious concern 

c=J Areas of some concern 

c=J Stable terrain 

[:=J Nonvegetated 

land 

I Areas of Concern for Soil Degradation I 

Figure 7. Global areas of human-induced soil degradation (International Soil 

Reference and Information Centre, Wageningen, The Netherlands, unpublished 

map, /990). Most of the degradation is caused by water and wind erosion resulting 

from agricultural activities, overgrazing, deforestation, and wood-gathering 

( Oldeman and others, 1991 ). Adapted from simplified version, p. 117, World 

Resources Institute ( 1992 ). 

extensive pollution of public lands in the United States 

from mine wastes, oil and gas production, landfills, and 

other sources (National Academy of Sciences, 1992). 

Even where problems of contaminated soil and water can 

be corrected, the cleanup costs are measured in billions 

of dollars (U.S. Congress, 1993; World Resources Institute, 1987; California Commission for Economic Development, 1985), and some lands and bodies of water have 

been so badly contaminated as to be unusable by humans 

for the foreseeable future, or to present overwhelming 

cleanup problems (U.S. Congress, 1991a,b). Indeed, in 

the case oflong-lived contaminants and irreplaceable soils 

(ones formed under climatic conditions that no longer 

exist), we may necessarily come to an appreciation that 

avoidance of degradation is the only viable alternative. 

Geologic Processes at the Land Surface 13 
RESEARCH CAN HELP 

For a growing populace to live safely on the land 

and to protect land resources requires improved knowledge of how the earth's surface works. We need to 

understand the nature and rates of the surface geologic 

processes themselves and the magnitudes, frequencies, 

and causes of the geologic events they govern. So 

armed, we can evaluate and to some extent forecast 

the likely geologic effects and interactions of natural 

and human-caused changes to the land surface. Appropriate actions to moderate negative effects can then 

be taken. 

To achieve a higher level of knowledge requires research into some basic questions. How do rates of soil 

regeneration compare with rates of erosion or with 

rates of chemical degradation caused by fertilizers and 

pesticides used in various farming and forestry practices? Given time, soil eroded away or chemically degraded by pesticides will restore itself, but an important question is whether the amount of time needed is 

really available in terms of human life expectancy. 

Moreover, a restored soil may not function in the same 

way or as well as the soil that was lost. For example, 

extensive areas of the Midwestern United States have 

a natural mulch formed by windblown dust (loess) 

derived from wind erosion of outwash plains of continental glaciers that no longer exist (see box). Loess 

soils are more productive than those derived from underlying rock materials. Once lost, they are gone forever . Such irreversible loss has already occurred in 

areas of thin loess cover. A blanket of loess, rarely 

more than half a meter thick, contributed substantially 

to the prosperity of early farming in New England but 

has largely been lost through erosion of the agricultural lands, exposing less fertile underlying gravels and 

sands (Kaye, 1976). 

Other questions that arise include: How are the natural functions of watersheds impaired by erosion, sedimentation, and chemical contamination of surface materials? What are the repeat intervals and durations of 

high water levels in inland lakes, and peak runoffs in 

major river systems, information that is essential for 

minimization of flood damage? What factors affect the 

long-term integrity of landfill and toxic-waste sites, 

and how and where should such sites be located? 

As answers come to these and other questions, we 

can gain a firmer and more comprehensive appreciation of surface processes, their rates, and their effects. 

Ultimately, the goal is the ability to model processes 

14 Research Can Help 

and make forecasts that help us manage our land and 

water resources prudently. It is well to bear in mind, 

however, that theoretical models of the complex processes operating at and near the earth's surface are 

based on idealized assumptions that must be continually reexamined. A modern trend toward mathematical formulation of surface processes, in the form of 

supposedly predictive models, has come increasingly 

into question, especially with regard to hydrologic processes (Oreskes and others, 1994; Konikow and 

Bredehoeft, 1992). The lesson is that idealized models can complement but not substitute for thorough 

evaluation of the geologic record (Baker, 1994 ). 
200 

km. 

~Dune Fields 

'):)::: · Loess 6 Sand Sheefs 

• Cities 

.;::; ;. Physiographic Boundaries 

Geologic Processes at the Land Surface 15 
PROCESSES: PRINCIPLES, RATES, 

AND SENSITIVITY TO CHANGE 

To improve our knowledge of surface processes, basic 

research is needed on a spectrum of topics. Information 

is needed on the mechanics and rates of the geologic processes of erosion, transport, and temporary storage of 

eroded materials on hill slopes and in stream courses, and 

of sedimentation in the long-term resting places of eroded 

materials, such as standing bodies of water and desert 

playas. Variations in these and other processes, particularly during the geologically recent Holocene Epoch (from 

10,000 years ago to the present) and late Pleistocene Epoch (from 1.7 million to 10,000 years ago) (fig. 8), need 

investigation. Strata formed during these times provide 

detailed records of an environment that was sometimes 

much like the one in which we now live and sometimes 

very different, showing major variations on a thousandyear scale. These records provide an opportunity to learn 

more about frequencies and magnitudes of environmental change and thresholds of change than can be gleaned 

from historical records alone (Koltermann and Gorelick, 

1992). Improved knowledge is needed of the mechanisms 

and rates of chemical and physical weathering and soil 

formation; of the processes of infiltration, runoff, evaporation, and evapotranspiration of water at the land surface; of the movement of shallow groundwater and its 

Eras Periods Epochs Millions of years 

before present Life forms 

Holocene 

QUATERNARY 0.01·--- Humans_{M 1---- 0~ Pleistocene 1.7 ---- ------------------------------------ ~0 Pliocene 

1--- ~a 5 ·---

~ Miocene 24 ·--- TERTIARY Oligocene 

~ 38 ·---

• basses become abundant 

Olo Eocene 55 ·--- Horses first appear % Paleocene 

~ 66 Flowering plants first appear Extinction of CRETACEOUS dinosaurs 

~ ---- 138 ----

~ JURASSIC Birds first appear 

~ ---- 205 ---- _.. ~ Mammals first appear TRIASSIC Many * .._, '() 240 Dinosaurs subdivisions first appear 

PERMIAN but not Conifers first appear 

differentiated ---- 290 ---- Reptiles first appear Oldest rocks dated 

3,800 million years 

PENNSYLVANIAN here Coal-forming swamps 

r---- 330 ----

MISSISSIPPIAN 

r---- 360 ·---- Amphibians first appear 

DEVONIAN ili r---- 410 ----

SILURIAN Land plants first appear 

r---- 435 ---- ... 1 . Vertebrates first ORDOVICIAN "'f~"'' appear (fish) f---- 500 ·----

' First abundant fossil record CAMBRIAN 

570 (marine invertebrates) 

Origin of earth Scanty fossil record 4-5,000 million years PRECAMBRIAN Primitive marine plants and invertebrates; • one-celled organisms 

Figure B. Geologic time scale. The earth is about 4.5 billion years old. The rock layers containing most of the evidence for 

development of life date from about 570 million years ago to the present, as illustrated in this diagram. These rock layers-like 

pages in a long and complicated history book-record events that have shaped the planet on which we live. The geologic record 

representing the Late Tertiary and Quaternary Periods, in particular the Pleistocene Epoch (about 1.7 million to 10,000 years 

ago) and the Holocene Epoch ( 10,000 years ago to the present) are both the most complete and the most relevant records on 

which to base predictions about our future. Modified from Molenaar (1987). 

16 Processes: Principles, Rates, and Sensitivity to Change 
interaction with soil particles; and of the role of plants 

and animals, including people, in all of these processes. 

The nature and rates of surface processes vary with 

the materials on which they act. Wind and water erosion in the arid Western United States etch every variation in rock strength onto cliff faces-soft layers are 

cut back and hard ones stand out boldly. Acid rain acting on old graveyards in the Eastern United States 

leaves granite tombstones relatively well preserved 

whereas many limestone tombstones are so deteriorated as to be illegible. 

External factors such as tectonic deformation and climate also control the nature and rates of surface processes. 

Deformation of the earth's surface by fault movement or 

by uplift of mountain ranges accelerates water erosion 

and landsliding by increasing the slope of the surface. 

Recent studies have shown that important climate changes 

at scales of decades, centuries, and millenia have punctuated Holocene time (Dickinson, 1995), despite the broadly 

held view that the Holocene is a period of drying after a 

relatively cooler and wetter Pleistocene. We do not as yet 

understand the reasons for these fluctuations. Regional 

analysis of ancient flood frequency and size also indicate 

markedly changing patterns of local precipitation through 

time (Webb and Betancourt, 1992; Ely and others, 1993), 

which are little understood yet may have drastic effects 

on human projects. 

By integrating an improved understanding of the processes in action today with the recent geologic record, 

the frequency and magnitude of geologic events such as 

floods, windstorms, and land deformation that may drastically alter our living environment can be better assessed. 

The predictive capability that this allows will help us plan 

for inevitable natural events. 

Knowledge Gaps 

In the past decade, much progress has been made in 

characterizing sequences of soils that formed from similar parent materials under similar climatic conditions. In 

these cases, the degree and nature of soil development 

depends primarily on time. Further study of soil sequences 

in different climatic zones and on different parent materials allows us to begin to assess average rates of soilforming processes and to determine the climatic conditions of their formation (Bockheim, 1980; Harden, 1988). 

The geochemistry and timing of weathering processes, 

however, are still poorly known. It is likely, for example, 

that soil formation takes place in a series of discontinuous steps rather than as a smoothly continuous process, 

but we cannot now describe these steps with any precision. Such knowledge is essential to understanding the 

consequences of soil degradation and the processes of 

soil formation and regeneration. 

A great deal is now known about stream hydraulics and 

the nature and amount of sediment transported by the 

Nation's rivers, but we still know little about how a unit of 

sediment is transported from the top of a hill to the base of 

the slope and into a river system. We cannot predict whether 

that unit will remain intact or where any of it will be at any 

time, how it has been chemically or physically transformed, 

where it may temporarily come to rest, and what characteristics it will have in the next increment of time. 

Much remains to be learned about the chemistry and 

physics of microscopic mineral-water interfaces, which 

govern such things as weathering rates and the formation 

of calcrete and silcrete (hard, relatively impervious layers composed of chemically precipitated calcium carbonate and silica at the base of certain soil horizons). Processes of adsorption of water on soil particles, chemical 

interchanges between pore water and solid components 

of the soil, and distribution of heat in soils are all vital 

parameters in development and productivity of soils. The 

processes are highly vulnerable to change from human 

activities. A single passage of an army tank on maneuvers over desert soil can dramatically alter water infiltration, soil moisture, and heat distribution, and consequently 

biological productivity, for more than 50 years (fig. 9). 

The physical and chemical processes that control transmission and adsorption of water also control the distribution of toxic substances in soils because most toxic substances can either be dissolved or carried as particles by 

groundwater. To understand how to stop or slow the movement of toxic chemicals in groundwater, we must appreciate how varying adsorption characteristics control the 

rate of movement, some toxic elements and compounds 

being readily adsorbed onto soil and rock surfaces and 

others not (Devinny and others, 1990). Research into such 

processes has wide application to agricultural and rangeland management and also to toxic- and radioactive-waste 

disposal. Other applications include predicting movement 

of fluids in petroleum and natural-gas reservoirs and predicting chemical reactions between ore-bearing fluids and 

their host rocks to produce mineral deposits. 

These important processes have received little attention because their study requires teams representing 

widely varying scientific disciplines, and such teams are 

difficult to assemble and maintain as funding for scientific research shrinks. Moreover, we have only recently 

developed some of the necessary tools, such as high-resolution electron microscopes with analytical capability and 

ion microprobes, which can amass detailed information 

on the chemical interchanges among mineral, water, and 

Geologic Processes at the Land Surface 17 
organic components of the soil, and which can directly 

image minute features of mineral-fluid interfaces (fig. 10). 

Empirical observations have been used over the years 

to develop methods for predicting rates of soil erosion, 

leading to the Universal Soil Loss Equation, which is 

stated as 

A=RKSLCP 

where A= soil loss per unit area, R = rainfall factor, K = 

soil-erodibility factor, S = slope-gradient factor, L =slopelength factor, C = cropping-management factor, and P = 

erosion-control-practice factor (Wischmeier, 1976; 

Wischmeier and Smith, 1978). This equation attempts to 

predict the loss of soil from an area in terms of factors 

<;=Figure 9. Impacts of military maneuvers in the Mojave 

Desert of California. A, Lichen crust (dark growth on ground 

between shrubs) on sandy soil destroyed in tracks left by light 

tank traffic in World War 11-vintage (1942-43) maneuvers has 

not recovered in 50 years. These intershrub organic crusts are 

important in stabilizing the soil, fixing nutrients, and holding 

moisture. Photographed in 1986 by Doug Prose. B, Annual 

plants and surface texture have not recovered in tracks left by 

the single pass of a tank in the maneuvers of 1942-43. 

Photograph by Howard Wilshire, 1987. C, Face of a trench 

dug across a single 1942-43 tank track, showing persisting 

compaction of the soil. Soil compaction inhibits infiltration of 

water and plant growth. Photograph by Doug Prose, 1984. 

1t Figure 10. This highly magnified back-scattered electron 

image of a thin slice of soil shows mineral and rock fragments 

with concentric coatings of alternating calcite and opal 

laminae. The scanning electron microscope has allowed new 

approaches to the study of interfaces between fluid and solid 

particles in soils, which control availability of nutrients and 

water for plant growth. The bar scale is 10 J.Lm long. 

that assess rainfall, erodibility, slope and uninterrupted 

length of slope, vegetative cover, and erosion-control planning. There is a parallel equation for wind erosion. Since 

the 1970's, erosion prediction has been the single most 

widely used tool for soil-conservation planning in the 

United States, and the predictions based on the Universal 

Soil Loss Equation have been applied to agricultural and 

grazing lands, construction sites, road cuts, mine dumps, 

and other disturbed sites (fig. 11 ). 

Tests reveal, however, that these predictions are not uniformly successful. Predictions of erosion of mine spoils 

prove reasonably accurate, whereas wide discrepancies 

emerge between predicted and measured erosion of graz18 Processes: Principles, Rates, and Sensitivity to Change 
ing lands (Trieste and Gifford, 1980). The equation incorrectly assumes steady runoff flow (uniform sheet flow 

across a hill slope as opposed to flow concentrated in small 

channels) and does not deal adequately with interactions 

that control detachment of soil particles from the surface 

and their transportation and deposition (Lane and others, 

1988). The equation cannot become universally applicable 

until it accurately incorporates processes of sediment detachment and the mechanics of sporadic or concentrated 

flow. For more accurate prediction of erosion, we need to 

know how the shape of the land surface affects overland 

flow of water and how the intrinsic sensitivity of different terrains governs their susceptibility to erosion. 

Figure 11 . Erosional problems caused by human activities. A, 

Mineral Park copper mine, Mohave County, Ariz.; erosion of 

mine spoils pollutes surface and groundwater. B, Gullies and 

depositional fans, dry land farm area, State Highway 46, San 

Louis Obispo County, southern California; episodic soil loss has 

diminished the productivity of these farmlands. C, Overgrazing 

by sheep, Tehachapi Mountains, Calif., has caused severe soil 

loss. D, Erosion of land cleared for housing development, 

Interstate Highway 15, north of San Diego. E, Gullies were 

initiated when coastal chaparral was stripped for flax crops, 

near Santa Cruz, Calif. F, Gullies formed where runoff from 

maintenance roads and pads for electricity-generating windmills 

is concentrated (see Wilshire and Prose, 1987). Photographs by 

Howard Wilshire. 

Geologic Processes at the Land Surface 19 
Figure 12. Overgrazing can cause extensive 

soil exposure and accelerated erosion. A. Near 

Bikaner, northwestern India. Virtually all 

understory plants have been stripped, and 

much soil has been lost to wind and water 

erosion. Photograph by Carol Breed. B. in the 

Temblor Range, southern California, intensive 

cattle grazing on steep slopes exposed the soil 

to erosion, causing formation of gullies. 

Photograph by Howard Wilshire. C, In the 

Tehachapi Mountains, southern California, the 

fence line (diagonally across center of 

photograph, from upper left to lower right) 

separates heavily grazed land (light colored 

due to stripping of vegetation in 1977 

windstorm) from still-vegetated (dark) 

ungrazed land. Photograph by California 

Department of Transportation. 

20 Processes: Principles, Rates, and Sensitivity to Change 
Figure 13. Soil modification and highly accelerated erosion were caused 

by military and recreational vehicles in the California desert. A. Hillside 

scarred by recreational motorcycles, northwestern Mojave Desert (Webb 

and Wilshire, 1983). Photographs by Howard Wilshire. B, Tracks left by 

tank exercises in 1942-43 and 1964 in the eastern Mojave Desert, 

photographed in 1985 (Prose, 1986). 

Thresholds and Irreversible Change 

Our ability to understand and predict the rates and effects of erosion and other surface processes has been 

greatly advanced by the concept of thresholds. A threshold is a point at which processes or rates in surface systems change rapidly or abruptly. There are many kinds of 

geologic thresholds, such as those that govern whether a 

glacier advances or retreats, and when rocks begin to melt 

when heated. Of particular importance are those thresholds that lead to irreversible change, such as the breaking 

of rocks under stress, or to long-term loss, such as soil 

erosion (Coates and Vitek, 1980). 

The abrupt change associated with thresholds contrasts 

sharply with the gradual change, or even constancy, that 

most people perceive for landscape. Not widely appreciated, for example, is that sandy soils in arid lands commonly are stabilized by lichen mats that form crusts only 

a few centimeters thick, or by barely visible masses of 

algal filaments that bind soil particles in the upper 2 to 3 

em. Other soils or surface sediments in arid lands may be 

cemented by hard inorganic crusts of clay, carbonate, or 

salt. A single pass of a bicycle or a hiker is often sufficient to break the surface and expose loose soil below to 

wind erosion. Thus, even slight disturbances of the natural crusts on arid-land soils can exceed a threshold and 

lead to active erosion of a previously stable ground surface. The sand released by the disturbance can itself abrade 

the surrounding crusted soils, spreading the damage. Practices that exceed such thresholds, ranging from livestock 

grazing in the Saharan fringe (Mabbutt, 1984; Dregne, 

1983) to off-road traffic in the Western United States 

(Webb and Wilshire, 1983), have contributed to desertification throughout the world. Grazing and off-road vehicle damage is illustrated in figures 12 and 13. 

A study of soil slips and resultant flows that formed 

during a severe rainstorm in the San Francisco Bay region in 1982 established a threshold value of rainfall intensity above which widespread debris flows occurred 

(see box, next page) (Cannon and Ellen, 1988; Cannon, 

1988). This threshold and a network of recording rain 

stations provide the basis for debris-flow alerts issued by 

the U.S. Geological Survey through the National Weather 

Service in the San Francisco Bay region. Other important 

hydrologic thresholds include the threshold of critical stream 

power that separates erosional from depositional modes in 

rivers (Bull, 1979) and the threshold of average annual 

precipitation in semiarid lands that controls the vegetative cover of hillslopes and hence their sensitivity to erosion (Langbein and Schumm, 1958; Schumm, 1968). Studies of the latter threshold found that erosion rates are greatest in regions with average annual precipitation of 30 to 40 

em, and thus are greater in semiarid regions than in arid 

or humid ones. When precipitation is less, runoff is too 

small to cause much erosion except where the precipitation is delivered by thunder storms. More precipitation supports sufficient vegetation to reduce erosion rates. 

Geologic Processes at the Land Surface 21 
RAINFALL THRESHOLDS FOR DEBRIS FLOWS 

Rainfall thresholds determine whether debris flows are triggered during 

rainstorms. Subthreshold storm rainfall (below left) simply raises groundwater 

levels in hillside soils. (Soils are indicated in the diagrams by a dotted pattern, 

ground water by gray shading. X's indicate impermeable bedrock below the 

soils.) Rainfall conditions above the threshold (right) raise groundwater levels 

high enough to trigger failure in soil types that can liquify. The resulting mass, 

which flows suddenly and rapidly downslope, can destroy buildings and claim 

lives within moments after movement starts. The debris flow pictured at lower 

right destroyed a horne in Marin Co., Calif., in January 1982. Photograph by 

Stephenson Ellen. 

22 Processes: Principles, Rates, and Sensitivity to Change 

Prepared by Stephenson Ellen and Bruce Rogers 
A study of soils developed on carefully dated lava flows 

in the Mojave Desert unexpectedly identified a threshold 

of wind erosion of dry lakes. Soils formed on lavas that 

range in age from about 150,000 to 20,000 years old are 

similar, indicating that they were all produced not by a 

continuous process of soil formation but instead by dust 

fallout from short wind-erosion events triggered by drying of lake beds and playas in the last 20,000 years 

(McFadden and others, 1984 ). 

Thresholds in surface systems thus may have an immediate bearing on people and their pursuits. Thresholds 

are also fundamental to understanding the processes themselves and to interpreting the geologic record of their frequency. Most commonly the geologic record is interpreted 

in terms of uniformitarian principles, embodied in the 

saying "The present is the key to the past." The concept 

of thresholds, which has received rapidly increasing attention in the past decade, indicates that abrupt changes 

may take place in normal system operations, and that repeat intervals of such changes may be long compared to 

human life expectancy. The geologic record teaches an 

appreciation of past-and future-events from a longer 

view than can be observed in the present. 

Important thresholds may be vulnerable to both climate change and human activities. A warmer, drier climate resulting from greenhouse warming would make 

land now above the vegetation-precipitation threshold 

more vulnerable to erosion through reduction of vegetative cover. Accelerated erosion in tum exacerbates water 

and air pollution. With sufficient understanding, we might 

be able to predict when such thresholds will be exceeded 

and thus to accommodate or even prevent the change. 

The obvious tactic to ameliorate damage from climate 

change would be to reduce greenhouse gas emissions, but 

other actions also are feasible, such as reducing grazing 

loads to give vegetative communities better chances to 

adapt. 

Another example is found in studies of the effects of 

dams on the Colorado River (Turner and Karpiscak, 1980; 

Lucchitta, 1991 ). The dams act as sediment barriers. Most 

sediment entering reservoirs settles out in the reservoirs; 

the sediment-free water released from the dams has great 

power to erode and transport sediment below the dams. 

The practice of accommodating peak daily demands for 

electricity by large discharges of water results in swamping of beach sands, which may slump into the river when 

the water levels drop; in other cases, former sand beaches 

have become marshlands. Resupply of sediment from the 

main river system is now prevented by the dams, but tributaries remain unaffected and continue to deposit sand, 

gravel, and boulders at their mouths. Although sand is 

redistributed, the river lacks the peak flood power to move 

the larger sediment. As a consequence, rapids are gradually becoming bigger at the confluences of tributaries and 

the river. Before the dams were built, there was a dynamic equilibrium between annual seasonal high discharges and low water such that sediment delivered by 

the tributaries was constantly redistributed downstream. 

Now, even during major periods of high water, such as 

occurred in spring of 1983, the river lacks the power to 

redistribute the boulder deposits that make up the rapids. 

The public works projects have artificially raised thresholds that, under previous natural conditions, were regularly exceeded so that erosion and deposition were balanced. 

Geologic Processes at the Land Surface 23 
TECHNIQUES 

The study of surface processes is based on observations of both natural and experimental systems. Hypotheses are drawn from observations, then tested, refined, 

and tested again. Such study addresses the fundamental 

nature of processes, their organization into systems, and 

their variation across the landscape and over time. 

In the early part of this century, sciences that dealt with 

the land surface-geomorphology, soil study, and allied 

fields-were largely descriptive. As these sciences have 

matured, the focus has shifted from simple description to 

comprehending how the processes work, and findings and 

principles have become ever more likely to be expressed 

quantitatively. These changes were led by progress in the 

mathematical description of streamflow, and they are facilitated by the application of the threshold concept to 

geomorphology, by the improved ability to determine the 

ages of Pleistocene and Holocene deposits and events, 

and by recent advances in the geochemistry of stable isotopes that allow dating of surface exposure as described 

in a following section. 

Clear understanding of specific surface processes requires field study of ancient and modern examples, field 

and laboratory experimentation, and theoretical analysis. 

We can observe active surface processes directly in the 

field by monitoring test areas, by conducting experiments, 

and by studying events of extreme magnitude, intensity, 

or duration. These events are particularly valuable for 

stimulating the discovery of new concepts. 

Figure 14. Catastrophic Nevados Huascardn avalanche, Peru, 1970, was triggered by an earthquake. A, Debris that was derived from elevations 

between 5,400 m and 6,500 m traveled 16 km to elevations at 3,000 m and lower. The avalanche had an average velocity of 280 km per hour, and 

large boulders launched into the air reached velocities estimated to be about 1,000 km per hour (Plajker and Ericksen, 1978). Photograph 

courtesy of George Plajker. B, The town ofYungay lay in lower middle ground (compare photographs A and B). Some of the low hills not 

overtopped by the 1970 avalanche were formed by deposits of previous avalanches, including some prehistoric ones much larger than the 1970 

avalanche. Photograph by George Plafker. 

24 Techniques 
Figure 15. Wind erosion and deposition. A. About 1 m of soil was 

eroded in 7 hours from this plowed field in eastern New Mexico in the 

windstorm of February 1977 (McCauley and others, 1981 ). The storm 

removed all the crop, the plow furrows, and the loose soil, leaving the 

streamlined knobs ("yardangs, " about 1m long) of the harder subsoil. 

Photograph by Carol Breed. B, Aerial photograph of the Portales, N. 

Mex., area in 1983, showing highly active wind-blown sand in three 

large circular fields (light colored) with center-pivot irrigation systems. 

In contrast, uncultivated parabolic sand dunes surrounding the circular 

fields are mostly stabilized by vegetation (Muhs and Maat, 1993). 

Photograph courtesy of Daniel Muhs. C, Deep erosion (as much as 0.5 

m into bedrock) of grazed land (foothills in background) and 

agricultural/and (middle ground), and deposition of sand in an alfalfa 

field (foreground), was caused by a severe windstorm of 24-hours 

duration in the southern San Joaquin Valley, Calif., December 1977 

(Wilshire and others, 1981). Photograph by Howard Wilshire. 

Knowledge Gained from Extreme Events 

Comparison of the effects of 1969 Hurricane Camille 

and 1972 Tropical Storm Agnes on the Eastern United 

States demonstrated strong contrasts in landscape response under conditions of intense rainfall (Williams and 

Guy, 1973; Costa, 1974; Wolman and Gerson, 1978). In 

the mountainous Blue Ridge Province, the intense rainfall caused debris flows; comparable rainfall in the hilly 

Piedmont Province caused massive flooding. Most deaths 

in the Blue Ridge Province were caused by boulders and 

trees carried by debris flows (Williams and Guy, 1973), 

whereas most deaths in the Piedmont Province were 

caused by drowning. The different responses to intense 

rainfall of the two areas resulted from differences in physiographic features such as basin size, shape, and slope; 

altitude; land use; soils; and surficial and bedrock geology (Costa, 1974). 

The 1970 Nevados Huascanin debris avalanche in Peru, 

triggered by an earthquake, taught that flows of soil and 

rock debris can travel at extreme speeds and spill over 

major topographic barriers (Plafker and Ericksen, 1978) 

(fig. 14). The avalanche overtopped a hilf 700 m high 

and destroyed the town of Yungay, killing an estimated 

18,000 people. 

Accelerated wind erosion resulting from agricultural 

and grazing practices led to complete stripping of the soil 

over large areas in both New Mexico and California in 

1977 during brief, violent windstorms (fig. 15; frontispiece). Dust from the storm in New Mexico was tracked 

by satellite across the Eastern United States and over the 

Atlantic Ocean (McCauley and others, 1981). Most of 

the dust carried in this storm came from plowed lands in 

areas known to be highly susceptible to wind erosion. 

The soils, stable in their natural vegetated condition, had 

originally formed on sediments deposited by wind about 

10,000 to 22,000 years ago. Breaking of the surface soils 

and removal of natural vegetation made the land vulnerable to wind erosion; because the storm struck in February, substantial areas were unprotected by crops. 

The California windstorm, in the San Joaquin Valley, 

carried and deposited sands and gravels so coarse (rock 

fragments as large as 10 em across were bounced across 

the surface) they previously were thought impossible for 

wind to move (Wilshire and others, 1981 ). This discovery revealed a need to establish criteria to distinguish riverborne from wind-borne gravels to correctly interpret the 

geologic record. The same storm formed desert pavements 

(surfaces coated by thin pebble layers) in hours, contradicting previous assumptions that thousands of years were 

required to create them. The storm also taught that windborne dust can be a causal factor in disease epidemics 

Geologic Processes at the Land Surface 25 
Figure 16. Mount St. Helens, before and after the 

eruption of May 18, 1980. A, View from Johnston 

Ridge, one day before the May 18 eruption. 

Photograph by Harry Glicken. B. View from 

Johnston Ridge, September 10, 1980, showing 

massive changes in the shape of the volcano and 

destruction of the forests seen in photograph A, 

due to the May 18 eruption. Photograph by Harry 

Glicken. C, View from Johnston Ridge, March 30, 

1987, showing major stream-channel incisions in 

the surface created by the May 18 eruption. In 

this setting. physical modifications of the surface 

have occurred more rapidly than biological 

regeneration. Photograph by Lyn Topinka. 

because it spread valley fever, 

coccidioidomycosis, a lung disease 

caused by a fungus endemic in creosotebush soil, hundreds of kilometers to the 

north end of California's Great Valley. 

Study of the 1980 eruption of Mount 

St. Helens revealed how swelling of the 

volcano's north flank due to magma rising inside the volcano culminated in an 

enormous landslide that released a lateral blast of hot volcanic gas and rock 

debris. The eruption killed 57 people, 

triggered debris flows that temporarily 

stopped shipping on the Columbia River 

and disrupted highways and rail lines, 

and transformed a landscape of dense 

green forest into a volcanic wasteland 

(Wright and Pierson, 1992; fig. 16). Continued study is documenting the nature 

and rate of hydrologic, geologic, and biologic recovery on the severely modified 

landscape in the vicinity of the volcano 

(Foxworthy and Hill, 1982; fig. 16C). 

Such information will provide new insights into surface processes that can be 

applied to interpreting the geologic 

record and will prove invaluable for planning for future large-scale land-surface 

modifications from either natural or human causes. The lessons from Mount St. 

Helens led to the discovery that huge 

catastrophic slope failures are typical at 

growing volcanoes worldwide (Francis 

and Self, 1987; Moore and others, 1989). 

26 Techniques 
Lower limit 

/_ offan 

---- c 

/'~~ ~ t J 

/ ....... \ '· \ I ~., ·,. \ ! :§ 

\ ' -:i~:~ r( ~}--- "--

Sta o+Oi>'·, ... ~ ,.,, 

Sta 3+00 -#"..::-~reSta_9+00 ,.._ ...... _____ ·- -~""""'-- ' ~IIIIIIIHIIIINI~ ·-~'--- lfllfHII"""'~,.·-· tlt~lllttltQ '', ...... -• ...,,,,. 

' i Sta -(0+85) '""'"'""~~---·-·-· ·-., '-\.~-~~·,. ' Sta 12+00 ·, ~ Sta 6+00 .• ,,,, .. ~\ '- '·- \ ~""""""mumu,,, Sta 15+00 ,. 

. !'"""""'"""""' ' 'i, = -, \ v . ., """""'""'""'"i''"'"'"'""'''~"' '\ v '· \.... • -·""' ........ .-·- -·-·/ ~Ill i ' ·""' =""''"""lllllllllllllllllt;:c-:._ ,, ~,,, \:_ ~ "! : 1111,1

1. • lfiiiiiUIIIIIttlt 

.,; V""'"'J ''''""""·,·~,, 

1111111111111111111111111111111 Indicates approximate location 

of steeper slopes 

0 

I 

'\magnetic) 

20 40 60 80 100 METERS I I I I I 

Watershed 

divide 

Figure 17. Example of a Vigil Network study site, at Last Day Gully, "')lo. Gully is shown in two halves to better fit page. Each bar denotes location 

of cross section measured between two bench marks. Station numbers are elevations (in feet) with reference to Station 0+00; arrows near top of 

gully indicate inflow; arrows at bottom indicate channel directions on alluvial fan at mouth of gully. From Osterkamp and Emmett (1992). 

Long-Term Monitoring 

Long-term monitoring of both natural and disturbed test 

areas is an important way to study processes and their 

rates. The Vigil Network of sites (Emmett and Hadley, 

1968; Osterkamp and others, 1990) was started in 1962 

to establish baseline monitoring of hydrology and landscape change in selected hydrologic and geomorphic settings, now numbering 82, dominantly in the United States 

and Sweden (Osterkamp and others, 1991). The aim of 

this program is to establish reference points that can be 

systematically remeasured over long periods of time in 

order to detect and quantify changes in the land surface 

and in the distribution of surface and near-surface water. 

Preservation of the records for use in the future is a paramount goal, because changes that take place over the period of one or several lifetimes cannot be evident in the 

data gathered to date, or they may be wrongly interpreted 

in terms of our present understanding of the systems involved. Measurements that are being recorded include 

shape, depth, and width of stream channels; thickness of 

channel fill; hillslope profiles; reservoir sedimentation; 

and vegetation type and cover (fig. 17). These and other 

measurements made over long periods of time allow assessment of causal relationships between changes in the 

landscape and land-use and climatic factors. Even over a 

short period of time this monitoring has proved useful. 

Emmett ( 197 4) used 10 to 12 years of geomorphic records 

from eight Vigil Network sites to show that an arroyocutting cycle, known to have begun about 1880 in the 

arid and semiarid southwestern United States, had reversed and that channels were filling. Other programs such 

as the Long-Term Ecological Research program 

(Bhowmik, 1987) make many of the same kinds of measurements as the Vigil Network. Integrating and expanding these programs internationally through the auspices 

of the United Nations would provide major benefits for 

understanding global changes that affect surface processes. 

Geologic Processes at the Land Surface 27 
Large-Scale Experimentation 

Recently developed facilities for large-scale experiments on infiltration, runoff, landslides, and debris flows 

are likely to provide important new information about 

basic processes. At the National Research Institute for 

Earth Science and Disaster Prevention in Tsukuba, Japan, a large-scale rainfall simulator has been built to conduct experiments on the effects of rainfall on surface processes. The rainfall simulator consists of a building about 

80 m long by 50 m wide and four stories high. The entire 

building is mounted on railroad tracks so that it can be 

moved to different experimental plots, some designed to 

study infiltration and runoff, others to study landslides. 

International groups of scientists have collaborated at the 

Japanese facility to examine the conditions that tum landslides (a general term for masses of rock or soil moving 

downhill) into debris flows (swift downhill flows of water-saturated rock debris; Iverson and LaHusen, 1989). 

The U.S. Geological Survey and U.S. Forest Service 

built a large flume at the H.J. Andrews Experimental Forest in Oregon in 1991 to conduct experiments on debris 

flows. The flume is allowing, for the first time, experimental study of all stages of the debris-flow process, from 

initiation through deposition (Iverson and others, 1992). 

Among the research objectives are testing of existing 

mathematical models (Iverson and LaHusen, 1993), development of new models for interpreting and forecasting debris-flow behavior, and development of improved 

technologies for mitigating the destructive effects of debris flows (Iverson and others, 1992). 

Dating Geologically Young Events 

Advances in methods of determining the age of geologically young materials can provide the time control 

crucial to identifying synchronous events and to calibrating time sequences. Accurate dating of soils lets us determine recurrence rates of El Ni.fio global-climate events 

and of landslides or other sudden geologic events, and 

makes possible time correlations across climatic boundaries so that we can determine effects of time and climate 

on soil formation. 

The carbon-14 system has long been used to date organic materials. Carbon-14 is produced in the atmosphere 

by a variety of nuclear reactions and is incorporated in 

C02 molecules by exchange with stable carbon isotopes 

(Faure, 1977). Organisms adsorb carbon-14 in C02 from 

the atmosphere while alive. Upon the organism's death, 

the adsorption of carbon-14 ceases and the carbon-14 in 

the dead tissue declines by radioactive decay with a halflife of 5,730 years. The time of death can be determined 

by comparing the activity of carbon-14 in the dead tissue 

28 Techniques 

with that in living tissue. The potassium-argon (K-Ar) 

method of dating, based on decay of 4°K to 40 Ar, has wide 

application to surface processes because potassium is an 

important element in many rock-forming minerals. A 

derivative method that measures the ratio of 40 Arf39 Ar is 

simpler and can be used with very small samples, thereby 

eliminating problems of measuring absolute abundances 

of K and Ar and of sample inhomogeneity. A promising 

new application is extension of 40 Arf39 Ar dating from 

older materials into the age range (tens of thousands of 

years) of carbon-14 (Hu and others, 1994 ). Other highly 

reliable and flexible dating techniques include 

tephrochronology and fission-track dating (Rosholt and 

others, 1991). Tephrochronology (Sarna-Wojcicki and 

Davis, 1991) identifies volcanic ash deposits by detailed 

chemical analysis of volcanic glass particles; it can be 

used to distinguish even individual ash beds erupted from 

the same volcano. Because ash beds represent an instant 

in time and may cover huge areas, both on land and on 

the ocean floor, they are extremely valuable time markers that allow correlation across climatic boundaries and 

measurement of such phenomena as rates of sedimentation in many natural environments (fig. 18). Fission-track 

dating (Naeser and Naeser, 1988) makes use of naturally 

occurring impurities of uranium-238 in certain minerals 

such as apatite (fig. 19) and zircon and in fresh volcanic 

glass. Spontaneous fission of the uranium-238 releases 

particles whose paths through the mineral or glass are 

preserved as fission tracks that can be examined under 

the microscope. Because heating above a certain temperature anneals fission tracks, thus resetting the fission-track 

clock, fission tracks can be used to determine how long a 

sample has been at temperatures cooler than the annealing temperature of the mineral examined. They can be 

used, for example, to determine the age of young volcanic glasses or to determine the time of uplift of rocks to 

levels near the earth's surface where temperatures are 

cooler than the annealing temperature. 

Promising new and emerging dating techniques for geologically young materials (Rosholt and others, 1991) include thermoluminescence (Wintle and Huntley, 1982; 

Lamothe and others, 1984 ), electron-spin resonance 

(Grun, 1989), rock-varnish chemistry (Dorn, 1982; 

Harrington and Whitney, 1987), and measurement of several short-lived cosmogenic isotopes such as chlorine-36, 

beryllium-10, aluminum-26, and helium-3 (Nishiizumi 

and others, 1986, 1989; Phillips and others, 1986, 1990; 

Lal, 1988). 

The property of thermoluminescence (TL) is imparted 

by natural nuclear radiation, which causes the release of 

electrons within mineral grains. The electrons accumu-
Lava Tecopa 

Creek B Lake 

Great 

Salt Lake Ventura 

AshBed'p'·~·mmmm~~~--~~~mq--------~~mmmq 7.9m Bishop/···· 

Ash Bed 

9 em /1 ,000 yr 

11.0 m 

270 em /1 ,000 yr 

Figure 18. Applications of tephrochronology. Cores from three lake 

deposits revealed the presence of widespread volcanic ash beds, the 

Bishop ash bed that is 758,000 years old and the Lava Creek Bash 

bed that is 665,000 years old. These beds were identified by 

tephrochronologic techniques, thus allowing correlation from core 

to core, and were dated by 40Ar!39Ar laser fusion techniques. The 

amount of sediment that accumulated in the 93,000 years between 

ash falls varies from 7. 9 m in Tecopa Lake in southern California , 

to JJ m in the Great Salt Lake in Utah, to 253 m in an area of rapid 

marine deposition near Ventura, southwestern California (data from 

Sarna-Wojcicki and others, 1987b; Eardley and others, 1973). 

I 

I ., 

·' , 

, 

late in "electron traps." During laboratory heating, these 

electrons are moved to sites within the mineral structure 

from which light is emitted. The intensity of the emitted 

light is a measure of the time that has elapsed since the 

mineral grain became shielded from sunlight. Electronspin resonance (ESR) dating also measures the effects of 

natural radiation on mineral grains. These two techniques 

can be used, for example, to date the formation of mineral grains in subsurface soil horizons, or to determine 

the age of a lava flow that buried measured mineral grains 

and thereby started their TL and ESR "clocks." 

Several approaches have been made to dating rock varnish, which is a thin coating of iron-manganese oxides, 

clay minerals, and minor organic material that accumulates on exposed rock surfaces in arid environments. The 

ability to accurately date these surfaces would provide 

important information about rates of climate change. 

Cation ratios change with age and are measured on 

samples of varnish scraped from rocks or by scanningelectron microscopy on varnish still adhering to rock 

samples. The ratios have been calibrated by dating of the 

same rock units by other methods. (Attempts to date the 

earliest organic component of varnish by the carbon-14 

method using accelerator mass spectrometry on scraped 

samples (Dorn, 1991) have not yet proved reliable and 

require additional research (Reneau and others, 1991)). 

The dates that have been acquired so far suggest that alluvial surfaces covered by varnished desert pavement have 

been stable for tens of thousands to hundreds of thousands of years. These time scales indicate the huge recovery times required when these surfaces are disturbed. 

The chronology of soil formation is being pursued by 

new techniques using isotopes produced by cosmic radiation, such as aluminum-26 and beryllium-1 0 

(Nishiizumi and others, 1986, 1989), chlorine-36 (Phillips 

and others, 1986, 1990), and helium-3 (Phillips and others, 1986; Ceding, 1990; Kurz and others, 1990). These 

techniques allow dating of young deposits and geomorI 

Figure 19. This photomicrograph of a 

thin slice through an apatite crystal 

shows fission tracks formed by particles 

released by spontaneous fission of 238U 

enclosed in the crystal when it formed. 

The abundance of tracks and source 

uranium allows determination of the age 

of the crystal when it last cooled through 

its annealing temperature (about 100°C). 

Width of photo is about 150 Jlm. 

Photograph by Charles Naeser. 

Geologic Processes at the Land Surface 29 
c D 

I 

B E 

A 

Figure 20. Computer mapping using geographic information systems (GIS) technology permits the systematic combination of different mapped 

features of the earth's surface to generate new maps. In this example, a map, A, showing areas of the Santa Cruz Mountains, northern California, 

that are at risk from fast-moving, fluid landslides called debris flows (or mudslides) has been generated by combining maps ofB, rainfall that 

triggered debris flows in a severe storm in 1982 (warmest colors indicate greatest rainfall); C, locations (red dots) of debris flows triggered by the 

1982 storm; D, steepness of hills/opes (warmest colors indicate steepest slopes); and E, geologic materials, including soils and bedrock (Mark, 

1992). In map A, warmest colors indicate greatest debris-flow hazard. Designed by Stephenson Ellen. 

phic surfaces, and determination of weathering and erosion rates. The isotopes record how much time the material has been exposed to cosmic rays, that is, how long it 

has been exposed at the land surface. The beryllium isotope beryllium-I 0 is produced both at the land surface 

and in the atmosphere, from which it is delivered to the 

surface by precipitation. Under appropriate conditions and 

soil acidity (pH about 5 to 8, slightly acid to very slightly 

basic), bery llium-1 0 is adsorbed by fine-grained soil components, so that its concentration in a soil profile can be 

used to estimate the minimum age of the soil (Pavich and 

others, 1984). 

Significant problems remain with all methods of dating young materials. Ages estimated for volcanic rocks 

in southern Nevada on the basis of K-Ar and 40 Arf39 Ar 

isotopic clocks (Turrin and Champion, 1991) are five to 

eight times older than ages based on landform and soil 

30 Techniques 

development (Wells and others, 1990) and cosmogenic 

isotopes (Wells and others, 1991 ). These discrepancies, 

and the need for their resolution, take on a very practical 

meaning when applied, for example, to the problem of 

determining the recurrence rate of past volcanism to assess whether future volcanism is likely to disrupt the highlevel radioactive waste repository proposed for Yucca 

Mountain, Nevada (Turrin and Champion, 1991; Wells and 

others, 1990). Variations over time in flux of cosmic radiation, and contamination by young carbon of carbon-14-dated 

material, even after sample collection (Bradley, 1985), are 

among the many problems that require further study. 

Cartographic and Satellite-Based Aids 

The U.S. Geological Survey has undertaken a program 

of digitizing topography throughout the country from 

1 :24,000-scale maps. This program offers the opportu-
Figure 21. Landsat infrared images of part of the Amazon rainforest in the state of Rondonia, Brazil, showing the dramatic increase in deforestation 

between 1975 and 1986 (see Stevens and Kelley, 1992; Skole and Tucker, 1993). A, In 1975. B, The same region in 1986. Red areas are forested, blue 

areas are stripped of forest. Linear patterns are roads along which access to the cleared areas was gained. 

nity to numerically describe and classify terrains and landforms, such as drainage basins (Moore, 1991), in sufficient detail to build direct, quantitative ties to process and 

history. Emerging geographic information systems (GIS) 

technology is creating new ways to store, analyze, and 

manipulate a host of interrelated data in map form (Wadge, 

1988; Dikau, 1989). Using GIS computer programs, researchers can create large databases of information about 

a region, subsets of which can be displayed in map form. 

Each subset of graphically displayed information forms 

a layer that can be overlaid on other layers from the data 

set. One layer of information about a given region might 

show topography, a second might show show average 

annual rainfall at various points, a third might show what 

types of soil are found in various areas, a fourth might 

show deposits of past landslides, a fifth might show types 

of vegetation, and a sixth might show human structures. 

Layers can be overlaid to highlight and study functional 

relations between the different types of information. For 

example, layers can be combined to examine the effects 

of soil type, topography, and rainfall on the likelihood of 

landsliding (Mark, 1992) (fig. 20) or to assess the potential for landslide damage to human structures. Traditionally, combination of such information layers was created 

manually on paper and transparent overlays, a process 

that took much time and yielded products that could not 

easily be modified. Using GIS software, such layers can 

be created electronically, and easily manipulated and 

mathematically related. Analysis can be quicker, more 

imaginative, and vastly more thorough. The benefits 

achieved from these approaches to mapping much exceed their modest costs (Bemknopf and others, 1993). 

Satellite-based remote sensing provides a comprehensive view of large areas of the earth's surface. Use of remote sensing is receiving increasing emphasis in national 

and international programs that monitor the global environment and changes brought about both by natural events 

such as drought and by human activities such as deforestation (Short and Blair, 1986; Stevens and Kelley, 1992) 

(fig. 21). Linkage of the large-scale view of surface effects with detailed on-the-ground studies of surface processes can provide a basis for regional and global models 

that explain the evolution of the land surface and predict 

future changes (Franklin, 1991). 

Geologic Processes at the Land Surface 31 
CONCLUSION 

We cannot remain blind to the almost daily warnings 

from a wide spectrum of scientists about the gradual decay of the earth's human life-support systems under everincreasing population load. Land resources and land hazards profoundly affect the lives of every person on earth. 

Yet, we proceed as if fully confident of our ability to undo 

any damage that we inflict on the land, secure in a mistaken belief that the land can meet all of our desires and 

remain all that it was. 

The extent of our ignorance of how natural surface systems operate underscores a need for action. Earth scientists studying the land's surface need to work toward two 

main goals: 

32 Conclusion 

• To understand-comprehensively-the origin and 

evolution of life-sustaining resources at the earth's land 

surface, the rates of their formation and decay, and their 

sensitivity to change. We must learn to assess accurately 

the present condition of these resources and of the interdependent systems of which they are a part and to predict 

what their future condition will be. 

• To explain our research to decision makers and the 

public so that sound land-use policies can be formulated. 

We must offer honest and convincing explanations of the 

carrying capacity of land-surface resources, by defining 

in defensible scientific terms the rates of their formation 

and replenishment. 
Acknowledgments 

This report represents a study by the Subcommittee on 

Land Surface Geologic Processes for the Geologic Division Science Advisory Committee of the U.S. Geological 

Survey. Subcommittee members J.E. Costa, J.W. Harden, 

R.L. Schuster, and L.D. Carter helped formulate thereport, and R.E. Wallace, J.C. Dohrenwend, J.C. Yount, S.D. 

Ellen, K.L. Pierce, M.C. Reheis, E.H. Helley, R.D. Watts, 

S.M. Colman, R.J. Pike, R.K. Mark, J.C. Tinsley, and L.P. 

Gough also contributed. We thank them and many other 

colleagues who discussed with us their views or helped 

review the report. Special thanks to Warren Hamilton for 

his insights and help, to Bruce Rogers and Susan Mayfield 

for their expert help with computer graphics, and to Sara 

Boore for designing the book. Thanks also to Sue Luttner 

Coonen, who improved a late draft of the manuscript. 

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Geologic Processes at the Land Surface 39 
Glossary 

absorb To take up and hold mechanically, as water in soil pores. 

acid rain Rainwater altered to a toxic or corrosive composition by adherence or solution of pollutants in the air. 

adsorption Adherence of molecules or ions in solution to surfaces of solids; adsorbed fluids are more strongly 

bound than absorbed fluids. 

alluvial deposits Materials deposited by running water. 

arid Dry; said of a climate, hot or cold, having less than about 25 em of annual precipitation as water or its equivalent as snow. 

arroyo Small, intermittent stream channel with steep or vertical walls; term applied to such ephemeral stream 

channels in arid and semiarid areas of the southwestern United States. 

ash (volcanic) Fine-grained material ( <4 mm) erupted from volcanoes. 

avalanche A type of landslide in which snow or rock debris slides or falls rapidly down steep slopes. 

background radiation Radiation from natural sources such as cosmic rays and the earth's naturally occurring 

radioactive substances. 

baseline monitoring Provision of a set of measurements with which subsequent measurements can be compared to 

assess change. 

bedrock Hard or consolidated rock, either exposed at the surface or lying beneath soil or other unconsolidated 

surficial deposits. 

chaparral Vegetation community dominated by dense growth of shrubs. 

chemical weathering Alteration of rocks and minerals by chemical reactions that transform them into new chemical combinations that are stable at the earth's surface. 

climate The combination of temperature, precipitation, and their seasonal variations that characterizes a region. 

correlation Establishment of a relation between one geologic phenomenon and another, for example by demonstrating that isolated deposits were formed in the same event, in the same time period, or in the same type 

of environment. 

cosmogenic Originating outside of the Earth-Moon system. 

debris flow A form of landslide in which water-saturated soil and rock debris flow rapidly downstream or 

downslope on hillsides. 

deposition The accumulation of rock material that was suspended in or transported by water or air, or moved by 

gravity. 

desert An area, hot or cold, characterized by low annual precipitation (<25 crnlyr), generally with small and sparse 

vegetation. 

eolian Relating to wind, such as transportation by wind currents. 

erosion Removal of soil or rock materials by running water (either by physical detachment or solution), wind, or 

mass movement under the influence of gravity. 

evaporite A chemical deposit formed by evaporation of water or some other solvent. 

evapotranspiration Loss of water by evaporation from the soil and transpiration from the leaves and stems of 

plants. 

flume A natural or artificial channel through which water is conveyed. 

fluvial Pertaining to rivers. 

geomorphology The study of the nature, origin, and development of present landforms and their relation to 

underlying rocks. 

40 Glossary 
glacier A large mass of stationary or moving ice formed, at least in part, on land. 

glass (volcanic) Liquid rock cooled too rapidly to permit crystallization of minerals. 

groundwater Water occurring beneath the surface of the ground in the saturated zone, where interconnected pores 

are all filled with water. 

hydrologic Pertaining to water. 

hydrosphere The waters of the earth, including surface water and groundwater. 

infiltration The movement of fluids into soil or rock through fractures and pores. 

isotopes Atoms of the same element that have different atomic weights. 

landform A physical feature of the earth's surface having a characteristic shape formed by natural processes. 

landslide A general name applied to mass movement of rock or soil downslope under the influence of gravity. 

outwash Rock debris washed out from the front of glaciers by melt water. 

parabolic dune AU- or V-shaped dune formed when the middle part of a dune has moved forward with respect to 

the ends of the dune; open end faces upwind. 

playa A dried-up, vegetation-free, flat-floored area composed of thin sheets of clay, silt, and sand representing the 

lowermost part of a shallow, undrained desert lake basin. 

pore water Water residing in the interstices of soil or rock. 

precipitation The discharge of water (as rain, snow, hail, sleet, dew) from the atmosphere to the earth's surface. 

river basin The entire area drained by a river and its tributaries. 

runoff That part of precipitation that flows from the surface rather than sinking into it. 

saline zone A zone on or within the soil in which salts have accumulated. 

sedimentation Deposition of rock particles and organic debris from water or air, or accumulation of chemical 

precipitates from water. 

semiarid Relatively dry; said of a climate, hot or cold, with about 25 to 50 em of annual precipitation. 

sensitivity (of a landscape) The intrinsic responsiveness of a landscape to external forces, such as extreme rain or 

wind. Governed largely by the character of the rock and soil materials and by landforms. 

slope Degree of inclination of the land surface. 

soil Layer of unconsolidated material derived from weathered rock or rock materials that have been transported by 

wind, water, or gravity; term usually reserved for materials in which land plants can root. Weathered-rock 

materials that make up soil have generally, but not necessarily, been altered by physical, chemical, and 

biological processes. 

surface water Standing or running bodies of water at the earth's surface. 

topsoil Uppermost layer of soil that is especially rich in organic material and (or) is in a physical state especially 

conducive to plant growth. 

transportation (of sediments) Movement of rock and mineral debris by dragging (traction), bouncing (saltation), 

or suspension in water or air. 

uplift Change in relative elevation of a land mass, either by raising the land mass or dropping adjacent land. 

weathering Chemical or mechanical transformation of rock materials by atmospheric, groundwater, or biological 

activity. 

wetland Land that is permanently or episodically saturated or covered with water, such as swamps or tidal flats. 

yardang Sharp-crested ridge sculpted by wind-driven sand; ranges from centimeters to kilometers in long dimension, oriented parallel to prevailing wind direction. 

Geologic Processes at the Land Surface 41