Geologic Field-Trip Guide of Volcaniclastic Sediments from 

Snow- and Ice-Capped Volcanoes—Mount St. Helens, 

Washington, and Mount Hood, Oregon

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2017–5022–F
Cover: Top. Photograph of open breach of the Mount St. Helens crater taken from the Pumice Plain looking south-southeast. Eroded deposits

in foreground are 1980 pyroclastic density current deposits. The smooth ramp leading up to crater floor is the sliding surface of the 1980 debris 

avalanche. The composite lava dome (1980–1986 and 2004–2008) is partly visible (largely in shadow) at back of crater. USGS photograph by 

T. Pierson, 2012. Bottom. Photograph of Mount Hood summit looking northeast from Zigzag Canyon, which has incised the debris fan on the 

southwest flank of the volcano. Stratified late Pleistocene and Holocene volcaniclastic sediments, emplaced by block-and-ash flows and 

lahars, are visible in the canyon walls overlying a lava flow near the bottom of the canyon. USGS photograph by T. Pierson, 2003.
Geologic Field-Trip Guide of Volcaniclastic 

Sediments from Snow- and Ice-Capped 

Volcanoes—Mount St. Helens, Washington, 

and Mount Hood, Oregon

By Thomas C. Pierson, Lee Siebert, Christopher J. Harpel, and Kevin M. Scott

Scientific Investigations Report 2017–5022–F

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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U.S. Government.

Although this information product largely is in the public domain, it may also contain copyrighted materials as noted 

in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation: 

Pierson, T.C., Siebert, L., Harpel, C.J., and Scott, K.M., 2018, Geologic field-trip guide of volcaniclastic sediments from 

snow- and ice-capped volcanoes—Mount St. Helens, Washington, and Mount Hood, Oregon: U.S. Geological Survey 

Scientific Investigations Report 2017–5022–F, 97 p., https://doi.org/10.3133/sir20175022F. 

ISSN 2328-0328 (online)
iii

Preface

The North American Cordillera is home to a greater diversity of volcanic provinces than any 

comparably sized region in the world. The interplay between changing plate-margin interactions, 

tectonic complexity, intra-crustal magma differentiation, and mantle melting have resulted 

in a wealth of volcanic landscapes. Field trips in this series visit many of these landscapes, 

including (1) active subduction-related arc volcanoes in the Cascade Range; (2) flood basalts 

of the Columbia Plateau; (3) bimodal volcanism of the Snake River Plain-Yellowstone volcanic 

system; (4) some of the world’s largest known ignimbrites from southern Utah, central Colorado, 

and northern Nevada; (5) extension-related volcanism in the Rio Grande Rift and Basin and 

Range Province; and (6) the spectacular eastern Sierra Nevada featuring Long Valley Caldera 

and the iconic Bishop Tuff. Some of the field trips focus on volcanic eruptive and emplacement 

processes, calling attention to the fact that the western United States provides opportunities to 

examine a wide range of volcanological phenomena at many scales. 

The 2017 Scientific Assembly of the International Association of Volcanology and Chemistry of 

the Earth’s Interior (IAVCEI) in Portland, Oregon, marks the first time that the U.S. volcanological 

community has hosted this quadrennial meeting since 1989, when it was held in Santa Fe, New 

Mexico. The 1989 field-trip guides are still widely used by students and professionals alike. This 

new set of field guides is similarly a legacy collection that summarizes decades of advances in 

our understanding of magmatic and tectonic processes of volcanic western North America.

The field of volcanology has flourished since the 1989 IAVCEI meeting, and it has profited from 

detailed field investigations coupled with emerging new analytical methods. Mapping has 

been enhanced by plentiful major- and trace-element whole-rock and mineral data, technical 

advances in radiometric dating and collection of isotopic data, GPS (Global Positioning System) 

advances, and the availability of lidar (light detection and ranging) imagery. Spectacularly 

effective microbeam instruments, geodetic and geophysical data collection and processing, 

paleomagnetic determinations, and modeling capabilities have combined with mapping to 

provide new information and insights over the past 30 years. The collective works of the 

international community have made it possible to prepare wholly new guides to areas across the 

western United States. These comprehensive field guides are available, in large part, because 

of enormous contributions from many experienced geologists who have devoted entire careers 

to their field areas. Early career scientists are carrying forward and refining their foundational 

work with impressive results. 

Our hope is that future generations of scientists as well as the general public will use these 

field guides as introductions to these fascinating areas and will be enticed toward further 

exploration and field-based research. 

Michael Dungan, University of Oregon

Judy Fierstein, U.S. Geological Survey

Cynthia Gardner, U.S. Geological Survey

Dennis Geist, National Science Foundation

Anita Grunder, Oregon State University

John Wolff, Washington State University

Field-trip committee, IAVCEI 2017
iv

Col orado Riv er

Green River

Rio Grande

S nake River

Sacramento River San Joaquin River

Great 

Salt Lake

Salton

Sea

Lake

Tahoe

Mono

Lake

CALIFORNIA

NEVADA

OREGON

IDAHO

MONTANA

WYOMING

UTAH

NEW MEXICO

ARIZONA

COLORADO

WASHINGTON

CANADA

MEXICO

Reno

Bend

Idaho Falls

Bozeman

Creede

Denver

Santa Fe

Portland

Seattle

Los Angeles

San Francisco

Klamath 

Falls

Pullman

Salt Lake

City

Columbia River

Columbia

River

Mount Rainier (A, B)

Yellowstone (P, Q)

Valles 

Caldera (R)

Southern Rocky Mountain 

Volcanic Field (R)

Mount St. Helens (C, D, E, F)

Mount Hood (F, G)

 Sand Mountain

Volcanic Field (H)

South Sister (I)

Newberry Volcano 

(H, I, J2, M)

Crater Lake (J1, M)

Medicine 

 Lake 

 (I, K1)

Mount Shasta (K3)

Lassen 

 Peak (K2)

Long Valley 

Caldera (L)

Mammoth

Mountain (L)

Mount 

 Baker

Glacier 

 Peak

Mount 

Adams

Mount 

Jefferson

PACIFIC

OCEAN

Columbia

Plateau (M, N, O)

Basin 

and

Range 

Province (M, N)

High Lava Plains 

(M, O)

130° 120° 115° 110° 105°

120° 115° 110° 105°

125°

45°

35°

45°

50°

40°

40°

35°

30°

EXPLANATION

Volcano visited by field trip 

(see list on next page)

Volcanic field visited by field trip 

(see list on next page)

Other notable volcano

0

0 100 200 300 400 MILES

100 200 300 400 500 600 KILOMETERS

Letters A–R refer to chapters of the geologic field-trip 

guide collection published as U.S. Geological Survey 

Scientific Investigations Report 2017–5022

Map of the western United States showing volcanoes and volcanic fields visited by 

geologic field trips scheduled in conjunction with the 2017 meeting of the International 

Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) in Portland, 

Oregon, and available as chapters in U.S. Geological Survey Scientific Investigations 

Report 2017–5022. Shaded-relief base from U.S. Geological Survey National Elevation 

Dataset 30-meter digital elevation model data.
v

Chapter 

letter Title

A Field-Trip Guide to Volcanism and Its Interaction with Snow and Ice at Mount 

Rainier, Washington

B Field-Trip Guide to Subaqueous Volcaniclastic Facies in the Ancestral Cascades 

Arc in Southern Washington State—The Ohanapecosh Formation and Wildcat 

Creek Beds

C Field-Trip Guide for Exploring Pyroclastic Density Current Deposits from the 

May 18, 1980, Eruption of Mount St. Helens, Washington 

D Field-Trip Guide to Mount St. Helens, Washington—An overview of the Eruptive 

History and Petrology, Tephra Deposits, 1980 Pyroclastic Density Current 

Deposits, and the Crater

E Field-Trip Guide to Mount St. Helens, Washington—Recent and Ancient 

Volcaniclastic Processes and Deposits

F Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and IceCapped Volcanoes—Mount St. Helens, Washington, and Mount Hood, Oregon

G Field-Trip Guide to Mount Hood, Oregon, Highlighting Eruptive History and 

Hazards

H Field-Trip Guide to Mafic Volcanism of the Cascade Range in Central Oregon—A 

Volcanic, Tectonic, Hydrologic, and Geomorphic Journey

I Field-Trip Guide to Holocene Silicic Lava Flows and Domes at Newberry Volcano, 

Oregon, South Sister Volcano, Oregon, and Medicine Lake Volcano, California

J Overview for Geologic Field-Trip Guides to Mount Mazama, Crater Lake Caldera, and 

Newberry Volcano, Oregon

J1 Geologic Field-Trip Guide to Mount Mazama and Crater Lake Caldera, Oregon

J2 Field-Trip Guide to the Geologic Highlights of Newberry Volcano, Oregon

K Overview for Geologic Field-Trip Guides to Volcanoes of the Cascades Arc in Northern 

California

K1 Geologic Field-Trip Guide to Medicine Lake Volcano, Northern California, Including Lava 

Beds National Monument

K2 Geologic Field-Trip Guide to the Lassen Segment of the Cascades Arc, Northern California

K3 Geologic Field-Trip Guide to Mount Shasta Volcano, Northern California

L Geologic Field-Trip Guide to Long Valley Caldera, California

M Field-Trip Guide to a Volcanic Transect of the Pacific Northwest

N Field-Trip Guide to the Vents, Dikes, Stratigraphy, and Structure of the Columbia 

River Basalt Group, Eastern Oregon and Southeastern Washington

O Field-Trip Guide to Flood Basalts, Associated Rhyolites, and Diverse Post-Plume 

Volcanism in Eastern Oregon 

P Geologic Field-Trip Guide to the Volcanic and Hydrothermal Landscape of 

Yellowstone Plateau, Montana and Wyoming

Q Field-Trip Guide to the Petrology of Quaternary Volcanism on the Yellowstone 

Plateau, Idaho and Wyoming

R Field-Trip Guide to Continental Arc to Rift Volcanism of the Southern Rocky Mountains—Southern Rocky Mountain, Taos Plateau, and Jemez Volcanic Fields of 

Southern Colorado and Northern New Mexico 
vi

Contributing Authors

Boise State University

Brittany D. Brand 

Nicholas Pollock

Colgate University

Karen Harpp

Alison Koleszar

Durham University

Richard J. Brown

Eastern Oregon University

Mark L. Ferns 

ETH Zurich

Olivier Bachmann

Georgia Institute of 

Technology

Josef Dufek

GNS Science, New 

Zealand

Natalia I. Deligne 

Hamilton College

Richard M. Conrey

Massachusetts Institute of 

Technology

Timothy Grove 

National Science 

Foundation

Dennis Geist (also with 

 Colgate University and

 University of Idaho)

New Mexico Bureau of 

Geology and Mineral 

Resources

Paul W. Bauer

William C. McIntosh

Matthew J. Zimmerer

New Mexico State 

University

Emily R. Johnson

Northeastern University

Martin E. Ross

Oregon Department of Geology 

and Mineral Industries

William J. Burns

Lina Ma

Ian P. Madin

Jason D. McClaughry

Oregon State University

Adam J.R. Kent

Portland State University

Jonathan H. Fink (also with 

University of British Columbia)

Martin J. Streck

Ashley R. Streig

San Diego State University

Victor E. Camp

Smithsonian Institution

Lee Siebert

Universidad Nacional 

Autónoma de San Luis Potosi

Damiano Sarocchi

University of California, Davis

Kari M. Cooper 

University of Liverpool

Peter B. Kokelaar

University of Northern Colorado

Steven W. Anderson

University of Oregon

Ilya N. Binderman

Michael A. Dungan

Daniele Mckay (also with 

Oregon State University and 

Oregon State University, 

Cascades)

University of Portland

Kristin Sweeney 

University of Tasmania

Martin Jutzeler 

Jocelyn McPhie

University of Utah

Jamie Farrell 

U.S. Army Corps of 

Engineers

Keith I. Kelson

U.S. Forest Service

Gordon E. Grant (also with 

Oregon State University)

U.S. Geological Survey

Charles R. Bacon

Andrew T. Calvert

Christine F. Chan

Robert L. Christiansen

Michael A. Clynne

Michael A. Cosca

Julie M. Donnelly-Nolan

Benjamin J. Drenth

William C. Evans 

Judy Fierstein

Cynthia A. Gardner

V.J.S. Grauch

Christopher J. Harpel

Wes Hildreth

Richard P. Hoblitt

Peter W. Lipman

Jacob B. Lowenstern

Jon J. Major

Seth C. Moran

Lisa A. Morgan

Leah E. Morgan

L.J. Patrick Muffler 

Jim O’Connor

John S. Pallister

Thomas C. Pierson

Joel E. Robinson

Juliet Ryan-Davis

Kevin M. Scott

William E. Scott

Wayne (Pat) Shanks

David R. Sherrod

Thomas W. Sisson

Mark Evan Stelten

Weston Thelen

Ren A. Thompson

Kenzie J. Turner

James W. Vallance

Alexa R. Van Eaton

Jorge A. Vazquez

Richard B. Waitt

Heather M. Wright

U.S. Nuclear Regulatory 

Commission 

Stephen Self (also with University of 

California, Berkeley)

Washington State University

Joseph R. Boro

Owen K. Neill

Stephen P. Reidel 

John A. Wolff 

Acknowledgments

Juliet Ryan-Davis and Kate Sullivan 

created the overview map, and 

Vivian Nguyen created the cover 

design for this collection of field-trip 

guide books. The field trip committee 

is grateful for their contributions.
vii

Contents

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

Context and Terminology .....................................................................................................................1

Defining Flow Types at Volcanoes .....................................................................................................2

Sediment-Gas Flows ...................................................................................................................2

Sediment-Water Flows ...............................................................................................................2

Criteria for Differentiating Volcaniclastic Deposits .................................................................................4

Pyroclastic Density Current Deposits ...............................................................................................4

Pyroclastic-Fall Deposits.....................................................................................................................5

Volcanic Debris-Avalanche Deposits................................................................................................5

Lahar Deposits.......................................................................................................................................6

Volcano-Fluvial Deposits .....................................................................................................................7

Glacial Deposits Composed of Volcaniclastic Particles ................................................................8

Volcaniclastic Deposits of Mount St. Helens............................................................................................9

Holocene Eruptive Activity..................................................................................................................9

Deposits in the Toutle River Drainage Basin..................................................................................11

Flow Processes in 1980 and Associated Deposits...............................................................11

March 1982 Lahar and Associated Deposits ........................................................................15

Post-1980 Sediment Redistribution.........................................................................................15

Toutle River Lahars from Older Eruptive Periods..................................................................18

Deposits in the Lewis River Drainage Basin ..................................................................................18

Volcaniclastic Deposits of Mount Hood...................................................................................................19

Holocene Eruptive Activity................................................................................................................21

Timberline Eruptive Period .......................................................................................................21

Old Maid Eruptive Period..........................................................................................................23

Deposits in the Sandy River Drainage Basin .................................................................................23

Deposits in the White River Drainage Basin..................................................................................26

Polallie-age Flow Deposits.......................................................................................................26

Old Maid-age Flow Deposits....................................................................................................26

Road Log and Description of Field Trip Stops .........................................................................................27

Day 1: Debris-Avalanche and Lahar Deposits in the Toutle River Valley, Mount St. Helens..27

Day 2: Pyroclastic density current, pyroclastic-fall, lahar, and volcano-fluvial deposits in 

Lewis River drainage, Mount St. Helens ...........................................................................49

Day 2: Alternative Route 1 ........................................................................................................61

Day 2: Alternative Route 2 ........................................................................................................63

Day 3: Hyaloclastites, glacial tills, and other diamicts in the Hood River and Sandy River 

basins, Mount Hood..............................................................................................................67

Day 4: Distal lahar and volcano-fluvial deposits downstream of Mount Hood ........................79

Stratigraphy of Timberline-age Section.................................................................................89

Historical accounts of the Sandy River delta during and following the Old Maid 

eruptive period..............................................................................................................89

Acknowledgments.......................................................................................................................................89

References Cited..........................................................................................................................................91
viii

Figures

 1. Quaternary chronologic and morphologic history of Mount St. Helens ........................................10

 2. Map showing full extent of volcaniclastic sediment deposition by the May 18, 1980, and

later 1980 eruptions of Mount St. Helens .....................................................................................12

 3. Aerial photograph of Mount St. Helens taken from the International Space Station in 2009....13

 4. Shaded-relief map showing the impacts of various volcanic and volcaniclastic processes

to the Mount St. Helens area in 1980 ............................................................................................14

 5. Composite lithostratigraphic sections of the May 18, 1980, lateral blast pyroclastic surge

deposit.................................................................................................................................................15

 6. Plot of suspended sediment yields in surrounding rivers since the May 1980 Mount

St. Helens eruption ...........................................................................................................................16

 7. Shaded-relief maps showing topographic changes in the North Fork Toutle River valley 

after the May 1980 Mount St. Helens eruption............................................................................17

 8. Photograph of an outcrop of pre-1980 volcaniclastic deposits along the Swift Creek

riverbank .............................................................................................................................................18

 9. Two photographic views of Mount Hood .............................................................................................19

10. Map showing Mount Hood and the three main river systems that drain the volcano................20

11. Simplified surficial geologic map of the Mount Hood debris fan showing recognized

deposits of Old Maid and Timberline age.....................................................................................22

12. Photographic examples of secondary lahar deposits originating as remobilized domecollapse pyroclastic density current deposits in the upper Sandy River canyon................22

13. Longitudinal profile of the Sandy River channel, starting at Crater Rock—the source

volcanic vent—and continuing downstream to the confluence with the 

Columbia River...................................................................................................................................23

14. Old Maid Flat, the main sediment source of the Sandy River ..........................................................24

15. Schematic stratigraphic relations between Timberline- and Old Maid-age deposits along 

the Sandy River at Oxbow Regional Park.....................................................................................25

16. Photograph of sediment fill in the lower Sandy River valley at Oxbow Regional Park...............26

17. Photograph of thick, incised sediment fill composed of pyroclastic density current and

lahar deposits in the head of White River canyon .....................................................................26

18. Map of locations of planned stops and optional stops for the Mount St. Helens area on

Days 1 and 2 of the field trip............................................................................................................27

19. Shaded-relief map of proximal volcaniclastic deposits at Mount St. Helens...............................28

20. Historical photographic view of Silver Lake with the snow-capped pre-1980 cone of

Mount St. Helens in the distance...................................................................................................29

21. Reconstructed lahar-flow cross sections at peak discharge (assuming synchroneity with 

peak stage) of lahars PC1 and PC3 in the North Fork Toutle River valley at Kid Valley ......30

22. Pine Creek eruptive period stratigraphy based on 1980–83 exposures in the North Fork

Toutle River valley .............................................................................................................................30

23. Photographs of Pine Creek-age deposits.............................................................................................31

24. Photograph of vertical tree mold encased by flood plain facies of PC1 and PC3 lahar

deposits...............................................................................................................................................32

25. Conceptual diagram of origin and transformation of the South Fork Toutle River lahar ............33

26. Photographs of lahar deposits at Harry Gardner Park ......................................................................33

27. Aerial photographic view of the North Fork Toutle River lahar on the afternoon of 

May 18, 1980.......................................................................................................................................34

28. Stage hydrograph at USGS gaging station 14243000, Cowlitz River at Castle Rock, for

May 18–19, 1980.................................................................................................................................34
ix

29. Photographs of North Fork Toutle River lahar deposits.....................................................................35

30. Photograph of the 575-meter-wide sediment retention structure on the North Fork Toutle 

River.....................................................................................................................................................36

31. Sketch of features visible from Castle Lake viewpoint......................................................................36

32. Line drawings and photographs depicting 1980 debris-avalanche kinematics ...........................39

33. Summary of 1980 debris avalanche components before and after edifice collapse ..................40

34. Photographs of barren Mount St. Helens 1980 debris-avalanche deposit shortly after

emplacement .....................................................................................................................................41

35. Sketch of the view towards Mount St. Helens from the Science and Learning Center at

Coldwater............................................................................................................................................41

36. Photograph of rusted logging equipment damaged and partly buried by the lateral

blast......................................................................................................................................................42

37. Photograph of logs of trees blown down by 1980 lateral blast........................................................42

38. Views of the 1980 eruption impacts from Johnston Ridge Observatory ........................................43

39. Photograph of Spirit Lake looking east from Harry’s Ridge..............................................................44

40. Map of the Coldwater Lake area............................................................................................................45

41. Photographs of the 1980 debris-avalanche deposit near Coldwater Lake....................................46

42. Schematic longitudinal cross section through the middle part of the debris-avalanche

deposit located within the North Fork Toutle River valley ........................................................47

43. Cross-sectional profiles of the North Fork Toutle River.....................................................................48

44. Photograph of a low terrace in the North Fork Toutle River valley .................................................48

45. Locations of Day 2 planned stops and optional stops on the south and southeast sides of

Mount St. Helens...............................................................................................................................49

46. Schematic cross section of Lewis River valley...................................................................................50

47. Failure of the Swift Reservoir power canal dam.................................................................................51

48. Cross-sectional view of deposits of the Swift Creek eruptive stage exposed in the Lewis

River valley near the mouth of Swift Creek..................................................................................52

49. Topographic map of the southeast quadrant of Mount St. Helens showing Day 2 stops ..........53

50. Photographic view of the south flank of Mount St. Helens between 8:33 and 8:34 a.m. on

May 18, 1980.......................................................................................................................................54

51. Map of volcanic slopes and river channels on west, south, and east flanks of Mount St.

Helens that were impacted by May 18, 1980, lahars..................................................................55

52. Pyroclastic-fall, pyroclastic density current, and lahar deposits at Stratigraphy Viewpoint 

along the Pine Creek-Muddy River debris fan margin ..............................................................56

53. Close-up photographs of two volcaniclastic units at Stratigraphy Viewpoint .............................58

54. Signs of passage left by 1980 lahars other than major deposits .....................................................59

55. Cross-sectional photographic view of Pine Creek-age lahar deposits and volcano-fluvial

layers in quarry at Stop 8.................................................................................................................59

56. Photograph of a Cougar-age debris-avalanche deposit at Cedar Flats.........................................60

57. Lake-level record for Swift Reservoir on May 18, 1980, and reconstructed hydrographs for

the Pine Creek and Muddy River lahars as they entered Swift Reservoir............................61

58. Evidence of lahars in the lower Muddy River......................................................................................62

59. Colorized digital elevation model of the Portland area......................................................................64

60. Large landslides in Columbia Gorge......................................................................................................65

61. Photograph of headscarps of the Bonneville landslide and Red Bluffs landslide in the

background, with the truncated toe of the Bonneville landslide deposit in the left 

foreground..........................................................................................................................................67

62. Locations of planned stops and optional stops for field trip Days 3 and 4 at Mount Hood........68
x

63. Deposits of a late Pliocene basaltic hyaloclastite delta, located along Interstate 84, just

west of Hood River, Oregon.............................................................................................................69

64. Photographs of edifice-collapse deposits from the Hood River lahar at Mount Hood...............70

65. Location of planned stops and optional stops on Days 3 and 4 that are proximal to Mount

Hood.....................................................................................................................................................71

66. Photographs of the Eliot Branch lahar source and deposit .............................................................72

67. Eliot Branch channel before and after deposition of the Eliot Branch debris flow, with the 

Parkdale lava flow largely blocking the channel........................................................................73

68. Impacts of Polallie Creek debris flow on December 25, 1980, on the east side of

Mount Hood........................................................................................................................................74

69. Photograph of small glaciers remaining on Mount Hood .................................................................75

70. Photographs of glacial till deposits at Bennett Pass .........................................................................75

71. Photographs of lithic pyroclastic density current and secondary lahar deposits in the

upper White River valley..................................................................................................................77

72. History of debris-flow damage to the Oregon Route 35 bridge crossing the White River.........78

73. Photograph of the Timberline Lodge on the southwest flank of Mount Hood in winter.............79

74. Photographs of volcaniclastic deposits of mainly Old Maid age exposed at Old Maid Flat,

about 15 kilometers downstream of Mount Hood ......................................................................81

75. Photographic view of Sandy River from Jonsrud viewpoint outside of Sandy, Oregon.............82

76. Shaded-relief map of the lower Sandy River with geology of the Sandy River delta shown ....83

77. Simplified cross sections along the low-gradient depositional reach of the Sandy River in

Oxbow Regional Park .......................................................................................................................84

78. Digital elevation model with 2-meter resolution of the Sandy River at Oxbow Regional Park..85

79. Photographs of sediments emplaced during the Old Maid eruptive period at Oxbow

Regional Park.....................................................................................................................................87

80. Holocene stratigraphy and chronology at Stop 9, as exposed in a riverbank outcrop along

the west bank of the Sandy River delta........................................................................................88

81. Photographs of lithofacies within the uppermost Timberline-age lahar deposit exposed at 

the mouth of the Sandy River..........................................................................................................90

Tables

1. Categories of rock fragments comprising volcanic sediments, based on fragment origin.......3

2. Time-discrete flow events and mechanical flow types involving two-phase flows of solid 

and fluid mixtures at volcanoes .................................................................................................3

3. Major eruptive, noneruptive geomorphic, and glacial events recorded at Mount Hood 

during the past 100,000 years ...................................................................................................21
xi

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as °F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as °C = (°F – 32) / 1.8.

Conversion Factors

[International System of Units to U.S. customary units]

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

meter (m) 1.094 yard (yd) 

kilometer (km) 0.6214 mile (mi)

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic meter (m3

) 264.2 gallon (gal) 

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 1.308 cubic yard (yd3) 

cubic kilometer (km3

) 0.2399 cubic mile (mi3) 

Flow rate

meter per second (m/s) 3.281 foot per second (ft/s) 

meter per second (m/s) 2.237 miles per hour (mi/h)

meter per year (m/yr) 3.281 foot per year ft/yr) 

centimeter per year (cm/yr) 0.3937 inch per year (in/yr)

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

Mass

metric ton (t) 1.102 ton, short [2,000 lb]

metric ton (t) 0.9842 ton, long [2,240 lb]

ton, short [2,000 lb] 0.9072 metric ton (t)

Geologic Field-Trip Guide of Volcaniclastic Sediments 

from Snow- and Ice-Capped Volcanoes—Mount St. 

Helens, Washington, and Mount Hood, Oregon 

By Thomas C. Pierson1, Lee Siebert2, Christopher J. Harpel1, and Kevin M. Scott1

1

U.S. Geological Survey.

2

Smithsonian Institution.

3

The term sediment, in the singular, is usually applied to particulate material 

held in suspension in water or deposited from suspension. The term in the 

plural is generally applied to all types of fragmental deposits, not just those 

carried by a flowing fluid (Neuendorf and others, 2005).

Introduction

This field guide for the International Association of 

Volcanology and Chemistry of the Earth’s Interior (IAVCEI) 

Scientific Assembly 2017 focuses on volcaniclastic sediments 

from Mount St. Helens in Washington and Mount Hood in 

Oregon. The trip spends four days in the field and includes nine 

stops at each volcano. For completeness, this guidebook also 

includes sixteen optional stops in the Mount St. Helens area and 

three in the Mount Hood area. These two volcanoes provide 

excellent depositional records of the broad spectrum of volcanic 

hazards that involve the flow or fall of volcaniclastic particles. 

At the field-trip stops we will contrast and compare the different 

types of deposits—sediments that can be frustratingly difficult 

to distinguish from one another. Correct identification of deposit 

origin leads to a better understanding of hazards that can impact 

vulnerable communities at particular volcanoes. 

During this four-day field trip, we will spend two days 

in the Mount St. Helens area and two days near Mount Hood. 

On the first day, we will visit debris-avalanche and lahar 

deposits in the Toutle River valley in Washington. The second 

day will focus on pyroclastic density current, pyroclastic-fall, 

lahar, and volcano-fluvial deposits in the Lewis River drainage 

system near Mount St. Helens. On the third day, we will visit 

hyaloclastites, glacial tills, and other diamicts in the Hood River 

and Sandy River basins near Mount Hood. The fourth and 

final day will focus on distal lahar and volcano-fluvial deposits 

downstream of Mount Hood. 

What follows is an in-depth introduction to the deposits 

studied on this field trip. We provide criteria that are observable 

in the field to aid in differentiating between pyroclastic density 

current, pyroclastic-fall, debris-avalanche, lahar, water-flood, 

and glacial deposits. We also introduce the Holocene eruptive 

histories of Mount St. Helens and Mount Hood and discuss the 

processes responsible for deposit emplacement. Field-trip stops 

and features of interest along the route are described in detail in 

a road log that provides daily cumulative mileage. 

Context and Terminology

Volcaniclastic sediments3 encompass fragmental geologic 

deposits composed of volcanic rock particles transported and 

deposited by pyroclastic processes, geomorphic processes, 

or a combination of the two. Thus, they record the history of 

both pyroclastic processes that are typically hot and dry and 

volcano-hydrologic processes that are typically cool and involve 

mixtures of fragmental rock debris and water. The bulk of 

volcaniclastic sediments are produced by explosive eruptions 

that generate large volumes of tephra and other fragmental rock 

debris. Explosive eruptions are most common at volcanoes 

of intermediate and silicic composition in volcanic arcs. All 

processes emplacing volcaniclastic sediments can be hazardous, 

but some can be more hazardous than others.

Terminology for volcaniclastic particles and deposits is not 

entirely consistent. Here, we adhere to the definitions used by 

White and Houghton (2006). Primary volcaniclastic particles 

and deposits are those emplaced by or during the eruption of 

rock that is fragmented during the eruption process itself or 

during gravitational collapse of still-hot lava domes or flows. 

Specific depositional processes include pyroclastic density 

currents (PDCs)—including block-and-ash flows—tephra 

fall, and primary lahars. Secondary volcaniclastic particles 

and deposits result from the remobilization and redeposition 

of primary deposits by gravity-driven mass flows or through 

mobilization and transport by flowing water, ice, or air. Major 

depositional processes in this case include debris avalanches, 

lahars, water floods, and glaciers.

The geologic, sedimentologic, and stratigraphic characteristics of primary volcaniclastic deposits have been classified and 

well documented over the years, but the characteristics of secondary volcaniclastic deposits have received much less attention (for 

example, Fisher, 1961, 1966; Walker, 1971; Wright and others, 

1980, 1981; Fisher and Schmincke, 1984; Cas and Wright, 1987; 

McPhie and others, 1993; Branney and Kokelaar, 2002; White and 

Houghton, 2006). Most of this attention came after a string of late 

20th century eruptions demonstrated the danger and long reach of 

processes emplacing these sediments (Manville and others, 2009): 
2 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

the May 18, 1980, eruption of Mount St. Helens (United States), 

the November 13, 1985, eruption of Nevado del Ruiz (Colombia), 

and the June 15, 1991, eruption of Mount Pinatubo (Philippines). 

Most detailed descriptions of secondary volcaniclastic deposits 

are process-specific or location-specific. Reviews and overviews 

that have generalized the characteristics of secondary volcaniclastic deposits more broadly have been limited in scope and detail, 

and they do not make direct detailed comparisons with primary 

volcaniclastics (for example, Smith, 1986; Cas and Wright, 1987; 

McPhie and others, 1993; Manville and others, 2009).

Fragmental volcanic deposits are notoriously difficult to 

identify and interpret in the field for a variety of reasons (Orton, 

1996):

• A wide variety of both eruptive and noneruptive 

depositional processes can produce fragmental deposits 

with similar characteristics.

• Depositional processes commonly evolve over short 

periods and distances, causing deposits with different 

characteristics to overlap spatially and grade into each 

other.

• Textural and compositional variations can have multiple 

causes.

• Mineral grains and glass fragments are susceptible to 

chemical alteration in harsh volcanic environments, which 

can change deposit appearance.

• Localized flow paths and variations in sediment supply and 

transport can result in spatially irregular deposits.

• Deposits are highly erodible and can have ephemeral 

residence times in the geologic record.

• Deposits are vulnerable to disruption and burial by 

subsequent volcanic or volcano-tectonic activities and 

processes.

As most field volcanologists already recognize, 

fragmental deposits emplaced subaerially by primary 

volcanic processes (PDCs and tephra fall), those emplaced by 

secondary volcano-hydrologic processes (debris avalanches, 

lahars, and muddy floods), and even those emplaced by alpine 

glaciers on the flanks of volcanoes, can all look similar in 

outcrop. This field trip is organized to provide opportunities 

to visit a wide range of deposit types, so that the critical and 

often subtle physical differences among these deposits can 

be examined and discussed. Except for the hyaloclastites to 

be seen on Day 3, the sediments visited during this field trip 

were deposited in subaerial settings. Deposition in marine or 

lacustrine environments is complicated by size- and densitycontrolled settling behavior of particles through a water 

column, which affects grain-size distributions and stratification 

differently than on land. 

Classification of deposits composed of volcanic rock 

fragments has been based on clast-forming processes (Fisher, 

1961, 1966; Fisher and Schmincke, 1984), on transport and 

depositional processes (Cas and Wright, 1987), and on a 

combination of both (McPhie and others, 1993; White and 

Houghton, 2006) (table 1). Regardless of the classification 

scheme, volcanologists agree that rock fragments that originate 

at volcanoes as solidified particles of erupting lava and then 

move to their place of deposition by nonvolcanic geomorphic 

processes constitute a separate and important category of 

volcanic deposits—termed secondary volcaniclastic deposits 

(Orton, 1996; Manville and others, 2009).

Defining Flow Types at Volcanoes

Primary and secondary volcaniclastic deposits are commonly 

emplaced by flow processes involving a solid phase (volcaniclastic 

particles) and a fluid phase (water or gas). The spectrum of 

two-phase gravity flows occurring subaerially at volcanoes are 

generally grouped into four basic categories of time-discrete flow 

events—pyroclastic density currents, debris avalanches, lahars, 

and floods. In terms of flow mechanics, we can describe four 

basic flow types, but flow types do not always have a one-to-one 

correlation with flow events (table 2). These flow types comprise 

two varieties of sediment-gas flow and two varieties of sedimentwater flow:

Sediment-Gas Flows

• Dry to damp granular flow. The dominant fluid phase is 

gas; particle-particle interactions primarily control flow 

behavior.

• Dry to damp fluid flow. The dominant fluid phase is 

gas; fluid properties primarily control flow behavior.

Sediment-Water Flows

• Water-saturated granular flow. The dominant fluid 

phase is water; particle-particle interactions primarily 

control flow behavior.

• Water-saturated fluid flow. The dominant fluid phase is 

water; fluid properties primarily control flow behavior. 

Dilute lahars (hyperconcentrated flows) behave as 

non-Newtonian fluids, but water floods behave as 

Newtonian fluids.

Boundaries between flow types are not sharp, either in 

terms of flow behavior or the characteristics of the deposits 

they emplace. Each flow type has the potential of transforming 

from one to another as the proportions of fluids to solids or 
Introduction 3

Table 1. Categories of rock fragments comprising volcanic sediments, based on fragment origin. The two basic types of volcanic 

sediments—sediments composed of particles of volcanic rock—are volcaniclastic and epiclastic sediments. Blank spaces indicate 

origins that were not addressed by the referenced authors.

Origin of volcanic rock fragments White and 

Houghton (2006)

McPhie and 

others (1993)

Cas and Wright 

(1987)

Fisher and 

Schmincke (1984)

Fragments of volcanic rock formed and dispersed by any 

process

Volcanic Volcaniclastic Volcaniclastic Volcaniclastic

Rock fragments ejected from a volcanic vent, regardless of 

eruption cause, fragment origin, or fragment composition

Primary 

volcaniclastic

Pyroclastic Pyroclastic Pyroclastic

Particles formed by fragmentation and ejection of magma 

in an eruption

Pyroclastic

 (juvenile)

Pyroclastic

Particles formed by steam-driven explosion Pyroclastic Hydroclastic

Particles formed by quench-shattering of lava in contact 

with water, water-saturated sediment, or ice

Autoclastic 

(hyaloclastic)

Autoclastic Hydroclastic

Fragments of lava formed by mechanical fragmentation of 

flowing (and solidifying) lava flows or domes

Pyroclastic 

(autoclastic)

Pyroclastic Autoclastic

Fragments of previously formed rock (volcanic or 

nonvolcanic) incorporated into a pyroclastic deposit

Lithic Lithic Lithic Lithic

Lithic fragments of nonvolcanic rock or volcanic rock not 

related to the erupting volcano

Accidental lithic Accidental 

lithic

Accidental lithic Accidental lithic

Lithic fragments of volcanic rock originating not from the 

erupting volcano but from a previous eruption

Accessory lithic Accessory

 lithic

Accessory lithic Accessory lithic

Lithic fragments of nonvesiculated juvenile magma in 

contrast to pumice from the same eruption

Cognate lithic Cognate lithic Cognate lithic Cognate lithic

Primary volcaniclastic rock fragments remobilized from 

an unconsolidated deposit at its initial site of deposition 

and transported to another site (one or more times) by 

nonvolcanic geomorphic processes

Secondary 

volcaniclastic

Resedimented 

volcaniclastic

Epiclastic Secondary 

pyroclastic 

(reworked 

pyroclastic)

Particles formed by physical breakdown of lithified 

volcanic or nonvolcanic rock, whether fresh or weathered

Epiclastic Volcanogenic 

sedimentary 

(epiclastic)

Epiclastic Epiclastic

Table 2. Time-discrete flow events and mechanical flow types involving two-phase flows of solid and fluid mixtures at volcanoes. 

1

Small amounts of gas (typically air) are commonly found in water-saturated sediment mixtures (as much as ~5 percent by volume) and similarly small 

amounts of liquid water may be found in dry flows.

2

 Relative importance indicated by mathematical symbols: >>, much greater than; >, greater than; ≈, approximately equal.

Flow event Flow type1 Dominant 

fluid phase Factors that determine flow behavior2

Debris avalanche Dry to damp granular flow Gas Particle interactions >> fluid properties

Concentrated pyroclastic density current (pyroclastic flow) Dry to damp granular flow Gas Particle interactions > fluid properties

Dilute pyroclastic density current (pyroclastic surge) Dry to damp fluid flow Gas Fluid properties > particle interactions

Concentrated lahar (debris flow) Water-saturated granular flow Water Particle interactions ≈ fluid properties

Dilute lahar (hyperconcentrated flow) Water-saturated fluid flow Water Fluid properties > particle interactions

Water flood Water-saturated fluid flow Water Fluid properties >> particle interactions
4 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

particle-size distributions change (through sediment entrainment 

or selective deposition) and as flow kinetic energy (manifest 

as granular temperature or turbulence) increases or decreases 

(Iverson and Vallance, 2001; Iverson, 2005; Pierson, 2005). 

Different flow types emplace subtly different sedimentary 

deposits that, in most cases, have characteristically identifiable 

sedimentologic, stratigraphic, and morphologic properties. 

Flows having sediment concentrations near the transformation 

boundaries between flow types, however, emplace deposits that 

can be particularly hard to identify.

Flow events involving two-phase flow in volcanic settings 

typically do not travel their entire flow paths as a single flow 

type; instead they commonly transform from one flow type to 

another (Scott, 1988a; Smith and Lowe, 1991; Vallance and 

Scott, 1997). For example, a crater-lake outburst flood can begin 

as a water flood, entrain sediment and transform over a short 

distance to become a debris-flow lahar, then transition more 

gradually to a hyperconcentrated-flow lahar through selective 

deposition of coarse sediment and dilution by water input from 

tributary streams, and finally change back to a water flood through 

continued dilution (Waitt and others, 1983; Pierson and Scott, 

1985; Pierson, 1999). As in this example, flow transformations 

can go in either direction, with the flow mixture becoming more 

concentrated or more dilute with time and distance traveled. 

Despite alternative definitions of the term flow transformation (for 

example, Fisher, 1983), transformations involving change from 

one flow type to another are not necessarily related to changes 

in hydraulic flow states or regimes, such as laminar/turbulent, 

subcritical/supercritical, steady/unsteady, or uniform/nonuniform 

flow transitions, but hydraulic changes may accompany changes in 

flow type nevertheless. Flow transformations have been observed 

or inferred in all types of volcanic granular and fluid flows.

Criteria for Differentiating 

Volcaniclastic Deposits

Pyroclastic Density Current Deposits

Pyroclastic density currents (PDCs) are density-stratified 

flows containing variably dispersed mixtures of juvenile rock 

fragments (usually hot) of varying particle size and density, plus 

some fraction of entrained accidental particles, magmatic gases, 

and entrained air (Fisher and Schmincke, 1984; Cas and Wright, 

1987; Carey, 1991; Freundt and others, 2000; Valentine and Fisher, 

2000; Wilson and Houghton, 2000; Branney and Kokelaar, 2002; 

Sarocchi and others, 2011; Sulpizio and others, 2014; Dufek 

and others, 2015). Most commonly driven by gravity, PDCs can 

also be driven by impulsive forces from phreatic or magmatic 

explosions (for example, base surges or directed blasts). Flow is 

typically highly turbulent when particle-gas mixtures are dilute 

and more laminar when highly concentrated. Such flows vary both 

temporally and spatially in bulk density, composition, temperature, 

transport mechanics, and velocity as they move. Emplacement 

temperatures for relatively small, dry PDCs have been measured 

as high as about 850 °C, but temperatures can reach 1,000 °C for 

large silicic ignimbrites (Hoblitt and Kellogg, 1979; Banks and 

Hoblitt, 1981; Voight and Davis, 2000; Charbonnier and Gertisser, 

2008; Bryan and others, 2010; Brown and Andrews, 2015). On 

the other hand, emplacement temperatures can be less than 100 °C 

for wet PDCs triggered by phreatic eruptions, phreatomagmatic 

eruptions, or eruptions through water bodies (Moore, 1967; Fisher 

and Waters, 1970; Lorenz, 1974).

PDC deposits exhibit a wide range of lithofacies that include 

poorly sorted diamicts, as well as stratified and unstratified gravels, 

gravelly sands, sandy gravels, and sands. This variability reflects 

the variety of eruption styles, rates, components, and depositional 

conditions that produce them (Sparks and others, 1973; Lorenz, 

1974; Wilson and Walker, 1982; Fisher and Schmincke, 1984; Cas 

and Wright, 1987; Branney and Kokelaar, 2002). PDC deposits 

can closely resemble lahar deposits in outcrop. Characteristic 

features to aid in the identification of relatively fresh (unaltered) 

PDC deposits include:

• Sediment mixture compositions wholly or mostly 

monomictic (composed of particles of a single rock 

species—that is, from a single magma batch).

• Lithofacies of concentrated PDCs typically massive, 

commonly coarse grained, and commonly graded 

(inverse, normal, or both).

• Lithofacies of dilute PDCs commonly stratified, with 

either sharp or diffuse bedding contacts, but can 

be massive; crosscutting contacts common within 

depositional units.

• Low-angle crossbedding common in finer grained 

deposits.

• Moderate to very poor sorting (We report sorting values 

(σ) as the Inclusive Graphic Standard Deviation of the 

particle size (φ), as defined by Folk, 1974).

• Consistency generally friable (dry)/soft (moist) owing 

to paucity of fine particles, unless cemented by vaporphase alteration or welding.

• Evidence of high emplacement temperature common 

(but not required), including fragments of charred 

vegetation, bread-crusted or prismatically jointed clasts, 

pinkish deposit surface coloration, welded zones, and 

uniform thermoremanent magnetic orientation of clasts. 

• Bubble vesicles typically absent, except in deposits 

from phreatic or phreatomagmatic eruptions, due 

to elutriation of fines; however, matrix may contain 

small voids between particles (a miniature open-work 

structure).

• Sub-2-millimeter (mm) size fraction (ash) dominated 

by crystals in concentrated PDC deposits and by vitric 

shards in dilute PDC deposits.
Criteria for Differentiating Volcaniclastic Deposits 5

• Subvertical gas-escape pipes common (elutriated conduits, 

often with surrounding cylindrical zones hardened by 

vapor-phase mineral sublimation and oxidation).

Pyroclastic-Fall Deposits

Subaerial pyroclastic-fall deposits are emplaced by the 

lofting of volcanic rock particles into the air by eruptive processes 

and their subsequent gravity-driven fallout through the air back to 

earth. Lofting is accomplished by (1) explosive ballistic ejection or 

pressurized jetting of solid rock fragments or molten lava spatter 

from volcanic vents and (2) suspension of relatively fine-grained 

particles in turbulent air currents by heat-driven convection and 

wind, either directly from source vents or from elutriation of 

ash from PDCs that are undergoing clast fragmentation during 

downslope movement (Walker, 1981; Fisher and Schmincke, 

1984; Cas and Wright, 1987; Wilson and Houghton, 2000; 

Houghton and others, 2000; Branney and Kokelaar, 2002).

Pyroclastic-fall deposits are typically better sorted than most 

volcaniclastic sediments and bedded lithofacies may superficially 

resemble volcano-fluvial deposits—the main difference being 

that the latter are found only as horizontal fills in channels and on 

valley floors, whereas air-fall tephras mantle the entire landscape 

with slope-parallel deposits. Sedimentologic variations reflect 

differences in eruption magnitudes, styles, rates, and components, 

as well as distance from the source vent, wind velocities, and wind 

directions (Walker, 1981; Fisher and Schmincke, 1984; Cas and 

Wright, 1987; Wilson and Houghton, 2000; Houghton and others, 

2000; Houghton and Carey, 2015). Characteristic features of fresh 

pyroclastic-fall deposits include:

• In time-equivalent stratigraphic units, particle mixtures 

usually are wholly or mostly monomictic (for example, an 

aphyric rhyolite pumice with 75 weight percent SiO2, or a 

hornblende- and plagioclase-phyric low-silica dacite). 

• In tephra sequences from prolonged eruptions or eruptions 

involving magma mixing, composition may be oligomictic 

(composed of a single rock type; for example, a rhyolite 

with compositional variation) or polymictic (composed of 

different rock types; for example, rhyolite and basalt).

• Depositional units can be stratified or massive, but 

stratification is common.

• Both lithic and juvenile particles are typically angular.

• Commonly moderately to well sorted.

• Bedding is plane-parallel with beds and laminae reflecting 

orientation of underlying topography; little to no 

deposition on slopes steeper than angle of repose unless 

wet (cohesive) or anchored by vegetation.

• Beds laterally continuous over long distances with 

relatively uniform thickness.

• Beds commonly graded (normal or inverse).

• Bedding contacts may be sharp but are commonly diffuse.

• Deposits well sorted with respect to aerodynamic size 

equivalence of particles (for example, fine dense ash 

particles may fall together with scoriaceous lapilli).

• Accretionary lapilli, cored lapilli, or ash pellets 

common, but not always well preserved.

• Intergranular voids common (size in proportion to 

particle size).

• Bubble vesicles commonly found only in fine ash from 

phreatic or phreatomagmatic eruptions; rarely found in 

ash layers subjected to heavy rainfall.

• Friable (dry)/soft (moist) consistency; loose packing.

• Leaves and fine twigs distributed throughout 

accumulations of coarse tephra in forested terrain (not 

in discrete layers); commonly such vegetal debris 

is progressively stripped from overhanging trees by 

abrasion from tephra fall.

Volcanic Debris-Avalanche Deposits

Volcanic debris avalanches are rapid, unsaturated, 

gravitational flows of non-coherent, unsorted granular mixtures 

of rock fragments (parts of failed edifice with or without 

juvenile lava) and ice, soil, wood fragments, and other debris 

entrained during transport, with varying amounts of air, 

volcanic gases, or liquid water in the dilated interstices between 

particles (Ui, 1983, 1989; Voight and others, 1983; Schuster 

and Crandell, 1984; Siebert, 1984, 1996; Ui and others, 2000; 

van Wyk de Vries and Davies, 2015). Volumetric water contents 

in these flows may vary from 0 to about 30 percent. Flow 

mechanics are principally controlled by inertial and frictional 

forces between moving particles and between the particles 

and the bed (Savage, 1984; Iverson and Vallance, 2001), but 

movement also involves rigid-body frictional sliding at the 

outset before disaggregation and deformation of the failed mass 

occurs.

Major lithofacies of volcanic debris-avalanche deposits 

include (1) mixed facies—disaggregated and well-mixed solid 

rock fragments ranging from clay-size grains to blocks larger 

than 10 meters (m) (that is, a mixture of clasts and matrix, 

where matrix is defined as particles less than 2 mm in diameter) 

and (2) block facies—deposits dominated by very large, highly 

fractured, unconsolidated to semi-consolidated pieces of the 

failed edifice that have been transported mostly intact to their 

point of deposition (Glicken, 1996). Characteristic identifying 

features include:

• Hummocky surface topography composed of irregularly 

shaped and distributed hills (hummocks) that are 

commonly tens to hundreds of meters in diameter and that 

have intervening closed depressions.
6 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

• Polymictic sediment composition (that is, composed of 

particles of multiple types of volcanic rock with or without 

accidental nonvolcanic rock fragments).

• Abundant large debris blocks composed of pervasively 

fractured, unconsolidated to semi-consolidated segments 

of the failed source volcanic edifice that commonly retain 

their original primary stratification.

• Unusually thick depositional units (commonly greater than 

100 m) that are devoid of primary stratification.

• Steep, sharply defined lateral and frontal margins.

• Abrupt color changes from the uniform neutral grays and 

browns of the mixed-material facies to more intense colors 

(typically black, white, tan, red, orange, or yellow) of 

relatively fresh or hydrothermally altered, intact rock units 

from the failed edifice.

• Jigsaw fracturing of blocks, even down to the size of sand 

grains.

• Angular shape of clasts and particles smaller than 2 mm in 

matrix; clasts commonly fractured extensively.

• Clastic dikes relatively common, with irregular margins 

and variable orientation.

• Plastic deformation of small blocks common—including 

elongation, folding, smearing, and contorting.

• Absence of true stratification, although intra-deposit 

shearing can produce subhorizontal shear zones that 

resemble bedding.

• Large clasts/blocks commonly clustered within outcrops 

(that is, large particles tend not to be randomly or 

uniformly distributed in outcrops).

• Mixed facies (small clasts and matrix) very or extremely 

poorly sorted.

• Absence of bubble vesicles and grading.

• Absence of strongly developed fabrics.

• Absence of fluid-escape structures (in most cases).

Lahar Deposits

Lahars are discrete, rapid, gravity-driven flows involving 

fully saturated, high-concentration mixtures of water and 

solid particles (fragments of rock, ice, wood, and other debris) 

originating from volcanoes (Smith and Fritz, 1989; Smith and 

Lowe, 1991; Vallance, 2000; Vallance and Iverson, 2015). Lahar 

flow types include both debris flow and hyperconcentrated flow—

two distinct flow types that commonly occur within a single lahar 

and both of which behave as non-Newtonian fluids (Crandell, 

1971; Smith and Lowe, 1991; Pierson, 2005). Thus, a lahar is not 

a flow type but rather a high-discharge flow event that commonly 

includes both debris flow and hyperconcentrated flow. When 

lahars occur in channels occupied by streams, their faster highconcentration flow fronts commonly push stream water ahead 

and mix with it. This creates wedges of muddy water, similar to 

bow waves, that become part of the lahars but precede the highconcentration parts. In some cases, these dilute “bow waves” 

produce the highest stages and peak discharges of lahars (Pierson 

and Scott, 1985; Cronin and others, 1999; Dinehart, 1999).

Deposits emplaced by the debris-flow phases of lahars are 

massive (unstratified) diamicts that can commonly be described 

as very/extremely poorly sorted muddy sandy gravels or muddy 

gravelly sands. In terrains affected by recent eruptions, fineparticle (silt and clay) content is commonly less than 10 weight 

percent, but the proportion can be as high as 25 weight percent 

if lahars originate from edifice failures (for example, Vallance 

and Scott, 1997). In levees or deposit margins where clasts are 

concentrated and matrix material has been partly or entirely 

removed, deposits typically have an openwork structure with 

virtually no fines or even sand. Single debris-flow depositional 

units are commonly graded but never stratified—this is a key 

distinguishing characteristic. Single flow units are seldom 

thicker than 1–2 m, unless ponded behind barriers or flow 

constrictions. However, sequences of multiple flow units can 

be many meters thick. If flow units are deposited in quick 

succession, such as by successive flow surges during a single 

lahar, sharp contacts may not form between flow units (Major, 

1997). Characteristic identifying features of deposits emplaced 

by debris flows include:

• Massive internal structure; no stratification (bedding).

• Very poor to extremely poor sorting (generally greater than 

2.0σ). 

• Both clast-supported and matrix-supported structures 

possible.

• Polymictic sediment compositions in most cases, although 

lahars can start as monomictic mixtures (for example, 

particles from a dome collapse) and become progressively 

oligomictic or polymictic with distance traveled as bed 

material from stream valleys is entrained.

• Matrix (particles less than 2 mm) and clasts packed tightly 

together in deposit interiors—no openwork structure 

except in flow levees and deposit margins, where fluid 

matrix has flowed or been washed away.

• Bubble vesicles (~1 mm) common throughout deposits.

• Debris-flow deposit outcrops relatively uniform and 

homogeneous in appearance; no abrupt changes in color, 

texture, or spatial distribution of clasts and matrix within 

single flow units.

• Both inverse and normal grading common, sometimes 

both in a single flow unit with inverse grading at base and 

normal grading at top; some deposits ungraded.
Criteria for Differentiating Volcaniclastic Deposits 7

• Clast fabric commonly weak, although strong fabric 

may be observed with elongate woody debris on deposit 

surfaces. Imbrication can develop in boulder bed forms. 

• Clasts generally not fractured.

• Uncharred wood fragments common (if trees encountered 

along flow path).

• No evidence of high heat (in most cases). Hot lahars are 

less than 100 °C, but remobilization of PDC deposits or 

sediments containing charcoal from wildfires may entrain 

charcoal fragments in lahar deposits.

• Relatively fine-grained basal layers (also called sole layers) 

and post-depositional dewatering cap layers may be 

present.

• Water-escape structures (subvertical elutriated pipes) may 

be present in deposits of relatively dilute noncohesive 

flows, but these are uncommon.

• Cobbles and boulders commonly show gross rounding, 

although surfaces not smoothed; pebbles typically are 

angular unless the lahar entrained stream gravel.

• Flat to slightly convex deposit surfaces, commonly 

studded randomly with boulders protruding above the 

surface of deposits.

• Extensive damage to trees growing near the main axis of 

flow, but relatively minor damage along flow margins; tree 

trunks typically coated in a thin layer of sandy mud.

• Lateral flow levees constructed of undifferentiated debris 

mixtures, concentrations of boulders (commonly with 

openwork structure), or battered logs.

• Hard (dry)/firm (moist) consistency—high degree of 

consolidation, particularly in cohesive debris-flow 

lahar deposits; deposits break off in chunks rather than 

individual particles when dug or struck with tools.

Deposits emplaced by the hyperconcentrated-flow phases 

of lahars may exhibit lithofacies that can range from (1) massive, 

relatively fine-grained, clast-supported gravelly sand diamicts to 

(2) faintly or diffusely horizontally bedded gravelly sands (with 

bedding commonly interrupted by isolated gravel lenses and 

outsized clasts) to (3) distinctly bedded alternating layers of sand 

and fine gravel. The coarser, more massive lithofacies generally 

occur along channel axes (channel facies) and are distinguished 

from sandier, more distinctly bedded lithofacies (lateral or 

overbank facies) along channel margins (Pierson and Scott, 1985; 

Scott, 1988a; Cronin and others, 2000). Hyperconcentratedflow deposits tend to become progressively finer grained and 

more distinctly bedded with distance from source. Characteristic 

sedimentologic features of hyperconcentrated-flow deposits 

include:

• Sorting values between those of debris-flow and waterflood deposits, namely in the range of about 0.9 to 

2.0σ; most deposits range between 1.1 and 1.6σ (Scott, 

1988a; Cronin and others, 2000).

• Deposits typically stratified, although channel facies 

deposits can be massive.

• Bedding is characteristically horizontal and plane 

parallel; contacts between beds are typically faint or 

diffuse.

• Low-angle crossbedding from migrating upper regime 

bed forms may be observed. High-angle (ripple- and 

dune-type) cross-stratification (that is, bedding angles 

steeper than about 15° from plane of channel bed) has 

never been reported.

• Isolated outsized clasts (cobbles and in some cases 

boulders) in outcrops of otherwise sandy beds are common; outsized clasts also common on surfaces of fresh 

deposits.

• Thin, bedding-parallel laminations of clayey silt to 

silty fine sand (cap layers) commonly mark contacts 

between depositional units. Flow pulses within a single 

lahar may emplace separate flow units with cap layers.

• Cap layers commonly deformed into dish structures—

water escape structures, formed by upward migration 

of pore fluid during deposit consolidation.

• Layers of bubble vesicles may be concentrated beneath 

silty cap layers (concentrated during upward migration 

of pore water).

• Soft-sediment deformation common (caused by 

undrained loading).

• Consistency slightly hard (dry)/slightly firm (moist). 

Massive deposits not as firm or hard as debris-flow 

lahar deposits. When dry outcrops are kicked or struck, 

deposits typically break off to produce some fragile 

chunks and some raveling of individual grains.

Volcano-Fluvial Deposits

Volcano-fluvial deposits are mainly emplaced by volcanic 

water floods—turbulent, high-discharge floods of muddy water 

transporting very high sediment loads from relatively small 

drainage basins in volcanically disturbed landscapes. However, 

volcano-fluvial deposits also include sediments emplaced by 

smaller discharge and much lower concentration streamflows, 

which commonly rework and change the character of muddy 

flood deposits. Volcanic water-flood deposits generally have 

more in common with flash floods in arid terrains than with 

meteorologic floods in large, well-vegetated drainage basins. 

Commonly triggered by heavy rainfall, they can also be 

generated by lake outbreaks and glacier outbursts. Like lahars, 

volcanic water floods are discreet flow events, but by definition 
8 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

these flows have sediment concentrations below the threshold 

of hyperconcentrated flow and behave as Newtonian fluids. 

Though generally not as destructive as lahars at equivalent 

discharges, volcanic water floods nevertheless typically 

transport enormous quantities of sediment and have significant 

geomorphic impacts on river channels.

Volcanic water-flood deposits are typically stratified and 

can range from bouldery conglomerates to finely laminated 

fine sands and silts, depending on channel slope, proximity to 

sediment sources, and discharges of the floods that emplaced 

them. Characteristic deposit features include:

• Most deposits well stratified—water-flow deposits 

nearly everywhere show distinct bedding with sharp 

boundaries between beds or laminae owing to abrupt 

changes in grain size during deposition. However, in 

deposits without much variation in grain size, bedding 

may be faint or appear massive. Strata commonly 

millimeters to centimeters in thickness. Bioturbation 

may obscure original bedding.

• Stratified deposits moderately to moderately well 

sorted—generally from 0.5 to 0.9σ for sandy deposits 

(Scott, 1988a; Letsinger, 1994). Deposits containing 

gravel can be more poorly sorted.

• Consistency very friable or loose (dry)/soft to very soft 

(moist). Sediment grains in dry outcrops ravel easily 

when disturbed.

• High-angle (greater than 15°) dune or ripple 

crossbedding common.

• Absence of bubble vesicles.

• Openwork structure common in gravelly facies.

• Imbrication of non-equant clasts common, with the 

flattest surface dipping upstream.

• Ungraded in most cases (upward fining may be 

observed locally).

• Polymictic sediment compositions; reworked epiclastic 

particles commonly mixed with fresh volcaniclastic 

particles.

Volcanic water-flood deposits are typically aggradational, 

rather than erosional, because the amount of sediment supplied 

to a river far exceeds its transport capacity. Fluvial reworking 

of such flood deposits can be surmised from localized 

disconformities, formed where reincision of the aggradational 

channel fill by post-flood lower discharge flow (either the 

tail of the flood itself or later flows) cuts across flood-deposit 

strata and redeposits beds that are commonly finer grained and 

more finely bedded.

Glacial Deposits Composed of Volcaniclastic 

Particles

Deposits emplaced by flowing glacial ice are termed tills and 

include two basic types: lodgment till and ablation till (Neuendorf 

and others, 2005). Lodgment tills are mixtures of fragmental 

rock debris that are overridden, deformed, and highly compacted 

by glaciers. Ablation tills consist of (1) supraglacial rock debris 

initially emplaced on glacier surfaces by nonglacial processes 

(mass movement or fluvial transport) and (2) englacial rock debris 

entrained within glacier ice, both of which are later lowered to the 

ground surface (without significant compaction) by ice stagnation 

and melting. These two types of tills can have very different 

sediment properties and deposit morphology. Alpine glacier 

deposits and landforms are the types most often encountered on 

the flanks of volcanoes having sufficiently high altitude or latitude.

Volcaniclastic deposits transported and redeposited by glacial 

ice typically have only a few differences distinguishing them from 

other volcaniclastic sediments. Both lodgment tills and ablation 

tills can closely resemble deposits of debris-flow lahars and debris 

avalanches. If only sediment textures in outcrop are examined, 

it may difficult to say whether a diamict has been emplaced by 

glacial ice; recognition of glacial landforms is also important. 

Characteristic features of alpine glacial tills include:

• Aggradational landforms, including thin irregular till 

sheets emplaced on ridges as well as in valleys, lateral and 

end moraines (may be found in valleys beyond base of 

volcanic edifice), and kettle holes (or ponds).

• Erosional features, including faceted, striated, bulletshaped clasts found within the deposit, as well as glacial 

striations, crescentic chatter marks, and polishing on 

surfaces of lava flows.

• Extreme compaction possible (but not required); 

compacted deposits may display fissility (flaky, 

subhorizontal foliation).

• Generally polymictic sediment mixture compositions 

(possibly monomictic or oligomictic near source).

• Lack of grading.

• Well-developed fabric in lodgment till; weak or no fabric 

in ablation till.

• Unusually high clay contents in matrix of tills from large 

glaciers.

• Crushed, flattened logs and vegetation found only beneath 

the diamict (not mixed in).

• Very thin deposits relative to areal coverage; deposits may 

be discontinuous.
Volcaniclastic Deposits of Mount St. Helens 9

Volcaniclastic Deposits of Mount St. 

Helens

Mount St. Helens is one of the youngest and most active 

volcanoes in the Cascades volcanic arc, as well as one of the 

most explosive (Mullineaux, 1996; Crandall, 1987; Clynne and 

others, 2005; Clynne and others, 2008) (fig. 1). Over its history 

it has produced a wide variety and large volume of volcaniclastic 

sediments. This mountain, known as Loowit by some native 

peoples, is an intermediate-composition stratovolcano, much of 

it built within the last 2,500 years (fig. 1B). The modern cone 

has been largely constructed of (1) a dacitic dome complex as 

its foundation; (2) andesitic to basaltic lava flows, scoriaceous 

pyroclastic units, and a few small dacitic lava domes forming 

its outer flanks; and (3) prior to the 1980 eruption, a central core 

composed of a large dacitic lava dome that was removed by the 

1980 debris avalanche. Today the edifice hosts a 2-kilometer 

(km)-wide and 600-m-deep, horseshoe-shaped crater (crater rim 

maximum elevation 2,550 m (8,363 feet, ft) above sea level. The 

crater is open to the north and partly filled by both a composite 

dacitic lava dome (extruded 1980–1986 and 2004–2008) and by 

an advancing glacier—arguably the newest glacier in the world 

(formed since 1986) and one of the few that is growing.

Holocene Eruptive Activity

Eruptive activity at Mount St. Helens includes four major 

eruptive stages dating back to about 300,000 years ago. Details 

of the eruptive activity and products are summarized below from 

Mullineaux (1996), Crandell (1987), Clynne and others (2005, 

2008), and Michael Clynne (written commun., 2012) (fig. 1A). 

Three Pleistocene eruptive stages were followed in the Holocene 

by the Spirit Lake stage, which is subdivided into seven eruptive 

periods, including the modern period that began in 1980. Ages are 

reported in calendar years (C.E.) or in thousands of years before 

present (ka). Refer to figure 1A for alphabetic names of tephra 

layers and sets.

1. Ape Canyon stage volcanism included explosive 

eruptions and lava dome emplacement west of the 

current edifice during two distinct periods, about 

300–250 ka and 160–35 ka. The explosive eruptions 

producing diagnostic biotite-bearing tephra set C around 

40 ka are among the largest eruptions at Mount St. 

Helens (Mullineaux, 1996). Extensive aggradational 

fill deposits containing Ape Canyon stage rocks are 

preserved in terraces in the lower Lewis River valley.

2. Cougar stage (28–18 ka) was probably the most active 

stage prior to the modern Spirit Lake stage. Activity 

included construction of dacite domes, large PDCs, tephra 

emissions, and a very large debris avalanche on the south 

flank of the volcano, followed by a large explosive eruption 

that culminated in the largest lava flow (andesite) at Mount 

St. Helens. Lahar and volcanic fluvial deposits caused 

aggradation in the middle and lower reaches of the Lewis 

River.

3. Swift Creek stage (16–12.8 ka) eruptive activity involved 

the growth of unstable lava domes and explosive eruptions 

producing two major ash layers and numerous PDCs and 

lahars. During this time, three extensive fans of pyroclastic 

debris were emplaced on the volcano’s flanks. This stage 

was followed by about 9,000 years of dormancy.

4. Spirit Lake stage (3.9 ka to present) has seven named 

eruptive periods separated by dormant intervals of 

decades to centuries.

A. Smith Creek eruptive period (3.9–3.3 ka) had major 

explosive eruptions that produced the voluminous set-Y 

tephras (Mullineaux, 1986, 1996). The Yn tephra layer 

(3.5 ka) was emplaced by the largest known explosive 

eruption of Mount St. Helens (approximately 10 times 

larger than the May 18, 1980, eruption), with a denserock equivalent volume of 4 cubic kilometers (km3

) 

that extends about 900 km north-northeast into Canada 

(Carey and others, 1995). At least four other Y tephras 

were erupted during this period.

B. The Pine Creek eruptive period, from about 

3.0–2.5 ka, was a period of major dome building, 

including the light-colored dacitic lava domes 

prominent in the current crater walls of Mount St. 

Helens. Tephra set P eruptions during the Pine Creek 

period were relatively modest in size. This period also 

included emplacement of two large debris avalanches 

in the north (Hausback and Swanson, 1990; Hausback, 

2000). Associated lahars in the North Fork Toutle River, 

formed by breakout(s) of the ancestral debris avalanchedammed Spirit Lake, were the largest Holocene 

volcaniclastic flows at Mount St. Helens (Scott, 1988b).

C. The Castle Creek eruptive period, from about 

2.2–1.9 ka, marked a change to variable composition 

eruptions that, in addition to dacitic lava domes, 

produced both explosive eruptions and lava flows of 

basaltic to dacitic composition, with associated lahars 

and PDCs. By the end of the Castle Creek period, the 

volcano had been transformed from a cluster of lava 

domes to a composite volcano with an elevation of 

about 2,450 m (fig. 1B).

D. The ensuing short-lived Sugar Bowl eruptive 

period, from 850–900 C.E., included eruption of the 
10 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 1. Quaternary chronologic and morphologic history of Mount St. Helens. A. Eruptive history of Mount St. Helens with Holocene 

Spirit Lake eruptive stage subdivided into eruptive periods at the right. Ages are given in calendar years (C.E.) or in thousands of years 

ago (ka). Note that time scales are not linear. B. Inferred morphologic evolution of Mount St. Helens edifice. Edifice-collapse events 

interrupted growth during the Cougar stage, the Pine Creek eruptive period, and the modern eruptive period. Both diagrams modified 

from Clynne and others (2005).

160 ka

250 ka

300 ka

28 ka

18 ka

16 ka

13 ka

4 ka

 1980 C.E. 1980 C.E.

1479 C.E.

1750

1800

1857

1 ka

2 ka

3 ka

4 ka

AGE, IN THOUSANDS OF YEARS (ka)

AGE, IN THOUSANDS OF YEARS (ka)

AGE, IN THOUSANDS OF YEARS (ka)

Dormant

Spirit

Lake

stage

Cougar

stage

Swift Creek

stage

Dormant

Dormant

Dormant

Butte Camp 

lava dome

?

Some

dormant

periods?

Dormant

Dormant

Kalama period

 

Castle Creek period

Pine Creek period

Smith Creek

period 

Dormant

Dormant

Dormant

Goat Mountain

dacite dome,

unnamed ashes

lahars, 

lava domes

older ashes, 

lava domes

“2-pumice pyroclastic 

flow,” debris avalanche,

lava domes

Swift Creek lava flow,

“white pumice”

Crescent and

Cedar Flats fans,

Swift Creek fan 

Goat Rocks lava dome, 

Floating Island lava flow,

tephra layer T

Basalt lava flows,

dacite lava domes and

pyroclastic flows,

andesite lava flows

Lahars, pyroclastic

flows, lava domes,

debris avalanches

Lava domes,

tephra layer Yn,

pyroclastic flows

Dacite domes, 

lateral blast

deposits

Summit dome, 

Worm complex

flows, domes

and pyroclastic

flows

 

Andesite lava flows

Tephra

set B 

Tephra

set P 

Tephra

set Y 

35 ka

Sugar Bowl period

Goat Rocks period

Tephra set J

Tephra set S

Tephra set K

Tephra set M

 Tephra 

set C

Tephra set X

Tephra set W

Tephra layer Z

Tephra layer D

Ape 

Canyon

stage

A
Volcaniclastic Deposits of Mount St. Helens 11

Figure 1.—Continued

men17-7185_fig 01b

10,000

8,000

6,000

2,000

4,000

ELEVATION,

IN FEET

Ape

Canyon

stage

Cougar

stage

Modern

period

Kalama

period

Castle Creek

period

Pine Creek

period

Swift Creek

stage

Spirit Lake stage

B

Sugar Bowl dome and other flank lava domes and a 

small north-northeast directed lateral explosion.

E. The Kalama eruptive period, from 1470–1720C.E.,

began with two closely spaced, large-volume 

eruptions (producing the Wn and We tephras) 

dendrochronologically dated at 1479 and 1482 C.E. 

(Yamaguchi and Hoblitt, 1995). These explosive 

eruptions were followed by PDC emplacement and the 

eruption of tephra layer X around 1500 C.E. Andesitic 

PDCs accompanied extrusion of the sinuous Worm 

Flows lava complex on the south and southeast flanks 

ending around 1570 C.E. Thereafter, a large summit 

lava dome grew to completely fill the crater between 

about 1620 and 1720 C.E.; summit dome growth was 

accompanied by lithic PDCs and lahars on all sides of 

the volcano. This sequence of activity brought Mount 

St. Helens to its pre-1980 configuration and maximum 

elevation of 2,950 m.

F. The Goat Rocks eruptive period, from 1800–

1857C.E., began with eruption of the T tephra in 

1800 C.E. Extrusion of the Goat Rocks lava dome on 

the north flank took place between 1831 and 1857 and 

was accompanied by minor explosive eruptions, which 

were witnessed by early explorers, settlers, and Native 

Americans (Mullineaux and Crandell, 1981). Minor 

phreatic explosions also were reported in 1898, 1903, 

and 1921, after the Goat Rocks eruptive period ended 

(Majors, 1980), and were likely related to the Goat 

Rocks hydrothermal system.

G. The modern eruptive period commenced in March 

1980 with intrusion of a cryptodome into the upper 

volcanic edifice, phreatic eruptions, and marked edifice 

deformation and culminated in the Volcanic Explosivity 

Index 5 Plinian eruption of May 18, 1980 (Lipman and 

Mullineaux, 1981). This eruption was triggered by a 

sector collapse of the north side of the volcanic edifice. 

The May 18 eruption was followed by five smaller 

explosive eruptions during the summer and fall of 1980. 

From October 1980 to October 1986, lava extrusion 

of highly viscous dacite built a 305-m-high (1,000-ft) 

lava dome in 17 eruptive episodes. Lava extrusion 

recommenced in October 2004 with the more or less 

continuous extrusion of semi-solid dacitic lava, which 

produced numerous lava spines and a new 455-m-high 

(1,500-ft) lava dome over the next 3+ years (until early 

2008). The closely monitored eruptions of this period 

have led to major new understandings of eruptive and 

volcaniclastic processes in volcanic arcs and have 

sparked the development of new monitoring techniques 

(Sherrod and others, 2008). A recent compilation of 

eyewitness accounts describes the impact of the May 

18, 1980, eruption on residents, visitors, first responders, 

public officials, and scientists (Waitt, 2015).

Physical impacts and deposits from the 1980 eruptions at 

Mount St. Helens, particularly the May 18 eruption, significantly 

changed the landscape surrounding the volcano and severely 

altered river channels draining the volcano (Lombard and others, 

1981; Major and others, 2000, 2009) (figs. 2, 3). Today, many 

of those features and deposits are being modified, eroded, or 

buried by ongoing surficial processes and recolonization by forest 

vegetation.

Deposits in the Toutle River Drainage Basin

The Toutle River and its tributaries drain the north and 

northwest flanks of the volcano and have accumulated volcaniclastic sediments from Mount St. Helens over multiple eruptive 

periods (figs. 2–4). Most of the volcaniclastic deposits from the 

1980 eruptions of Mount St. Helens are found in this drainage 

basin. Volcaniclastic deposits in the upstream half of the North 

Fork Toutle River basin are dominated by debris-avalanche 

and PDC deposits, but also contain pyroclastic-fall and lahar 

deposits. The southern fork and main stem of the Toutle River 

are dominated by lahar deposits.

Flow Processes in 1980 and Associated Deposits

The May 18, 1980, eruption began with a catastrophic 

rockslide and debris avalanche that was one of the largest globally 

in history, filling 60 square kilometers (km2

) of the North Fork 

Toutle valley with 2.8 km3 of debris and reaching as far as 28 km
12 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

downstream in 10–12 minutes (Voight and others, 1981, 1983; 

Glicken, 1996). Peak velocity at the base of the volcano was 

about 70 meters per second (m/s); the average velocity was 35 

m/s. High-velocity debris entering Spirit Lake pushed a water 

wave up-valley to an elevation 260 m above lake level. Its deposit 

redammed the lake outlet, raised the surface elevation of the 

new Spirit Lake about 60 m, and filled the upper North Fork 

Toutle River valley with hummocky, poorly sorted debris to a 

maximum depth of 180 m (average depth 45 m)—only a fraction 

of which has been eroded since. Although elevated pore pressures 

and seismically induced deformation may have contributed 

to the failure, modeling by Reid and others (2010) shows that 

progressive shear strength reduction due to cryptodome intrusion 

Figure 2. Map showing full extent of volcaniclastic sediment deposition by the May 18, 1980, and later 1980 eruptions 

of Mount St. Helens (J.J. Major, written commun., 2016). Labeled triangle symbols are locations of stream gages used in 

monitoring sediment yield: Tower Road (TOW), Kid Valley (KID), South Fork Toutle (SFT), fish trap (FTP), Green River (GRE), 

Clearwater Creek (CLW), and Muddy River (MUD). 

men17-7185fig 02

GRE

TOW

KID

SFT

MUD

Mount

St. Helens

CLW

FTP

Area swept by pyroclastic density current blast

Lahar deposit 

Debris-avalanche deposit

Pyroclastic density current deposit

Gaging station and identifier

Sediment retention structure

EXPLANATION

FTP

SRS

SRS

Elk Rock

Longview

Toutle

Cougar

Kid

Valley

Castle

 Rock

Kelso

WASHINGTON

WASHINGTON

OREGON

Map 

area

0 5 10 15 KILOMETERS

0 5 10 15 MILES

123°00' 122°30' 122°00'

46°30'

46°00'

Yale Lake

Swift Reservoir

C

Coweeman River

owlitz River

Silver Lake

Lewis River

Riffe Lake

Smith Creek

Clearwater Creek

Swift Creek

Pine Creek

Kalama River

Toutle River

Clear Creek

North

Fork Toutle River

Spirit

Lake

Muddy River

River

Green

Speelyai Creek

Lake Merwin

Columbia River

South Fork Toutle River

and gravitational loading could have provoked edifice collapse, 

even in the absence of transiently elevated pore-fluid pressures or 

seismic disturbance.

Rapid unloading of the hydrothermal-magmatic system 

in the upper edifice by retrogressive edifice collapse triggered a 

catastrophic directed volcanic explosion. A wide variety of terms 

has been applied to this relatively dilute PDC, but we follow 

others in using the term “lateral blast” to acknowledge the directed 

component and initial explosive thrust of the density current 

(Hoblitt and others, 1981). The lateral blast devastated an area of 

600 km2

 over a broad arc west-northwest to east-northeast of the 

volcano, removing trees proximally, causing extensive radial and 

flow-parallel tree blowdown, and leaving singed standing trees at 
Volcaniclastic Deposits of Mount St. Helens 13

Figure 3. Aerial photograph of Mount St. Helens taken from the International Space Station in 2009. View is to the west. Crater 

breach opens to the north and adjacent lava domes from 2004–2008 (left) and 1980–1986 (right) are visible inside the crater. The 1980 

debris-avalanche deposit is the thick valley-fill sediment extending westward within the North Fork Toutle River valley (toward top 

of photograph). The largely still unvegetated “blast zone” extends across most of the right half of the photograph. NASA astronaut 

photograph ISS018-E-5643, acquired October 28, 2008, available at http://earthobservatory.nasa.gov/.

men17-7185_fig 03

the distal margins (Hoblitt and others, 1981; Moore and Sisson, 

1981; Waitt, 1981).

Lateral-blast deposits vary across the upper North Fork 

Toutle River basin. Their grayish color and lithic-rich composition 

contrast with the whitish pumiceous deposits emplaced later by 

May 18 PDCs and open-vent tephra fall. Typical sections include 

a basal fines-poor A1 layer that can include organic material 

with pebble-size cryptodome dacitic fragments, a friable A2 

layer, portions of which display bedded surge layers, and a thin 

overlying A3 air-fall layer that may contain accretionary lapilli 

(Waitt, 1981) (fig. 5). Hoblitt (2000) noted evidence for a second 

blast explosion that may have been phreatomagmatic because of 

lower proportions of juvenile clasts.

Following emplacement of the debris avalanche and the 

lateral blast, a vertical open-vent phase of the May 18 eruption 

produced a Plinian eruption column and dacitic pyroclasticfall deposits—including pumice lapilli in proximal areas and 

ash in distal areas downwind to the east (Sarna-Wojcicki and 

others, 1981). Secondary PDCs derived from remobilization 

of lateral-blast deposits on steep valley walls ponded in South 

Coldwater Creek valley, covering much of the debris-avalanche 

deposit that overtopped Johnston Ridge (Hoblitt and others, 

1981; Fisher and others, 1987; Fisher, 1990). Collapse of the 

eruption column produced pumiceous PDCs that descended the 

north flank of Mount St. Helens, traveling 7–8 km from the vent 

and formed a pyroclastic debris apron south of Johnston Ridge, 

locally known as Pumice Plain (Rowley and others, 1981; 

Criswell, 1987). Secondary small-scale phreatic eruptions originating from water-ice interaction with still-hot pumice deposits 

produced explosion pits rimmed by bedded pyroclastic surge 

layers. Smaller pumiceous PDCs formed during subsequent 

explosive eruptions between May 25 and October 16, 1980 

(Christiansen and Peterson, 1981). Two types of lahars occurred 

in the Toutle River drainage basin on May 18: (1) a granular, 

fines-poor lahar, triggered almost instantly by rapid snowmelt 

produced by PDCs scouring snow and ice and (2) a muddy, 

slightly more fines-rich lahar (3–5 weight percent silt and clay), 

triggered by liquefaction of the debris-avalanche deposit over 
14 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 4. Shaded-relief map showing the impacts of various volcanic and volcaniclastic processes to the Mount St. Helens area in 1980

(Lipman and Mullineaux, 1981). Shaded relief from 2009 U.S. Army Corps of Engineers 1-meter light detection and ranging (lidar) data.

Mount St. Helens

Johnston Ridge

Observatory

Green River

South Fork Toutle River

North Fork Toutle River

Coldwater Lake

Spirit

Castle Lake

Lake

0 1 2 3 4 5 KILOMETERS

0 1 2 3 4 5 MILES

Shaded relief from 2009 U.S. Army Corps of Engineers

1-meter light detection and ranging (lidar) data

Area affected by lateral blast with variable deposition

Zone of standing dead (singed) trees

Debris-avalanche deposit

May 18 and later pumiceous pyroclastic density current deposit

May 18 lahar deposit—Overlies parts of the debris-avalanche deposit

EXPLANATION

N

a period of hours. The granular lahar coursed down the South 

Fork Toutle River within minutes of the 8:32 a.m. onset of 

the May 18 eruption, starting as a debris flow and becoming 

progressively more dilute as it coursed downstream (Cummans, 

1981; Janda and others, 1981). It transformed fully to hyperconcentrated flow after mixing with North Fork Toutle River water 

at the confluence of the two forks. The larger fines-rich North 

Fork Toutle River lahar initiated from “mud springs” coalescing 

over several hours on the debris-avalanche deposit in the early 

afternoon of May 18. The mud springs were generated by 

seismically induced liquefaction of the fresh debris-avalanche 

deposit in the upper North Fork Toutle River valley (Fairchild, 

1985). The South Fork Toutle River lahar flowed about 43 km 

as a debris flow before transforming to hyperconcentrated flow. 

The North Fork Toutle River lahar, by contrast, traveled 93 km 

to the Columbia River entirely as a debris flow.
Volcaniclastic Deposits of Mount St. Helens 15

Weak traction

Weak turbulence

Blocking and drainback

Rapid sedimentation

High particle concentration

EXPLANATION

A B

A

A1

A2

A3

A2a

A2c

A2b

Unit

Layer

Bed

Gray sandy silt; contains pisolites

Fine to very fine sand, vaguely laminated to massive

Medium to fine sand, laminated

Coarse to medium sand, massive

Pebbly or granular very coarse sand or sandy 

granular gravel, massive

Small-pebble gravel with sandy matrix

Pebble gravel with or without sandy matrix;

scattered wood

Pebble or cobble gravel with or without sandy

matrix; much wood

Wood and (or) reworked regolith (not everywhere

present)

Soil, forest duff, or road surface

A3

A2b

A2a

A1

A0

1

0

Ash fall with accretionary lapili

Coarse to fine sand

Planar, wavy, or ripple cross-stratified

Pinch and swell

Present on steep slopes

Crude stratification

Matrix supported

Coarse tail graded

Poorly sorted

Drapes topography

Basal interse grading

Poorly developed sandwaves

Strongly erosive on A1

Gravel and coarse sand

Very pooly sorted

Coarse tail graded

Fines-depleted

Loosely packed

Thickens and coarsens into lows

Inverse to normal grading common

Absent from steep slopes

Sandwaves absent

Flotation of wood

Sheared, rich in wood and soil

Vertical distance, in meters

Figure 5. Composite lithostratigraphic sections of the May 18, 1980, 

lateral blast pyroclastic surge deposit, showing two slightly different 

interpretations. A. Generalized section modified from Waitt (1981). 

B. Generalized section modified from Druitt (1992).

March 1982 Lahar and Associated Deposits

The largest post-1980 lahar at Mount St. Helens took 

place in 1982. On March 19, nearly instantaneous melting of 

snow and ice by a large explosion during lava dome extrusion 

sprayed snow-covered crater walls with incandescent rock 

debris, triggered avalanches of rapidly melting snow, and 

released 4 million cubic meters (m3) of water and hot rock 

debris from the crater. The flowing mixture transformed to 

a debris flow through erosion and incorporation of sediment 

by the time it reached the base of the volcano (Waitt and 

others, 1983; Pierson and Scott, 1985; Pierson, 1999). Over 

the next 81 km that it traveled down the Toutle River, the 

lahar became progressively dilute as water from the overtaken 

river was ingested and coarse sediment was deposited 

(Pierson and Scott, 1985). The debris flow transformed to 

hyperconcentrated flow starting about 27 km downstream of 

the crater. At 43 km from the crater, the lahar had completely 

transformed to hyperconcentrated flow (Dinehart, 1999). 

The lahar reached the Cowlitz River confluence, 83 km 

downstream from the crater, in approximately 5 hours. Flowfront velocity averaged 4.6 m/s.

The hyperconcentrated phase of the 1982 lahar deposited 

an unusual, faintly stratified, coarse sand with isolated 

outsized clasts and gravel lenses—the first systematically 

described deposits emplaced by hyperconcentrated flow 

(Pierson and Scott, 1985). Unfortunately, almost all of these 

deposits have been eroded away. Similar deposits have since 

been described in lahar channels and in the geologic record at 

a number of other volcanoes worldwide, indicating that lahars 

are more common than had been recognized prior to 1985.

Post-1980 Sediment Redistribution

Large-scale erosion and mobilization of May 18 

volcaniclastic deposits (primary and secondary) flushed a huge 

volume of sediment down the Toutle River, requiring extensive 

and prolonged engineering intervention by the U.S. Army 

Corps of Engineers in order to prevent catastrophic inundation 

and flooding of downstream communities (Willingham, 2005). 

Without intervention, the lower Toutle and Cowlitz Rivers would 

have experienced significant channel aggradation.

Long-term monitoring of stream erosion and sediment 

transport since 1980 has revealed that initial sediment yields 

following the 1980 eruption were several hundred times above 

background and that the magnitude of yields depended on the type 

and degree of disturbance to subcatchments (Major and others, 

2000). Levels of suspended sediment were highest and lasted 

longest in the upper North Fork Toutle River drainage, which was 

impacted by the 1980 debris avalanche (fig. 6). In contrast, lower 

sediment yields of shorter duration occurred in the Green River 

catchment, which was deforested and covered only with thin 

lateral-blast (pyroclastic-surge) and pyroclastic-fall deposits. The 

sediment yields recorded immediately after the eruption declined 

sharply over the next five years, but have remained well above 

background levels until the present.
16 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

The initial rapid decline in sediment yield in the Toutle 

River basin was due to stabilization of tephra deposits 

remaining on hill slopes, as demonstrated by annual yields 

from the Green River basin (affected only by tephra deposition) 

returning to pre-eruption levels within five years (Collins 

and Dunne, 1986; Major and others, 2000). In other Toutle 

River subbasins, continuing high sediment yields were caused 

primarily by channel erosion of debris-avalanche and lahar 

deposits rather than hillside erosion. Such channel erosion 

has continued to produce sediment yields about 10–50 times 

Figure 6. Plot of suspended sediment yields (in metric kilotons per square kilometer, kt/km2) in surrounding rivers since 

the May 1980 Mount St. Helens eruption (J.J. Major, written commun., 2016). Stream gage locations are marked in figure 

2. Gray-shaded bar graph depicts mean annual discharge (in cubic meters per second, m3

/s) at Tower Road station (TOW) 

on the Toutle River near its confluence with the Cowlitz River (blue line). Background blue shading shows the range of 

average annual yields from western Cascade Range rivers. The dashed horizontal line is the approximate median value 

of the range of average annual yields. SRS marks the completion date of the sediment retention structure; “spillway 

bypass” indicates when water and sediment began passing over the SRS spillway in 1998. 

1980 1985 1990 1995 2000 2005 2010 2015

Year

160

120

80

40

0

Mean annual discharge, 

in cubic meters per second (m3/s)

10,000

1,000

100

10

1

0.1

0.01

Suspended sediment yield, in metric kilotons

per square kilometer (kt/km2

)

SRS

Spillway

bypass

Mean annual discharge, in m3/s, at Tower Road stream gage (TOW)

Range of average annual yields from western Cascade Range rivers

Approximate median value of range of average annual yields

Suspended sediment yield, in kt/km2

, by river and stream gage

Toutle River—Tower Road stream gage (TOW)

North Fork Toutle River—Kid Valley stream gage (KID)

North Fork Toutle River—Fish trap stream gage (FTP)

South Fork Toutle River—South Fork Toutle stream gage (SFT)

Muddy River—Muddy River stream gage (MUD)

Clearwater Creek—Clearwater Creek stream gage (CLW)

Green River—Green River stream gage (GRE)

EXPLANATION

greater than probable pre-eruption levels for more than three 

decades.

In the North Fork Toutle River valley, channel erosion 

during the second decade after the eruption (1987–1999) was 

concentrated along a 6-km-long area at and upstream of a valley 

constriction near Elk Rock in the middle of the debris-avalanche 

deposit (fig. 7). During the following decade (1999–2009), erosion 

was more evenly distributed along the channel, incising into the 

middle and upper parts of the avalanche deposit with increasing 

lateral erosion of tall stream banks. In 1987–1989, construction 
Volcaniclastic Deposits of Mount St. Helens 17

Figure 7. Shaded-relief maps showing topographic changes in the North Fork Toutle River valley after the May 1980 Mount St. Helens eruption 

(Mosbrucker, 2014). A. Digital elevation models of topographic difference derived from comparison of aerial photographs taken in 1987 and 1999. 

B. Digital elevation models of topographic difference derived from comparison of 1999 aerial photographs with 2009 U.S. Army Corps of Engineers 

light detection and ranging (lidar) data. SRS, sediment retention structure, completed in 1989; N1, former small retention structure constructed in 

early 1980s. Selected long-term monitoring cross section sites, used for data verification, are labeled. 

Elk

Rock

Castle Lake

Spirit Lake

Coldwater Lake

Mount

St. Helens

SRS

N1

NF375

NF360

NF350

NF345

NF340

NF320 NF310

NF300 NF130 NF120

NF110 NF100

CA225

CA220

LO040

LO030

Tunnel

NF330

Elk

Rock

Castle Lake

Spirit Lake

Coldwater Lake

Mount

St. Helens

SRS

N1

NF375

NF360

NF350

NF345

NF340

NF320 NF310

NF300 NF130 NF120

NF110 NF100

CA225

CA220

LO040

LO030

Tunnel

NF330

>15 

10 to 15

5 to 10

3 to 5

1 to 3

Below detection level

>15

10 to 15

5 to 10

3 to 5

1 to 3

EXPLANATION

Change in elevation, in meters—Blue colors 

show deposition and brown colors show erosion

0 2 4 6 8 10 KILOMETERS N

0 2 4 6 MILES

A. Elevation change from 1987–99

B. Elevation change from 1999–2009
18 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

of a sediment dam—the sediment retention structure—forced 

sediment deposition within the valley downstream of these 

erosion-prone areas, burying the toe of the debris-avalanche 

deposit. 

Despite the focused erosion of the debris-avalanche 

deposit, the bulk of this massive sediment reservoir is still 

in place. As of 2010, stream erosion or local mass-failure 

processes have modified only about 20 percent of the debrisavalanche deposit planimetric area and only about 13 percent 

of the deposit volume has been eroded (J.J. Major, written 

commun., 2016). 

Toutle River Lahars from Older Eruptive Periods

The distal North Fork Toutle River valley downstream 

of the North and South Fork Toutle River confluence exposes 

lahar deposits spanning much of the history of Mount St. 

Helens (Scott, 1988a). Terraces near the confluence preserve a 

sequence of lahar deposits dating back to late Pleistocene time: 

(1) a lahar deposit of Swift Creek age (16–12.8 ka), (2) lahar 

deposits of Smith Creek age (3.9–3.3 ka), and (3) lahar deposits 

of Pine Creek age (3.0–2.5 ka). The four Pine Creek-age lahar 

deposits are interpreted to have been generated by outbreak 

floods released from the ancestral Spirit Lake, a lake repeatedly 

formed by damming caused by debris-avalanche deposits (Scott, 

1988a). The lahars contain clasts composed almost entirely of 

well-rounded river gravel, suggesting that they could only have 

been deposited by large floods entraining sediment from the 

Toutle River bed.

The recurrence interval of Toutle River lahars is one lahar 

every few centuries over the past 4,000 years, determined for 

lahars of sufficient volume to reach 50 km downstream. However, 

lahar occurrence is not randomly distributed, but concentrated in 

eruptive periods. The average recurrence interval of large lahars 

during the last 4,000 years, if only eruptive periods are considered, 

is 130 years (at least 15 lahars during the 1,930 years within 

eruptive periods) (Scott, 1989).

Deposits in the Lewis River Drainage Basin

A variety of Mount St. Helens volcaniclastic sediments 

can be found in the Lewis River basin, the system that drains 

the south and east flanks of Mount St. Helens (Crandell and 

Mullineaux, 1973; Major and Scott, 1988; Clynne and others, 

2008) (figs. 2, 8). Key tributaries affected by eruptions at 

Mount St. Helens include Smith Creek, Clearwater Creek, 

Muddy River, Pine Creek, and Swift Creek. In addition, three 

major hydropower reservoirs (Swift Reservoir, Yale Lake, 

and Lake Merwin) are found along the main stem of the 

Lewis River, which extends from the volcanic edifice to the 

Columbia River confluence, about 90 km downstream. All 

of the Pleistocene eruptive stages of Mount St. Helens and 

several eruptive periods of the Holocene Spirit Lake eruptive 

stage (fig. 1) are named for features and deposits in the Lewis 

River drainage.

A huge volume of volcaniclastic material was produced 

during the Cougar eruptive stage (28–18 ka). A very large 

Cougar-age debris avalanche originated from near the location of 

the present-day edifice (Mullineaux and Crandell, 1981; Clynne 

and others, 2008). The avalanche traveled about 17 km, filled the 

valley with a deposit 200–300 m thick and 1–2 km3 in volume, 

and dammed the Lewis River at the present-day site of the 

Swift Creek confluence (Clynne and others, 2008). Subsequent 

overtopping of the blockage and lake drainage caused flooding 

downstream to the Columbia River and contributed to aggradation 

of at least 75 m in the lower Lewis River valley. This flooding 

is analogous to what occurred in the North Fork Toutle River 

during the Pine Creek eruptive period. Avalanche occurrence was 

immediately followed by the explosive eruptions that produced the 

dacitic “two-pumice” PDC (24.4 ka) (Clynne and others, 2008). 

The PDC deposit buries the debris-avalanche deposit to depths 

of 100–200 m without evidence for intervening erosion or soil 

formation. After a hiatus of several thousand years, deposits from 

the Swift Creek eruptive stage (16–12.8 ka) buried Cougar stage 

deposits in the Lewis River drainage and produced the Cedar Flats 

debris fan southeast of the volcano. This feature is dominated by 

an assemblage of deposits, including lahars and dome-collapse 

PDC deposits, about 100 m thick. Following another dormant 

period, abundant lahar, PDC, and volcano-fluvial deposits of the 

Smith Creek (3.9–3.3 ka), Pine Creek (3.0–2.5 ka), and modern 

(1980 to present) eruptive periods were emplaced within in the 

Swift Creek, Pine Creek, and Muddy River drainages.

On the morning of May 18, 1980, after the start of the 

Mount St. Helens eruption, major snowmelt-generated lahars 

flowed down Pine Creek and Muddy River, depositing about 

1.4×107

 m3 of lithic debris in Swift Reservoir in less than an hour 

(Pierson, 1985). A smaller similar lahar descended Swift Creek 

Figure 8. Photograph of an outcrop of pre-1980 volcaniclastic 

deposits along the Swift Creek riverbank. The thick lowest unit 

was emplaced by the “two-pumice” pyroclastic density current 

that was erupted during the Cougar stage. The largest blocks 

visible are about 2 meters in diameter. The soil developed on this 

deposit (dark band) is overlain by a whitish lahar deposit of Swift 

Creek age. Smithsonian Institution photograph by L. Siebert, 2016.
Volcaniclastic Deposits of Mount Hood 19

at the same time and, during the afternoon, a pumiceous lahar 

was generated by snowmelt from column-collapse PDCs. On the 

northeast flank of the volcano, a complex assemblage of deposits 

from the lateral blast and resultant snowmelt lahars was emplaced 

in Smith Creek (Brantley and Waitt, 1988). As in the case of 

the lahars in Pine Creek and Muddy River, initial lahar flow 

velocities in the upper Smith Creek tributaries were high (greater 

than 30 m/s). Deposits emplaced in Smith Creek include ponded 

lateral-blast (PDC) deposits and secondary PDC deposits derived 

from valley sides and lahars that are capped by a tephra-fall unit 

containing accretionary lapilli (lateral blast unit A3) (fig. 5). This 

stratigraphy indicates emplacement of the entire Smith Creek 

sequence in less than 30 minutes (Brantley and Waitt, 1988).

Volcaniclastic Deposits of Mount Hood

Mount Hood is an intermediate composition stratovolcano, 

composed largely of andesite and some low-silica dacite 

(58.0–64.5 weight percent SiO2), and is capped by perennial 

snow and glacial ice. It is located along the crest of the Cascade 

Range about 120 km east of Portland, Oregon, and 56 km south 

of the Columbia River (Wise, 1969; Scott, Gardner, and others, 

1997; Scott, Pierson, and others, 1997; Scott and Gardner, 2017) 

(figs. 9, 10). The present volcanic edifice—composed of lava 

domes, lava flows, and interbedded volcaniclastic deposits—is 

approximately 50 km3 in volume (Sherrod and Scott, 1995). 

Its summit stands 3,427 m (11,241 ft) above sea level. Mount 

FIgure 9. Two photographic 

views of Mount Hood. A. 

Southwest flank of Mount 

Hood with key features 

labeled. Collapsing lava 

domes located at the site 

of present-day Crater Rock 

(dome remnant) produced 

the pyroclastic debris fan 

extending downhill during 

the Timberline (~450 C.E.) and 

Old Maid (1781–1793 C.E.) 

eruptive periods. Activity 

during the Polallie eruptive 

period (30,000–15,000 calendar 

years ago) sent flows down 

the White River. Modified 

from Pierson and others 

(2009). B. North flank of Mount 

Hood viewed from the Hood 

River valley looking south. 

Cooper Spur is thought to be 

part of the collapse scarp of 

the late Pleistocene Hood 

River debris avalanche and 

lahar. Smithsonian Institution 

photograph by L. Siebert, 2013.

A

B

Crater Rock Steel Cliff

Zigzag

Glacier

White River Glacier

Palmer

Snowfield

Polallie-age fan

Timberline Lodge

Timberline-age fan

Mississippi Head

Cooper Spur

Eliot

Dome

Eliot

Glacier
20 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Hood differs from Mount St. Helens and other Cascade Range 

volcanoes in the relatively uniform chemistry of its eruptive 

products over its 500,000 year history and in its lack of highly 

explosive eruptions. Effusive eruptions and lava dome growth 

dominate its recent activity, along with associated lithic PDCs 

and lahars.

Construction of the Mount Hood edifice began about 

1.5 million years ago (Ma) with eruptions from vents within 

several kilometers of the current vent. By about 500 thousand 

years ago (ka), lavas were added from other vents closer to 

the present summit (Sherrod and Scott, 1995; Scott, Gardner, 

and others, 1997; Scott and Gardner, 2017). The reversely 

magnetized 1.2 Ma basaltic to andesitic Sandy Glacier volcano 

underlies the west flank of Mount Hood (Wise, 1969). Lava 

flows dominate, but the upper part of Sandy Glacier volcano is 

composed of a thick sequence of diamicts that were probably 

deposited chiefly by block-and-ash flows and lahars. Prior to 

300 ka, several Pleistocene-age basaltic-andesite volcanoes 

grew on what is now the north flank of Mount Hood. Most 

Figure 10. Map showing Mount Hood and the three main river systems that drain the volcano.

OREGON

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

 MOUNT HOOD

West Fork Hood River

Middle Fork Hood River Hood River

White River

Sandy River

Zigza g River

Columbia River

East For k

Hood River

Sandy

River

Troutdale

Cascade Locks

Hood

River

The Dalles

Tygh

Valley

Parkdale

Sandy

N

216

197

197

26

35

14

14

14

Base from U.S. Geological Survey (USGS) 30-meter Landsat imagery

Roads from USGS’s The National Map

of the present Mount Hood edifice is composed of lava flows 

of Pleistocene age and most of the upper cone is formed of 

flows younger than about 150 ka (Scott, Gardner, and others, 

1997; Scott and Gardner, 2017). Pleistocene lava flows were 

overridden by glaciers during the Last Glacial Maximum 

(about 20 to 33 ka), and many remain partly buried by 

moraines.

Mount Hood has produced both effusive and moderately 

explosive eruptions during the past 100,000 years, including 

dormant periods of variable length (table 3) (Scott and Gardner, 

2017). Most volcanic activity since about 30 ka has consisted 

of lava-dome growth and collapse, which fed numerous PDCs, 

and the eruption of stubby lava flows on the summit and upper 

flanks. The last of the Pleistocene eruptive activity produced lava 

domes, PDCs, lahars, and tephra over the period from about 30 

to 15 ka—an interval of multiple poorly dated eruptive episodes 

broadly defined as the Polallie eruptive period, which coincides 

with maximum glacial ice cover on the volcano. Eruptive products 

during this interval impacted valleys surrounding the volcano.
Volcaniclastic Deposits of Mount Hood 21

Table 3. Major eruptive (orange-yellow), noneruptive geomorphic (beige), and glacial (blue) events recorded at Mount Hood during the 

past 100,000 years. Modified from Scott, Gardner, and others (1997).

Date or age Event Deposits

20th and 21st centuries Debris flows in Sandy, White, and Hood Rivers Debris-flow deposits

1859, 1865 C.E. Minor explosive eruptions Scattered pumice

Late 19th century Late neoglacial advance Prominent, sharp-crested moraines

1781–1793 C.E. Dome-building eruption of Old Maid eruptive 

period

Silica-rich andesite lava dome (Crater Rock); domecollapse pyroclastic density current and lahar 

deposits; minor tephra

~500 years ago Debris flows in Zigzag River Debris-flow deposits

~1 ka Debris flows in upper Sandy River (not syneruptive) Debris-flow deposits

~1.5 ka Dome-building eruption(s) of Timberline eruptive 

period

Silica-rich andesite lava dome; dome-collapse 

pyroclastic density current and lahar deposits; 

minor tephra

~7.7 ka Mafic eruption from satellite vent north of Mount 

Hood (near Parkdale)

Basaltic andesite lava flow

~20–11 ka Waning phases of Evans Creek glaciation Moraines

~20–13 ka Dome-building eruptions of Polallie eruptive period Silica-rich andesite lava dome; dome-collapse 

pyroclastic density current and lahar deposits; 

minor tephra

~25–20 ka Maximum extent of Evans Creek glaciation Belts of moraines in most valleys

~30–20 ka Dome-building eruptions Silica-rich andesite lava dome; dome-collapse pyroclastic density current and lahar deposits

~50–30 ka Lava-flow eruptions from central vent Andesite lava flows of Cathedral Ridge and Tamanawas Falls

~100–60 ka Sector-collapse debris avalanche from north flank of 

Mount Hood

Avalanche transformed to Hood River lahar and 

flowed to the Columbia River

[ka, thousand years ago; cm, centimeters]

Holocene Eruptive Activity

Dome-building eruptions occurred at Mount Hood 

during its two most recent eruptive periods: (1) the Timberline 

eruptive period about 1.5 ka, and (2) the Old Maid eruptive 

period that occurred slightly more than 200 years ago, with 

historically documented explosions (phreatic?) occurring 

throughout the mid-19th century (Crandell, 1980; Cameron 

and Pringle, 1986, 1987; Scott, Gardner, and others, 1997; 

Pierson and others, 2011; Scott and Gardner, 2017). Eruptions 

during both Holocene eruptive periods produced multiple 

dome-collapse lithic PDCs and lahars but very little air-fall 

tephra. The Timberline eruptive period began with a flank 

collapse that sent a large lahar down the Sandy River and left 

the active vent surrounded by a scarp-bounded, horseshoeshaped failure scar open to the southwest. Flows of primary 

and secondary volcaniclastic fragments during both eruptive 

periods were therefore directed southwest and emplaced 

on a steep pyroclastic debris fan on the southwest flank of 

the volcano (fig. 9A, 11). The Holocene debris fan extends 

6–10km downslope of the Holocene eruptive vent on the 

south flank of the volcano and sheds sediment into two of the 

three river systems draining Mount Hood—the Sandy and 

White Rivers (fig. 10, 11). In addition to sediment storage 

of primary PDC and lahar deposits on the debris fan itself, 

substantial volumes of these sediments were remobilized by 

lahars and floods during and following volcanic activity, and 

moved into the heads of confined canyons beyond the toe of 

the fan in both the upper Sandy and White River drainages 

(fig. 12). This combined fan and valley-head stockpile of 

stored sediment became the sediment source for decadeslong periods of post-eruption sediment remobilization. The 

third major drainage basin on Mount Hood, the northwardflowing Hood River, does not receive sediment from Holocene 

eruptions. The Holocene vent location is marked today by a 

remnant of the most recent lava dome, named Crater Rock.

Timberline Eruptive Period

Holocene eruptive activity at Mount Hood began about 

1,500 years ago and follows nearly 10,000 years of quiescence. 

Timberline-age eruptive products are dominated by the deposits 

of lithic PDCs and accompanying ash clouds and extend at least 

12 km from the vent (Scott, Gardner, and others, 1997). They 
22 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

14142500

35

35

84 26

84 30

26

26

N

Base modified from U.S. Geological Survey’s The National Map

Area enlarged

Sandy River

delta

MOUNT HOOD

West Fork Hood River

Middle Fork Hood River East Fork Hood

River

Hood River Bull Run River

Salmon River

Zigzag River

White River

Sandy River

River

West Fork Hood River

Middle Fork Hood River East Fork Hood

River

Hood River Bull Run River

Salmon River

Zigzag River

White River

Sandy River

River

Columbia River Sandy

0 5 10 15 MILES

0 5 10 15 KILOMETERS

Surficial geology

Deposits of Old Maid

eruptive period

Deposits of Timberline

eruptive period

Older or undifferentiated

fan deposits

Debris fan deposits

Sandy River reach

Upper reach

Middle reach

Lower reach

U.S. Geological Survey

stream gage and 

 number

EXPLANATION

14142500

Figure 11. Simplified surficial geologic map of the Mount Hood debris fan showing

recognized deposits of Old Maid and Timberline age. Modified from Pierson and others (2009).

Figure 12. Photographic examples of secondary lahar deposits originating as remobilized dome-collapse pyroclastic density current 

deposits in the upper Sandy River canyon. A. Timberline-age lahar deposit containing a large, prismatically jointed block, indicating 

a dome-collapse origin for the block. Trenching tool shovel handle is 55 centimeters long. B. A thick pink-colored lahar deposit of 

Timberline age, overlain by a ~50-centimeter-thick gray lahar deposit of Old Maid age. USGS photographs by T.C. Pierson, 2005. 

A B
Volcaniclastic Deposits of Mount Hood 23

were generated by multiple gravitational collapses of a growing 

lava dome, as inferred from radial fracturing and microvesicular 

texture observed within many clasts in these deposits. Lahars 

were mobilized by water from rapid snowmelt mixing with the 

PDCs and probably also by heavy rain (Crandell, 1980; Cameron 

and Pringle, 1986; Scott, Gardner, and others, 1997; Pierson and 

others, 2011). Deposits of Timberline age are found in all of the 

rivers that drain the debris fan but are sparse in the White River 

drainage, suggesting that a topographic barrier blocked the head 

of the White River during this period (Scott, Gardner, and others, 

1997). Timberline deposits within the fan itself are locally as thick 

as 100 m (Cameron and Pringle, 1986). The surficial distribution 

of Timberline deposits in the fan suggests that the source lava 

dome must have been at or very near the present site of Crater 

Rock on the upper southwest flank of the volcano.

The age of the Timberline eruptive period was originally 

defined from about 300 to 600 C.E., on the basis of four dated 

samples (Crandell, 1980). More recent unpublished dates have 

narrowed this range to a decades-long interval centered at about 

450 C.E. (Scott and Gardner, 2017; W.E. Scott, written commun., 

2016). Paleomagnetic data from the Mount Hood edifice, 

supported by stratigraphic and weathering evidence, suggest 

that Timberline eruptive activity was divided into two episodes 

separated by at least several decades (Scott, Pierson, and others, 

1997; Scott and Gardner, 2017).

Old Maid Eruptive Period

Another dome-building eruption occurred during the late 

18th century Old Maid eruptive period, when about 2×108 m3

dense-rock equivalent of hypersthene-hornblende dacite was 

extruded (Crandell, 1980). More than 90 percent of the extruded 

magma was converted to fragmental debris and deposited on the 

flanks of the volcano by multiple phreatic explosions and domecollapse lithic PDCs. About half of that debris was delivered to 

two headwater tributaries of the Sandy River.

Recent tree-ring dating, augmented by correlation of 

geochemical signatures from individual annual rings of trees 

affected by PDC ash clouds, indicates that Old Maid domebuilding began in 1781 C.E. and lasted until 1793 C.E. (Sheppard 

and others, 2010; Pierson and others, 2011). Other tree-ring 

evidence suggests that a PDC occurred between 1795 and 1805 

in the White River valley (Lawrence, 1948; Cameron and Pringle, 

1987). Eyewitness accounts of minor explosions (possibly 

phreatic) were reported in 1859 and 1865 by early settlers (Scott, 

Gardner, and others, 1997), but a precise end to the Old Maid 

eruptive period has not been defined.

Deposits in the Sandy River Drainage Basin

A variety of volcaniclastic sediments from Mount Hood are 

found in sequences of eroded fill terraces along the Sandy River. 

This river and its major sediment-transporting tributary, the Zigzag 

River, drain the northwestern part of the debris fan and the west 

flank of Mount Hood (fig. 12). The river is incised 100–500 m 

into late Tertiary bedrock and early to middle Quaternary volcanic 

sediments that were shed westward from the Cascades crest, 

forming the western piedmont of the Cascades at this latitude 

(Trimble, 1963; Sherrod and Scott, 1995). The river valley remains 

narrowly confined and relatively steep until just before it reaches 

its confluence with the Columbia River, about 87 km downstream 

(fig. 13). Here the Sandy River has built a 10 km2 delta extending 

about 3 km into the Columbia River valley. Drainage area of the 

Figure 13. Longitudinal profile 

of the Sandy River channel, 

starting at Crater Rock—the 

source volcanic vent—and 

continuing downstream to the 

confluence with the Columbia 

River. The upper, middle, and 

lower reaches of the river 

(shown in fig. 11) are indicated 

and selected cross sections of 

the valley within each reach 

are shown. Profile derived 

from USGS 30-meter digital 

National Elevation Dataset 

and USGS 1:24,000 digital line 

graph. Vertical exaggeration is 

9.3×. Modified from Pierson and 

others (2011).

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Elevation, in meters

0 10 20 30 40 50 60 70 80 90 100

Horizontal distance, in kilometers

50 40 30 20 10 0

River miles, in miles

3,000

3,400

3,800

4,200

3,200

3,600

4,000

FEET

Upper Old Maid Flat

1,000

1,200

600

800

500

700

900

1,100

1,300

FEET

? ?

Oxbow Regional Park

300

100

-100

0

500

200

400

FEET

USGS stream gage

14142500

USGS stream gage

14142500

Crater 

Rock

Sandy River

delta Lower reach

Middle reach

Upper reach

Lower reach

Middle reach

Upper reach

Debris fan
24 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

men17-7185_fig 14

A

B

Location of 

photograph in B

MOUNT HOOD

Old Maid Flat

N

Sandy River

Zigzag River

3 KILOMETERS

3 MILES

0 1 2

0 1 2

Shaded relief from U.S. Geological Survey 3-meter light detection and ranging (lidar) data

Figure 14. Old Maid Flat, the main sediment source of the Sandy River. A. Shaded-relief map of 

the west flank of Mount Hood, with the wide, flat-floored valley known as Old Maid Flat outlined. 

Shaded relief from USGS 3-meter light detection and ranging (lidar) data. B. Photograph of a thick 

(40–60 meters) sediment-fill terrace in the upper part of Old Maid Flat, taken approximately from 

the location marked in A. View looks to the southeast. USGS photograph by T. Pierson, 2005.
Volcaniclastic Deposits of Mount Hood 25

Sandy River is about 1,130 km2 upstream of a stream gage (USGS 

14142500) located 58 km downstream from Crater Rock (fig.13).

Peak discharge of record at this gage (in 1964) is 2,390 cubic 

meters per second (m3/s). Mean annual flow was 66 m3/s prior to 

flow regulation in 1915.

Wedges of volcaniclastic sediment that thin downstream 

have accumulated in the heads of the Sandy and Zigzag Rivers 

since the late Pleistocene to a combined thickness of as much as 

200 m (fig. 13), including some Polallie-age sediments that are 

mostly buried. Remnants of these valley fills have remained active 

sediment sources to the present time. They continue to provide 

above-normal sediment volumes to the Sandy River during winter 

storms, affecting bank stability and flood hazards along the river. 

The upper Sandy River sediment wedge is 16 km long; its slightly 

incised top surface forms a broad valley floor known locally as 

Old Maid Flat (figs. 13, 14). Sediments exposed in riverbanks 

at Old Maid Flat are dominated by lahar deposits and poorly 

sorted water-flood deposits of Old Maid age. Rapid deposition 

of these sediments engulfed standing forest trees, the evidence 

for which are numerous vertical shafts (tree wells) on Old Maid 

Flat that remain today after the wood rotted away. Most of the 

lahars recorded by deposits in Old Maid Flat did not travel much 

farther than 20–30 km downstream. Only four Holocene lahars 

are presumed to have traveled the full 87 km from the vent to the 

Columbia River: three Timberline lahars documented on the delta 

(Rapp, 2005) and one Old Maid lahar that was at least 3 m deep a 

few kilometers upstream of the river mouth and recorded by a thin 

paleoterrace deposit (Pierson and others, 2011).

Volcaniclastic deposits downstream of Old Maid Flat in 

the Sandy River valley are dominated by lahar and volcanofluvial deposits of Timberline age. The estimated bulk volume 

of fragmental material produced during this eruptive period 

is 1.0–2.0 km3 (Scott and Gardner, 2017). Timberline lahar 

deposits (with a veneer of Old Maid-age deposits) now fill the 

entire Sandy River valley floor in the area from Wildwood to 

Welches (fig. 10). Large lahar-transported boulders still dot the 

flat valley floor in this zone.

Deposits in the lower Sandy River valley (60–84 km 

downstream) and in the Sandy River delta are composed mainly 

of volcano-fluvial sediments. At Oxbow Regional Park (~68 km 

downstream), an aggradational valley-fill sequence of Timberline 

age is as much as 30 m thick (fig. 15), whereas a similar sequence 

emplaced during the Old Maid eruptive period is at most 23 m 

thick (figs. 15, 16) (Pierson and others, 2009). Aggradation at this 

Figure 15. Schematic stratigraphic relations between Timberline- and Old Maid-age deposits along the Sandy River 

at Oxbow Regional Park. The figure depicts present and former channel levels, exposed terrace surfaces, buried 

paleoterraces, and buried forests 68–70 kilometers downstream of Mount Hood. Modified from Pierson and others (2009).

Approximate river elevation

just prior to the Old Maid 

and Timberline 

eruptive periods

Present-day river elevation

Lahar deposits

Fluvial sand and gravel

Old Maid eruptive period (syn- and

post-eruptive deposits)

Timberline eruptive period (syn- and

post-eruptive deposits)

Older geologic units

Overbank flood deposits

Soil

EXPLANATION

0

10

–10

20

30

40

METERS
26 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

site during both the Timberline and Old Maid eruptive periods 

was rapid enough to bury entire old growth forests before the 

wood in the trees could rot. During the Old Maid eruptive period 

when data on timing are available, aggradation during the 12-year 

period of dome growth averaged about 2 meters per year but was 

likely episodic and faster for short periods of time, presumably in 

response to major winter storms. On the delta, older aggradational 

volcano-fluvial units dating to about 5.0 ka have been found in 

drill cores (Rapp, 2005), but no volcanic activity of that age has 

been documented. Some high terraces in the lower Sandy River 

valley are formed of Polallie-age (late Pleistocene) volcaniclastic 

deposits (Evarts and others, 2013).

Deposits in the White River Drainage Basin

Significant volumes of volcaniclastic deposits of Polallie 

and Old Maid age are preserved on the steep upper flank of 

Mount Hood within the White River drainage. Lahar deposits of 

Old Maid age are also found in a low-gradient reach 65–70 km 

downstream of the vent (Tygh Valley). Only the steep upper reach 

of the river and the lower reach spanning from Tygh Valley to the 

Deschutes River confluence are easily accessible by foot because 

the river flows for much of its length through a remote and deeply 

incised canyon in which sediment transport dominates over 

deposition.

Debris flows and hyperconcentrated flows still regularly 

occur in the upper White River valley, mobilized during 

rainstorms by bank erosion and slope failures of the abundant 

loose volcaniclastic debris in headwater V-notch canyons (fig.17).

Multi-day, high-intensity storms generated by “atmospheric 

Figure 16. Photograph of sediment fill in the lower Sandy River

valley at Oxbow Regional Park (67 kilometers downstream of 

Mount Hood), where an Old Maid-age aggradational sequence of 

volcaniclastic lahar and volcano-fluvial deposits can be seen on 

the riverbank. These deposits buried a forest that was growing on 

Timberline-age river terraces. The remains of trees killed in 1781 

C.E. are still standing, recently exhumed by bank erosion. USGS 

photograph by T.C. Pierson, 2009.

men17-7185_fig 16

Figure 17. Photograph of thick, incised sediment fill composed of

pyroclastic density current and lahar deposits in the head of White 

River canyon. View is to the north from the Timberline Trail, a few 

miles east of Timberline Lodge. Canyon walls are composed mainly of 

Polallie-age deposits, locally capped by Timberline-age deposits. The 

lower level inset terrace, with its eroded surface sloping eastward, is 

composed of deposits emplaced during the Old Maid eruptive period. 

USGS photograph by T.C. Pierson, 2007.

men17-7185_fig 17

rivers”, such as occurred in February 1996 and November 2006, 

are particularly effective in generating high-concentration flows. In 

recent years, such events have caused channel aggradation that has 

episodically buried the Oregon Route 35 bridge in bouldery debris. 

These episodes have been costly for the Oregon State Department 

of Transportation in their efforts to maintain the bridge crossing. 

However, a few years ago the bridge was raised and extended 

laterally to facilitate debris passage and prevent blockage.

Polallie-age Flow Deposits

Lahar and PDC deposits of Polallie age in the upper White 

River are remnants of a broad and discontinuous apron of late 

Pleistocene volcaniclastic debris. The current White River canyon 

cuts into this apron. Polallie-age deposition, synchronous with the 

Last Glacial Maximum at Mount Hood, did not emplace deposits 

on high-elevation valley floors, owing to occupation of the valleys 

by glacier ice that shielded their floors from direct deposition 

(Crandell, 1980). Some units of late Polallie age locally formed 

kame terraces at lower elevations as the glaciers shrank in size.

Old Maid-age Flow Deposits

Inset within upper White River canyon walls (composed of 

Polallie-age deposits) are two sequences of volcaniclastic deposits 

comprising a set of Old Maid-age syneruptive lithic PDC deposits 

and lahars and a slightly younger set of Old Maid depositional 

units comprising only lahar deposits (Crandell, 1980) (fig. 17). 

Both sequences were formed by deposition from dome-collapse 

PDCs, snowmelt lahars, and subsequent rain-triggered lahars.
Road Log and Description of Field-Trip Stops 27

5 Field-trip stop—Number is

planned stop; letter is

optional stop

EXPLANATION 0 5 10 15 KILOMETERS

0 5 10 15 MILES

N

30

5

5

432

4

411

505

504

506

504

503

503

Base from U.S. Geological Survey (USGS) 30-meter Landsat imagery

Shaded relief and roads from USGS’s The National Map

Mount St. Helens

Spirit

Lake

Swift Reservoir

Yale

Lake

Lake Merwin

North Fork Toutle River

South Fork Toutle River

Lewis River

A

1 2

3

C

D

5 E 4 F

G

H

6 J

K

7 L

8

M

9

I

B

A

1 2

3

C

D

5 E 4 F

G

H

6 J

K

7 L

8

M

9

I

B

Cougar

WoodlandWoodland

Kelso

Castle

Rock

Road Log and Description of Field-Trip 

Stops

This road log gives mileage and GPS coordinates for 

regular and optional stops associated with this field trip. 

Cumulative mileage is calculated for each day from the starting 

point given (number at left in log). Some of the optional stops 

require travel along spur roads off of the main trip route. The 

distances traveled for these spur routes are not included in the 

cumulative trip mileage.

Some of the stops in this guidebook are on public land and 

require permits, such as a Northwest Forest Pass or seasonal 

Sno-Park Permit. Stops on private land may require permission 

for access. Such stops are noted in the log.

Snowfall in the Cascades arrives early and stays late. Owing 

to snow cover, many of the stops in this guidebook generally 

will not be accessible by vehicle from October to June, and it is 

advisable to check the current conditions prior to any trip during 

the spring or fall.

Figure 18. Map of locations of planned stops (numbers) and optional stops (letters) for the Mount St. Helens area on Days 1 and 2 of the field trip. 

Day 1: Debris-Avalanche and Lahar Deposits in 

the Toutle River Valley, Mount St. Helens

This field trip log begins in Castle Rock, Washington, 

although the 2017 IAVCEI field trip begins and ends in 

Portland, Oregon. From Portland, drive 1.75 hours north on 

Interstate 5. Take Exit 49 for Castle Rock off I–5 and turn 

right (to head east) at the traffic light at the end of the exit 

ramp. Continue east to the next traffic light. You are now at the 

intersection of Mount St. Helens Way NE, Dougherty Drive 

NE, and Old Pacific Highway North. Start your odometer here 

and begin traveling east on Mount St. Helens Way NE, which 

becomes State Route 504 (S.R. 504). Watch for a sign advising 

that the Mount St. Helens Visitor Center at Silver Lake and 

Seaquest State Park are ahead.

Day 1 field trip stops emphasize deposits of the 1980 

eruption in the North Fork Toutle River (fig. 18). A more 

detailed map of stops proximal to the edifice shows the 

exposed 1980 debris-avalanche and pumiceous PDC deposits 

(fig. 19). 
28 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 19. Shaded-relief map of proximal volcaniclastic deposits at Mount St. Helens. Visible in the crater is the composite lava 

dome, extruded in 1980–1986 (northern third of dome) and again in 2004–2008 (southern two thirds of dome). Yellow numbers mark 

planned field trip stops and letters mark optional stops (as in fig. 18). Shaded relief from 2009 U.S. Army Corps of Engineers light 

detection and ranging (lidar) data; roads from the USGS National Map.

504

504

N

Coldwater Lake

South Fork Toutle River

Spirit Lake

North Fork Toutle River

Castle Lake

0 1 2 3 MILES

0 1 2 3 KILOMETERS

Debris-avalanche deposit

May 18 and later pumiceous pyroclastic density current deposit

May 18 lahar deposit—Overlies parts of the debris-avalanche deposit

Field-trip stop—Number is planned stop; letter is optional stop

EXPLANATION

5

Mount St. Helens

C

D

E 4

F

5

3

C

D

E 4

F

5

3

Shaded relief from 2009 U.S. Army Corps of Engineers

light detection and ranging (lidar) data

Roads from the U.S. Geological Survey’s The National Map
Road Log and Description of Field-Trip Stops 29

0.0 Head east on Mount St. Helens Way NE from the

 intersection with Dougherty Drive NE and Old

 Pacific Highway North.

5.1 Optional Stop A—Mount St. Helens Visitor Center

 at Silver Lake (lat 46.2952° N, long 122.8228° W)

 Pull off at the Mount St. Helens Visitor Center at 

 Silver Lake on the right side of the road. This was the

 original Mount St. Helens visitor center before others

 were constructed closer to the volcano. It is now part 

 of a Washington State park and contains exhibits 

 focusing on the 1980 eruption. A boardwalk trail

 provides access to Silver Lake, with views of Mount

 St. Helens in the distance (fig. 20). This lake, 12 km2 in

 area but only 1–3 m deep, was formed when deposits

 from two huge Toutle River lahars blocked a broad

 tributary valley about 2.6 ka during the Pine Creek

 eruptive period. The lake is drained by Outlet Creek,

 and lake level is controlled for the convenience of the

 large number of homes along the lakeshore. From the

 north shore of the lake the field trip route continues for

 about 15 miles (mi; 24 km) on the surface of this 

 extensive lahar deposit. 

 When you are finished at this stop, turn right out

 of the parking lot onto S.R. 504 and continue 

 traveling east. When you enter the town of Toutle, 

 watch for your next turn, which will be shortly after 

 passing the school on the north side of the road.

10.4 Turn right onto South Toutle Road. Your next turn is

 just on the left.

10.5 Stop 1—Pine Creek-age lahars at Outlet Creek 

 (lat 46.3238° N, long 122.7299° W) Turn left into the

 recycling center and follow the access road until it

 begins to loop around to the east. Park your car at the

 end of the access road. Walk about 200 ft east through 

 the forest to the edge of Outlet Creek to view 

 exposures of Pine Creek-age lahars. The outcrop is 

 located at lat 46.3241° N, long 122.7280° W. 

Outcrops on the north side of Outlet Creek expose

 three Pine Creek-age lahar deposits PC1, PC2, and

 PC3. Based on estimated peak flows, PC1 and PC3

 were the largest and second largest Holocene lahars

 from Mount St. Helens, respectively (Scott, 1988a,

 1988b, 1989). The PC1 lahar had a peak discharge of

 200,000–300,000 m3

/s (approximate average discharge

 of the Amazon River) and was about 30 m deep at

 distances of 30–50 km from the volcano (fig. 21) 

 (Scott, 1988b, 1989). Such a flow rate places it among

 the world’s largest floods (O’Connor and Costa, 2005).

 The only water source large enough to produce such

 large lahars would have been outbreak floods from

 precursors to Spirit Lake—the lake repeatedly formed

Figure 20. Historical photographic view of Silver Lake with the 

snow-capped pre-1980 cone of Mount St. Helens in the distance, 

50 kilometers to the east-southeast. Photograph by J. Boyd Ellis.

men17-7185_fig 20

 by debris-avalanche dams blocking off a small basin 

 on the north side of Mount St. Helens (Hausback and

 and Swanson, 1990; Hausback, 2000). A debris-

 dammed ancestral Spirit Lake as the source for the 

 Pine Creek-age lahars is also inferred from (1) 

 abundant well-rounded river cobbles and pebbles that

 show large-scale entrainment of sediment from the 

 Toutle River bed (such sediment entrainment is

 common in massive turbulent water floods), and (2)

 megaclasts of hydrothermally altered dacite in the 

 lahar deposits (indicating erosion of a debris-avalanche

 deposit). Thus, the Pine Creek-age deposits (figs.22,

 23) show what might have happened if the current 

 Spirit Lake, dammed in 1980, had not been engineered

 to prevent overtopping of the 1980 blockage.

 Deposits from the PC1 and PC3 lahars at Stop1

(and nearby) show that the lake-outbreak floods 

entrained sufficient sediment to form debris flows. 

PC1 deposits in this area are as much as 6–8 m 

thick and locally contain large fragile clasts of the 

unconsolidated debris-avalanche deposit that formed 

the lake blockage. Farther downstream, the flows 

transformed to hyperconcentrated lahars. Throughout 

this area “tree wells” (fig. 24) can be found marking 

where trees on terraces were encased by lahar deposits 

but remained upright. Subsequent decay of the wood 

left vertical shafts that provided preferred pathways for 

tree roots.

 When you are finished at the site, return to the car 

and depart the recycling center parking lot. Turn left 

onto South Toutle Road. When you cross the bridge 

over the South Fork Toutle River, you are close to your 

next turn (5 minute drive).
30 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Pre-Pine Creek

deposits

Approximate thickness of 

PC1 and PC3 lahar

overbank deposits

Pre-Pine Creek

deposits

Peak stage of PC3

Quarry

Peak stage of 1980

North Fork Toutle River lahar

Peak stage of PC1

?

?

Horizontal distance, in meters

0 200 400 600 800 1,000

Vertical scale, in meters

50

60

40

30

10

20

0

EXPLANATION

Active North Fork Toutle 

 River profile 

Boundary of Pine

Creek-age lahar

deposits—Queried

where uncertain

PC1

PC3

Figure 22. Pine Creek eruptive period stratigraphy based on 1980–83 exposures in the North Fork Toutle River valley about 2 kilometers 

northeast of Stop 1. Both flood plain and channel facies are shown. Note the vertical tree mold (“tree well”) formed in the flood plain 

facies. Modified from Scott (1988a).

Figure 21. Reconstructed lahar-flow cross sections at peak discharge (assuming synchroneity with peak stage) of lahars 

PC1 and PC3 in the North Fork Toutle River valley at Kid Valley, 10 kilometers northeast of Stop 1. The cross section of the 

May 18, 1980, lahar is shown for comparison in gray. Modified from Scott (1989).

“Ball-bearing bed”

Runout sand

Runout sand

Buried stumps—2,510±40

Trunk molds

Megaclasts of hydrothermally

altered dacite breccia (da)

Tree mold

Soil

PC3

PC2

PC1

DEPOSITS OF

PINE CREEK

ERUPTIVE PERIOD

FLOOD PLAIN FACIES CHANNEL FACIES

Vertical scale

METERS

3

2

1

0

PC4—Final lahar in sequence

0–1.5 meters

PC2—Locally pumice-rich lahar

0–2.0 meters

PC3—Second largest lahar of

Pine Creek age

PC1—Largest lahar of

Pine Creek age

Alluvial sand with cross-bedding

interbedded with runout sands

surrounding stumps that were

the source of limb and trunk

molds in the overlying lahar radiocarbon years

Normal graded bedding—Sediments

fining upward

Inverse graded bedding—Sediments

coarsening upward

EXPLANATION

da
Road Log and Description of Field-Trip Stops 31

Figure 23. Photographs of Pine Creek-age deposits. A. Lahar deposits PC1 and PC2 formed by breakouts of the ancestral Spirit Lake 

at Outlet Creek 2,600 years ago (Stop 1). Clast roundness is notable, indicating origin by entrainment in a water surge. Exposure is on 

the bank of Outlet Creek, where the lahars dammed and created Silver Lake. Note the 2-meter-wide pink fragile clast of the lakedamming debris-avalanche deposit (outlined) at right in PC1. USGS photograph by K.M. Scott, 2016. B. Two Pine Creek-age debrisavalanche deposits (white deposits indicated by arrows) exposed in the incised north flank of Mount St. Helens. The upper deposit 

probably dammed Spirit Lake and was breached by breakout floods that transformed into the PC1 lahar. USGS photograph by C.J. 

Harpel, 2003. C. Fines-depleted basal section “ball-bearing” bed of the PC1 lahar channel facies deposit on the North Fork Toutle River 

about 2 kilometers upstream of the South Fork confluence (eroded gravel-rich unit behind shovel; handle is 55 centimeters long). High 

shear and clast crushing are inferred from the “ball-bearing bed” (Scott, 1988a). USGS photograph by K.M. Scott, 1981. D. Lenticular 

structure in coarse-grained unit at the top of the PC1 lahar deposit protrudes into the PC3 deposit above, which is capped by the finer 

grained PC4 lahar deposit at the top of the section. Location along North Fork Toutle River near Pullen Creek (PC2 is missing at this 

locality). Orange backpack for scale. USGS photograph by K.M. Scott, 1981.

PC1

PC2

A B

C D
32 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

11.8 Turn left onto Fiest Road. Harry Gardner Park is to 

 your left.

12.1 Stop 2— North and South Fork Toutle River lahar

 deposits at Harry Gardner Park (lat 46.3247° N,

 long 122.7076° W) Turn left into Harry Gardner Park 

 and park your vehicle at the west end of the parking 

 lot. From the parking lot, walk about 450 ft down to 

 the river’s edge. Walk downstream along the river, 

 looking for good exposures of 1980 lahar deposits in 

 the cutbank. 

 The 1980 South Fork Toutle River lahar was 

triggered by rapid snowmelt at about 8:33 a.m., a 

minute after the start of the May 18 eruption. This flow 

began as a PDC, entrained meltwater from scoured 

snow and transformed to a highly concentrated lahar 

(debris flow), then became more dilute through 

incorporation of river water, likely transforming 

abruptly to hyperconcentrated flow when it joined the 

North Fork of the river (fig. 25). It traveled 49.8 km 

to the confluence of the North Fork and South Fork 

Toutle Rivers in less than 2 hours, arriving to this 

location at about 10:20 a.m. (Cummans, 1981). Mean 

peak flow velocity near the source was estimated to be 

about 28 m/s; in the lower South Fork Toutle River, 

it was about 7 m/s (Fairchild, 1985). The harrowing 

account of two people caught and transported 

downstream by this lahar (and survived) can be found 

in Waitt (2015, p. 198–201). The larger North Fork 

Toutle River lahar did not arrive at this site until about 

5:50 p.m. (Cummans, 1981).

South Fork Toutle River lahar deposits vary in 

mean grain size from 0.61 mm in the upstream reach 

to 0.31 mm in the downstream reach. Sorting varies 

from 3.3 to 2.8s. Fines contents range from 11–20 

weight percent silt and 2–3 weight percent clay. Nearly 

continuous exposures of South Fork Toutle River 

lahar deposits, overlain by North Fork Toutle River 

lahar deposits, can be seen in high cutbanks north of 

Harry Gardner Park (fig. 26). The park’s namesake, 

Harry Gardner, constructed Spirit Lake Lodge for 

Harry Truman, who was a media favorite for his 

newsworthy refusal to leave his lodge prior to the May 

18 eruption (Waitt, 2015). Here, the South Fork Toutle 

River lahar deposit is as much as 2 m thick and was 

deposited by a flow in transition between debris flow 

and hyperconcentrated flow. Note the angularity of 

coarse clasts in the South Fork Toutle River lahar in 

contrast with the rounded clasts in the Pine Creek lahar 

deposits viewed at Stop 1. The recognition in 1980 

of hyperconcentrated flow deposits as a downstream 

“runout facies” of debris-flow lahars was first made in 

a study of the South Fork Toutle River lahar deposits 

here by Scott and others (2002).

The larger 1980 North Fork Toutle River lahar 

(fig. 27) began in the early afternoon of May 18 and 

lasted 11 hours (Cummans, 1981; Janda and others, 

1981; Fairchild, 1985). Flow volume was about 0.14 km3

downstream of the debris-avalanche deposit and it was 

nearly that large (0.12 km3) when it reached the Toutle 

and Cowlitz River confluence, approximately 60 km 

from its source—a liquefaction zone on the avalanche 

deposit. It had a single broad flow peak that lasted about 

2.5 hours (fig. 28); near-source peak discharge was about 

7,200 m3/s, which slowed to about 6,000 m3/s at the 

mouth of the Toutle River (Fairchild, 1985). The North 

Fork Toutle River lahar flowed as a fully developed 

men17-7185_fig 24

Figure 24. Photograph of vertical tree mold encased by flood 

plain facies of PC1 and PC3 lahar deposits (shovel at contact). A 

horizontal log mold is visible in the upper right. Some trees were 

encased by deposits of the PC1 and PC3 lahars, indicating that 

these lahars (plus PC2) occurred in relatively quick succession—

before the dead vertical snags of those trees could decay and 

topple. Tree molds, often filled with loosely compacted material, 

are easily penetrated by modern tree roots. USGS photograph by 

K.M. Scott, 1982.
Road Log and Description of Field-Trip Stops 33

Figure 25. Conceptual diagram of origin and transformation of the South Fork Toutle River lahar. The flow likely started as a 

concentrated pyroclastic density current and transformed first to a debris flow, through entrainment of snow and ice, then gradually 

to hyperconcentrated flow, through dilution farther downstream (with major dilution at the North Fork Toutle River confluence), and 

ultimately to a surge of muddy water following wholesale mixing with river water at the confluence with the Cowlitz River (72 kilometers 

downstream from the volcano’s new crater). Modified from Scott (1988a).

Figure 26. Photographs of lahar deposits at Harry Gardner Park (Stop 2). A. The North Fork Toutle River lahar deposit (with protruding 

horizontal logs) overlies the South Fork Toutle River lahar deposit. The North Fork Toutle River lahar flowed some distance up the South 

Fork Toutle River. The contact is even with the man’s waist, at the center of the photograph. B. Closeup photograph of the contact, 

located just below the keys and coarse clast. Note that the underlying South Fork Toutle River lahar is finer grained and better sorted 

than the overlying North Fork Toutle River lahar. USGS photographs by K.M. Scott, 2016.

Distal phase Transition

interval

Lahar-runout

flow

Medial phase

downstream

Medial phase

upstream

Deflating pyroclastic surge

Peak flow deposits

Slope deposits from

pyroclastic surge

Basal flow Whaleback bars

(muddy matrix)

Berms and bars

(muddy matrix)

Basal avalanche deposits

(little mud matrix)

Glacier surface

scoured

Hyperconcentrated

streamflow deposits

Lahar

Slope deposits from

pyroclastic surge

Basal flow Whaleback bars

(muddy matrix)

Berms and bars

(muddy matrix)

Basal avalanche deposits

(little mud matrix)

Glacier surface

scoured

Hyperconcentrated

streamflow deposits

Lahar

A B
34 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 27. Aerial photographic view 

of the North Fork Toutle River lahar 

on the afternoon of May 18, 1980. 

This photograph was taken at the 

moment the lahar pushed the State 

Route 504 bridge off its support piers. 

The location is just downstream of 

the North and South Fork Toutle River 

confluence. The bridge deck was 

never found. Photograph by Roger 

Werth, The Daily News (Longview, 

Wash.), copyright 1980.

men17-7185_fig 27

North Fork Toutle River

lahar

Stage based on

periodic wireweight readings

South Fork Toutle River

lahar

Recorder Stream gage orifice

plugged by lahar

5 hours 10.5 hours

8 a.m.

12 p.m.

4 p.m.

8 p.m.

12 a.m.

4 a.m.8 a.m.

12 p.m.

4 p.m.6 p.m.

May 18, 1980 May 19, 1980

Time, in hours

16

14

12

10

8

6

Altitude, in meters

900,000

800,000

500,000

600,000

700,000

400,000

300,000

200,000

100,000

0

Sediment concentration,in milligrams per liter (mg/L)

Mount St. Helens eruption at 8:32 a.m.

EXPLANATION

Hydrograph at USGS gaging station 14243000

Stage measured by recorder

Stage based on periodic wire-weight readings

Measured sediment concentration

Figure 28. Stage hydrograph at 

USGS gaging station 14243000, 

Cowlitz River at Castle Rock, for 

May 18–19, 1980. A partial record of 

sediment concentration is plotted 

on the right axis. This plot shows 

the passage of the South Fork 

Toutle River and North Fork Toutle 

River lahars in the Cowlitz River 

just downstream of the mouth of 

the Toutle River. The first measured 

sediment concentration (at the 

tail of the North Fork Toutle River 

lahar) is in the hyperconcentrated 

flow range. For volcanic sediment, 

the transition from debris flow to 

hyperconcentrated flow typically 

occurs at about 1,400,000 milligrams 

per liter (mg/L), and the transition 

from hyperconcentrated flow to 

water flow occurs at about 300,000 

mg/L (Pierson, 2005). Modified from 

Scott (1988a).
Road Log and Description of Field-Trip Stops 35

debris flow about 90 km to the Columbia River, where 

it deposited 34–36 million cubic meters of sand and 

gravel over a reach of more than 10 km and reduced 

the depth of the Columbia River navigational channel 

from 12 m to 4 m (Schuster, 1981; Hubbell and others, 

1983; Fairchild, 1985). Most of this deposition occurred 

upstream of the Cowlitz River mouth, where it was 

carried on an incoming tide. Peak flow velocities varied 

from 6 to 12 m/s over the total flow distance, although 

the lahar “flood wave” progressed downstream at only 

1.5 to 2.1 m/s (Fairchild, 1985). Typical flow depths 

ranged from 5 to 8 m, although depths increased to as 

much as 18 m at channel constrictions (Fairchild, 1985). 

Back-calculated peak sediment concentration of the 

lahar was about 65 percent by volume; near the tail of 

the flow hydrograph, sediment concentration was about 

50 percent by volume (Fairchild, 1985) (fig. 28). One 

sample—collected just upstream of the mouth of the 

Toutle River at 8:45 p.m. on May 18—had a measured 

sediment concentration of 67 percent by volume (Scott, 

1988a).

Deposits of the North Fork Toutle River lahar 

have textures indicating that, at its peak, the lahar 

never transformed from debris flow to hyperconcentrated flow. This probably is owing to the relatively 

high concentration of fine sediment in the debris 

mixture—19–28 weight percent silt and clay, of which 

3–5 weight percent is clay (Scott, 1988a). Mean grain 

size of matrix samples decreases slightly from about 

0.8 mm to about 0.2 mm (particle size value of 0.2 to 

2.0φ), and sorting increases progressively from 4.4 

to 2.9s along the 90-km flow path of the North Fork 

Figure 29. Photographs of North Fork Toutle River lahar deposits. A. Clast-supported boulder bar in the North Fork Toutle River lahar deposit at 

the confluence of the two Toutle River forks (near Stop 2). A more fine-grained lahar deposit caps the core of the boulder bar. The high degree 

of rounding in several boulders attests to the entrainment of stream alluvium in these lahar deposits. Trenching shovel handle is 55 centimeters 

long. B. Fine-grained channel facies of the North Fork Toutle River lahar near Kid Valley. Rounded pre-lahar pebble/cobble alluvium is overlain by 

a resistant, light-colored “sole layer” at the base of the lahar deposit, with an average thickness of 20 centimeters (layer on which the hammer 

rests). The dark fine-grained lahar deposit behind the folding ruler (extended 48 centimeters) slopes down from the crest of a whaleback bar, 

which is partly visible in the upper left corner of the photograph. USGS photographs by K.M. Scott, 1980.

A B

Toutle River lahar. Outcrops of this deposit appear as 

massive, highly consolidated gravelly muddy sand 

along its full flow path (fig. 29).

13.8 Turn right onto S.R. 504. 

14.3 Coal Bank Bridge. This bridge replaces the original 

 S.R. 504 bridge that was destroyed by the 1980 North

 Fork Toutle River lahar (fig. 27). At least 35 lahars 

 spanning the Pleistocene and Holocene history of 

 Mount St. Helens have reached the Coal Bank Bridge

 area, which is about 55 km downstream of Spirit Lake

 and the volcano. Excellent exposures of lahar deposits 

 upstream of the bridge have in recent years been 

 obscured by vegetation growth and failure of cliff 

 faces.

20.5 Kid Valley bridge. This is the only bridge over the 

 North Fork Toutle River that survived the 1980 lahar. 

 After passing through Kid Valley (last service station 

 on S.R. 504) the road begins climbing in elevation. 

 Watch for the sign that advises the sediment retention 

 structure turnoff is approaching.

24.3 Optional Stop B—Sediment Retention Structure 

 (lat 46.3641° N, long 122.5595° W) Turn right onto

 Stewart Dam Road and follow until it ends in a parking

 lot (0.9 mi). A short trail leads to a viewing platform.

In an effort to keep massive volumes of 

volcaniclastic sediment from washing into the lower 

Toutle and Cowlitz Rivers, the U.S. Army Corps of 

Engineers (USACE) constructed a special type of dam 
36 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

that traps sediment but allows water to pass through 

to the upper North Fork Toutle River basin (Major 

and others, 2009). The 550-m-long and 56-m-high, 

earthen-cored sediment retention structure began 

trapping sediment in 1987 and construction was 

completed in 1989 (fig. 30). By 2000 the structure had 

trapped about 100 metric tons (t) of sediment—about 

30 percent of design capacity. However, by then, 

progressively larger amounts of sediment were passing 

over the spillway and reducing trap efficiency. In 2013 

the spillway was raised 2.1 m in order to improve 

sediment trapping efficiency. When you are finished, 

return to S.R. 504, turn right, and continue traveling 

east.

32.8 Hoffstadt Creek bridge and margin of the devastated 

 zone. One of the highest bridges in Washington State 

 (113 m [370 ft]), the bridge crosses Hoffstadt Creek 

Figure 30. Photograph of the 575-meter-wide sediment 

retention structure (SRS) on the North Fork Toutle River. The 

structure is located 35 kilometers downstream from Mount St. 

Helens, which is visible in the background, in the North Fork 

Toutle River basin. The SRS was designed to trap nearly 200 

million cubic meters of sediment and nearly reached capacity 

by 2012, after which the spillway was raised slightly to extend 

the lifespan of the sediment trap. USGS photograph by A.R. 

Mosbrucker, 2012.

men17-7185_fig 30

 near the western limit of the 1980 PDC (lateral blast)

 deposit, which is 25 km from the volcano. Replanted 

 forests beyond this point obscure evidence of the 

 devastated zone. Also, outside of Mount St. Helens 

 National Volcanic Monument, most tree blowdown 

 was removed by salvage logging in the years following

 the 1980 eruption.

43.4 Stop 3—Castle Lake viewpoint (lat 46.2968° N,

 long 122.2992° W) Pull into the Castle Lake 

 viewpoint on the right side of the road for a view 

 up-valley towards Mount St. Helens. Castle Lake 

 viewpoint provides an excellent view of the volcano 

 and the 1980 deposits that fill the North Fork Toutle 

 River valley (Major and others, 2009) (fig. 31). 

 Hummocky debris of the 1980 debris avalanche fills 

 much of the valley and blocks local drainages, forming

 Castle Lake to the south-southeast and Coldwater Lake

Figure 31. Sketch of features visible from Castle Lake viewpoint. Modified from Pringle (2002).

Spud

Mountain

Debris-avalanche hummocks

Ponds on the debrisMaratta sink—swampy avalanche deposit

remnant of a temporary

lake that breached its

dam in 1980

Quaternary

landslide

Minnie

Peak

Mount

Whittier

Recessional moraine

of Evans Creek age

The Dome

Coldwater

Peak

Trimline carved by the debris

avalanche as it flowed down

South Coldwater valley

The Spillover

Johnston

Ridge

Tertiary

dike

Pumice

Plain

Sugar

Bowl

dome

Mount St. Helens

Fan deposited during 

eruptions that produced Goat 

Rocks dome (1842–1857)

Science and Learning 

Center at Coldwater

Terraces

Castle

Lake

Castle Creek

North Fork Toutle River

Coldwater Creek

Coldwater falls

Maratta Creek

Coldwater

Creek

EAST SOUTH
Road Log and Description of Field-Trip Stops 37

 behind the ridge in the east (where the Science and

 Learning Center at Coldwater is visible). Concern 

 about potential rising waters overtopping and rapidly

 eroding these unconsolidated barriers and producing

 catastrophic floods prompted the USACE to create

 armored spillways for both Castle and Coldwater 

 Lakes.

Johnston Ridge to the southeast divides the North 

Fork Toutle River from South Coldwater Creek. The 

1980 debris avalanche had sufficient momentum to 

overtop Johnston Ridge at two locations, climbing as 

much as 380 m above the valley floor and flowing into 

the valley of South Coldwater Creek before merging 

with the main body of the avalanche in the North Fork 

Toutle River valley.

This viewpoint is perhaps one of the best 

locations to view the spatial distribution of the 1980 

debris-avalanche deposits and to understand their 

kinematics. The first few seconds of the catastrophic 

edifice collapse were captured in a series of dramatic 

photographs by Gary Rosenquist and permitted 

analysis of the movement of discrete slide blocks and 

onset of the lateral blast (Voight and others, 1981) 

(fig. 32). The distribution of debris-avalanche deposit 

components relates directly to the collapse and flow 

kinematics (fig. 33) (Voight and others, 1981; Glicken, 

1996). The initial failure of slide block I involved 

material from the upper edifice—primarily rocks of 

Castle Creek and Kalama age (fig. 33). It produced 

a deposit dominantly of block facies material that 

forms the hummocky terrain to the south of Coldwater 

Lake on the west end of Johnston Ridge. Field trip 

Stop 5 of Day 1 will explore this area in more detail. 

Downstream of this point, constriction of the valley 

hindered transport of slide block I. The deposit to 

the west is formed primarily of slide blocks II and 

III, which also involved older portions of the edifice 

(Pine Creek age). Slide block II, seen in photographs 

to have initiated immediately after slide block I, was 

disrupted by the initial lateral blast explosions, which 

provided additional momentum to the avalanche. 

The area below this viewpoint to the south and west 

reflects a transition to the more mobile debris from 

slide blocks II and III. This part of the landscape has a 

higher proportion of mixed-facies material and a more 

subdued topography (fig. 34).

From here, continue traveling east. Follow signs 

to the Johnston Ridge Observatory. 

46.0 Optional Stop C—The Science and Learning Center

 at Coldwater (lat 46.2976° N, long 122.2669° W) 

 At the overpass intersection, follow the signs directing

 you towards the Science and Learning Center at 

 Coldwater, about 0.3 mi ahead. The Center is only 

 open to the public in winter (November through May)

 and on weekends from 10:00 a.m. to 4:00 p.m. It is

 reserved for special events and educational programs

 from June through October. Formerly the U.S. Forest 

 Service Coldwater Ridge Visitor Center, the building 

 has been repurposed as the Science and Learning

 Center at Coldwater. Now operated by the nonprofit 

 Mount St. Helens Institute, the Center provides 

 in-depth and extended education programs to schools 

 and other educational groups visiting the volcano,

 with programs varying in length from 2 hours to 

 multiple days. The facility offers impressive views of 

 the avalanche-dammed Coldwater Lake, the 

 hummocky debris-avalanche deposit, and Mount St. 

 Helens (fig. 35). From here, you can access a short trail

 through the blast zone and a trail to Coldwater Lake.

From here, rejoin S.R. 504 and follow the signs 

towards Johnston Ridge Observatory. 

47.9 Coldwater Lake picnic area and boat launch

A short spur road to the left leads to a picnic 

area, boat launch, and restrooms at Coldwater Lake. A 

short trail with a boardwalk (0.2 mi) traverses around 

a lakeshore debris-avalanche hummock. Continue 

driving upvalley; the next stop is shortly after the road 

turns sharply to the left.

48.8 Optional Stop D—Logging equipment damaged by

 1980 lateral blast (lat 46.2858° N, long 122.2537° W)

Just after a broad sweeping turn to the left, pull into 

a small parking lot on the left side of the road. From 

the parking lot, a hiking trail leads up the ridge. After 

hiking 1.5 mi and gaining about 1,000 ft in elevation, you will have a view of the heart of the blast 

zone—the area devastated by the high-velocity, lowconcentration PDC—the lateral blast—that initiated 

the 1980 eruption. At the open viewpoint, you will 

see scattered logging equipment that was damaged by 

the 1980 eruption (fig. 36). A bulldozer can be found 

at about 3,450 feet elevation and other artifacts are 

nearby—including remnants of commercial logging 

equipment damaged or destroyed by the eruption. 

The blast stripped paint off the now-rusted surfaces 

of machinery facing the volcano. A metal screen on 

one of the machines trapped wood and rock fragments (as large as 10 centimeters, cm) that were 

entrained by the hot, turbulent blast cloud (Waitt and 

others, 1989).

When leaving the parking lot, turn left and 

continue on S.R. 504. The road proceeds along the 

South Coldwater Creek valley floor, swings sharply 

to the right (passing over the outlet to the tunnel now 

draining Spirit Lake), and then climbs the north side of 

Johnston Ridge. The next stop is at the end of the ridge 

where the road begins to curve to the ridge’s south 

side.
38 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Modified from Voight and others, 1981

s

k

n

l

o q q u s

k

o

q

n

u o

u

w

h

s

w

II

s

k

n

l

o q q u s

k

o

q

n

u o

u

w

h

s

w

II

s

u

w

q

s

c

h

i

II

k

l n n

l h i

h

l

k

i

i

k

k

i i

k

l m

m l

k

i

h h

h

I

l n n

l h i

h

l

k

i

i

k

k

i i

k

l m

m l

k

i

h h

h

I

b

II

c

d

d

e

f

g

h

d c d e f

g

c

d

e

f

g

I

c

d

d

e

f

g

h

d c d e f

g

c

d

e

f

g

I

Main summit scarp Summit crater

Slide I scarp

First explosion

Dogs

Head

Windy Pass Sugar Bowl

Goat Rocks

Original configuration

Surge

Original configuration

b

c

d

e

f

g

h

i

j

k

l

m

n

o

Surface points showing positions

in different Rosenquist frames

Top of slide block I—Solid where certain, dashed where inferred

Forsyth ice avalanche

Front of Forsyth ice avalanche lobes

Top of slide block II—Solid where certain, dashed where inferred

Line symbols—Drawn from Gary Rosenquist’s 1980 photographs; 

discontinuous where obscurred by ash or dust

EXPLANATION

m–n

n–o

o–p

p–q

q–r

r–s

s–t

t–u

u–v

v–w

Frame

interval

Cumulative time,

in seconds

13.0

16.7

19.1

21.6

24.1

27.5

28.8

31.8

34.2

36.2

c–d

d–e

e–f

f–g

g–h

h–i

i–j

j–k

k–l

l–m

Frame

interval

Cumulative time,

in seconds

0.8

1.6

2.4

3.3

4.2

6.3

7.5

8.6

10.0

11.6

A
Road Log and Description of Field-Trip Stops 39

Figure 32. Line drawings and photographs depicting 1980 debris-avalanche kinematics. A. Line drawings inferred from 

Gary Rosenquist’s time-lapse photographs depicting the onset of edifice collapse by incremental movement (top to bottom) 

of slide block I (red) and slide block II (blue) material (based on photograph frames b through o) over about 17 seconds. The 

top two drawings show sequential movement of slide block I and the onset of slide block II movement. By the time of the 

lower drawing, slide block I had dropped below the level of the foreground ridge, and the arrow labeled “surge” marks the 

direction of the lateral blast that initiated after passage of slide block I. Figure from Voight and others (1981). B. Rosenquist 

photograph c (8:32:47 a.m. PDT) depicting early collapse configuration. Slide block I, at lower right, was displaced 700 meters 

with a scarp 600 meters high and slid at a rate of about 50 meters per second. Slide block II, with its headscarp at the summit, 

was displaced 100 meters. C. Rosenquist photograph o (8:33:04 a.m. PDT), taken 16.7 seconds after photograph c, shows the 

lateral blast penetrating through the moving landslide blocks. Photographs B and C by Gary Rosenquist, copyright 1980. 

B

C
40 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 33. Summary of 1980 debris avalanche components before and after edifice collapse. A. Generalized north-south cross section 

of the Mount St. Helens cone showing the inferred May 17, 1980, pre-failure cryptodome topography. The failure planes for slide blocks 

I, II, and III as well as the post-1980 crater floor are also shown. B. Map of the 1980 debris-avalanche deposit showing the distribution 

of the primary lithofacies. The depositional extent of slide blocks I, II, and III are outlined. Four of the debris-avalanche lithofacies are 

sourced directly from the pre-1980 volcanic edifice (shown in A). Castle Lake viewpoint (Stop 3) and Hummocks Trail (Stop 5) are labeled 

on the map. Modified from Glicken (1996).

Distance, in kilometers

Elevation

0 1 2 3 4 5 6

4,000

5,000

6,000

7,000

8,000

9,000

10,000

III III

SOUTH NORTH

FEET

VERTICAL SCALE GREATLY EXAGGERATED

Edifice rock units (various ages)

Cryptodome dacite (May 18, 1980)

Goat Rocks dacite lava dome (1800

to 1857 C.E.)

Dacite summit lava dome (1600 to1750 C.E.)

Andesite lava flows and pyroclastic

deposits (1500 to 1857 C.E.)

Andesite and basalt lava flows and

pyroclastic deposits (2,000 years

before present to 1600 C.E.)

Older dacite unit (~38,000–2,500

years before present)

Slide-block-failure plane—Arrows show 

 relative motion

Post-1980 crater floor

Pre-March 1980 topography

Contact

Slide block number—Block III is youngest

EXPLANATION

II

EXPLANATION

pf

dd

dmx

dmu

dmd

dab

dod

Pyroclastic density current deposit (May 18 to October 18, 1980)

Pyroclastic flow deposits

Debris-avalanche deposit (May 18, 1980)

Distal lithofacies

Mixed block and matrix lithofacies

Modern undifferentiated lithofacies

Modern dacite lithofacies—Includes both the 1980 cryptodome 

 dacite and dacite summit lava dome

Andesite and basalt lithofacies

Older dacite lithofacies

Extent of slide-block deposition—Solid where certain, dashed 

where approximate, queried where uncertain

Contact

II Slide block number

0 1 2 3 4 KILOMETERS

0 1 2 MILES

A

B

N

Deposition aided by the initial

lateral blast explosion

Deposition by gravitational flow of

debris-avalanche blocks dd

II + III

dmx dab

Mount

St.Helens

crater

dod

dab I

dod

dod

Castle 

Lake

dmu

I

dmd

dab

dod

Coldwater

Lake

dmx

II + III

II + III

dab

dab

I

dmd

dod

dmu

dab

dmd

dab

pf

dmd

Spirit Lake

dmu

dab

dab

?

dd

dod

dmu

dmd

Stop 3

Stop 5

dmx

dab

A

A'

A A'
Road Log and Description of Field-Trip Stops 41

Figure 34. Photographs of barren Mount St. Helens 1980 debris-avalanche deposit shortly after emplacement. A. View looking north across 

North Fork Toutle River valley near Elk Rock, about 13 kilometers downstream of the Mount St. Helens crater. Relatively flat, gray terrain in 

foreground is the mixed block and matrix facies of the debris-avalanche deposit (unit dmx in fig. 33B). This unit partly liquefied just hours after 

emplacement and contributed a fine-grained slurry from mud springs of the avalanche deposit in the North Fork Toutle River lahar. Shallow 

channels, such as seen here, conveyed the lahar slurry downstream. Large hummocks in the middle ground are features of the block facies 

of the avalanche deposit (not differentiated in fig. 33B). Brown valley slopes in background show trees blown down by the lateral blast. The 

future site of Castle Lake viewpoint (Stop 3) lies above the upper left corner of the image. USGS photograph by K.M. Scott, July 1980. B. View 

north across the hummocky topography of the debris-avalanche deposit block facies (foreground) with numerous pond-filled depressions, 

composed primarily of andesite and basalt (unit dab in fig. 33B). Coldwater Lake (formed by tributary blockage by the avalanche deposit) is 

visible on the left on the far side of the avalanche deposit. Visible on the right is the flat-floored South Coldwater Creek valley—a valley partly 

filled by secondary pyroclastic density current deposits (remobilized valley-side lateral-blast deposits) and debris-avalanche material that 

overrode Johnston Ridge. USGS photograph, 1984.

Figure 35. Sketch of the view towards Mount St. Helens from the Science and Learning Center at Coldwater, formerly the Coldwater 

Visitors Center. Modified from Pringle (2002).

men17-7185_fig 34

A B

Hummocks of the 1980 

debris-avalanche deposit

South Coldwater delta

Trees blown down and 

damaged by the 

blast density current

Quaternary

landslide

Debris flow scars

Minnie

Peak

Recessional moraine of Evans Creek age

(22,000–11,000 years before present)

Coldwater

Peak MOUNT

WHITTIER

Spud

Mountain

Dip slope in

Tertiary

bedrock

Trimline

carved by

debris avalanche

Floating Island

lava flow

(1800 C.E.)

Plume of

volcanic gas

Shoestring

Glacier Lava

Dome

Sugar

Bowl

dome

Johnston

Ridge

Highway to

Johnston Ridge

Observatory

Harrys

Ridge

MOUNT ST. HELENS

Coldwater Lake

Castle

Lake

South Coldwater Creek

North Fork 

Toutle River

NORTHEAST SOUTH-SOUTHWEST
42 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 36. Photograph of rusted logging equipment damaged 

and partly buried by the lateral blast, located on Coldwater Ridge. 

Smithsonian Institution photograph by L. Siebert, 2015.

men17-7185_fig 36

51.9 Spirit Lake tunnel outlet

Just before S.R. 504 makes a nearly 180° turn 

to the right and crosses over South Coldwater Creek, 

South Coldwater Creek can be seen emerging from 

the Spirit Lake tunnel if you park on the shoulder and 

walk closer to the channel. The tunnel, finished in May 

1985, was designed to drain Spirit Lake water into 

South Coldwater Creek, maintaining the lake level at 

1,038 m altitude—and thereby prevent failure of the 

lake’s debris-avalanche natural dam and any potential 

catastrophic flooding. The 2.6-km-long tunnel, passes 

375 m below the crest of Harrys Ridge. Modifications 

to the tunnel were completed in 2016 after rock 

deformation threatened blockage in a section of the 

tunnel.

54.1 Optional Stop E—Loowit viewpoint (lat 46.2770° N,

 long 122.2301° W) Turn right into the Loowit 

 viewpoint for views of Mount St. Helens and the 

 upper North Fork Toutle River valley. This viewpoint, 

 just west of Johnston Ridge Observatory, provides the 

 closest road-side view of tree blowdown from the 

 1980 eruption on the western approach to the volcano.

 Tree blowdown—forest tree trunks knocked down, 

 sheared off near the ground surface, and oriented 

 radially away from the volcano—can be seen from an

 overlook below the parking lot. The Boundary Trail 

 heading west from Loowit viewpoint crosses areas 

 with some radially oriented blowdown and, after 

 traversing the south side of Johnston Ridge for about 

 a half mile, the trail enters a dramatic quarter-mileFigure 37. Photograph of logs of trees blown down by 1980 lateral

blast, viewed looking southeast from the Boundary Trail west of Loowit 

viewpoint. Note blowdown orientation is dominantly parallel to the 

ridge crest, located behind and to the left of this image. Smithsonian 

Institution photograph by L. Siebert, 2016. 

men17-7185_fig 37

 long section through a dense tract of old growth 

 blowdown. Here, ridge-parallel trees mark a change 

 in orientation as the blast cloud deflected around 

 Johnston Ridge (fig. 37). Mount St. Helens National 

 Volcanic Monument regulations require visitors here 

 and on other parts of the Boundary Trail (including 

 those mentioned in this guidebook) to stay on 

 established trails; removal of rock or plant materials is

 not allowed.

From the parking lot, continue along S.R. 504.

54.9 Stop 4—Johnston Ridge Observatory

 (lat 46.2771° N, long 122.2167° W) S.R. 504 

 terminates at the parking lot to the Johnston Ridge 

 Observatory. Park and make your way to the visitor 

 center. Here, you can view the Mount St. Helens crater, 

 which is actually an avalanche scar, watch a movie 

 about the 1980 eruption, briefly view exhibits, and 

 hike to the viewpoint on Eruption Trail. Note that you

 are in the Mount St. Helens National Volcanic 

 Monument, which is a fee site. This is our lunch stop;

 pick up boxed lunches from the support vehicle.

Johnston Ridge (formerly Coldwater Ridge) was 

named after USGS volcanologist David Johnston, 

who lost his life monitoring the volcano when the 

May 18 lateral blast overran his camp. Johnston Ridge 

Observatory, built not far from where David Johnson 

watched the blast cloud that morning, contains 

a number of well-designed exhibits that explain 

various aspects of the May 18 eruption. Geological 

and biological impacts of the eruption to the forested 
Road Log and Description of Field-Trip Stops 43

landscape, the geological context of the May 18 

and later eruptions, and USGS volcano monitoring 

techniques are also treated. A dramatic topographic 

model with 80,000 multi-colored lights sequentially 

portrays the emplacement history of the 1980 deposits 

(fig. 38A), and, combined with repeat showings of 

a movie about the eruption, provides an excellent 

overview of the events. A short climb on Eruption Trail 

leads to a good vantage point for seeing into the 1980 

crater, where the composite lava dome (extruded in 

two eruptive episodes in 1980–86 and 2004–08) and 

the dome-encircling Crater Glacier can be seen clearly. 

The debris-avalanche deposit and the thick pumiceous 

deposits of the Pumice Plain fill the intervening valley 

below (figs. 38B, C). Farther down the trail, on a loop 

that returns to the parking lot, is a memorial to victims 

of the eruption. 

Note that Optional Stop F is accessed by foot 

from Stop 4. Stop 5 is about a 10 minute drive away.

54.9 Optional Stop F—Boundary Trail to Harrys Ridge 

 (lat 46.2753° N, long 122.2166° W) The Boundary 

 Trail departs from the east side of the Johnston Ridge

 Observatory and continues east along the ridge. The 

 trail provides exceptional views of Mount St. Helens 

 and its surroundings. In less than a mile the trail 

 crosses a low point in Johnston Ridge topped by 

 debris-avalanche hummocks. Visitors can walk the 

 trail as far as time permits; even very short walks 

 provide outstanding views away from the crowds at 

 Johnston Ridge Observatory. For those with the better

 part of a day to spare, the Boundary Trail continues 

 3.5 mi to Harrys Ridge, which was named after 

Figure 38. Views of the 1980 eruption impacts from Johnston Ridge Observatory (JRO). A. Topographic model of the landscape around Mount 

St. Helens installed at the visitor center, with orange lights showing the extent of the May 18, 1980, lateral blast. Other colored lights sequentially 

show the extent of the debris avalanche, pumiceous pyroclastic density currents, ashfall, and lahars. B. View of Mount St. Helens from the 

JRO movie theater. Smithsonian Institution photographs A, B by L. Siebert, 2016. C. Annotated oblique light detection and ranging (lidar) image 

showing deposits of the 1980 and subsequent eruptions, with JRO in the foreground. Image from Dzurisin and Dreidger (2013).

 octogenarian Harry Truman who refused to leave his 

 lodge at Spirit Lake prior to the May 18 eruption. The 

 trail on Harrys Ridge continues right for another 0.6mi

 to the crest of the ridge. From here you’ll see 

 outstanding views of Spirit Lake—its surface still

 partly covered by a mat of floating logs from the 1980

 tree blowdown—and Mount Adams to the east (fig.39).

When you are finished, return to your car and 

begin to backtrack on S.R. 504.

61.5 Stop 5—Hummocks Trail (lat 46.2869° N, 

 long 122.2717° W) Watch for the sign marking the 

 Hummocks Trailhead and pull into the parking lot on 

 the left side of the road. Hike on the Hummocks and 

 Boundary Trails to see three types of deposits: (1) 

 the 1980 “lateral blast” deposit; (2) the 1980 debris-

 avalanche deposit; and (3) post-1980 lahar deposits, 

 as well as the morphologic evolution of the debris-

 avalanche deposit. 

This lengthy stop involves about 3 mi of hiking 

round trip, mostly on flat terrain (fig. 40). We start 

on the eastern entrance to the Hummocks Trail and 

hike 0.7 mi past debris-avalanche hummocks and 

pond-filled depressions until we reach the junction 

with the Boundary Trail. On the Hummocks Trail 

segment we will discuss the surface morphology 

of this large debris-avalanche deposit and postavalanche vegetation recovery of the initially barren 

landscape over the past nearly four decades. Trailside 

exhibit signs provide visitors with information about 

the avalanche, biological recovery, and associated 

research. At the trail junction, we take the Boundary 

Trail about 0.5 mi eastward and continue nearly to 

A B
44 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 39. Photograph of Spirit Lake looking east from Harry’s Ridge. The splintered stump of a tree 

broken off and removed by the lateral blast at Harrys Ridge is visible in the foreground, Spirit Lake to the 

east with its floating mass of blown down logs (bottom and middle left) in the middle ground, and Mount 

Adams in the background. The lake surface was raised about 60 meters by deposition of the debris 

avalanche. Smithsonian Institution photograph by L. Siebert, 1984.

Spirit Lake

C

Figure 38.—Continued
Road Log and Description of Field-Trip Stops 45

the base of Johnston Ridge, where a large protruding 

Tertiary dike is visible. The hummocky ridge just 

north of this trail is part of the broad levee that dams 

Coldwater Lake.

Flat areas between hummocks in this area 

consist of thin lahar deposits—remnant strands of the 

liquefied avalanche deposits that coalesced to form 

the voluminous North Fork Toutle River lahar. Tree 

blowdown from the lateral blast can be seen on the 

west end of Johnston Ridge, where deposits of the 

lateral blast can be found (fig. 41A).

The stratigraphy in this area sheds light on 

the sequence of events on the morning of May 18. 

It is known from satellite infrared sensors that the 

lateral blast reached the Coldwater Lake area about 

2.8 minutes after the eruption-triggering earthquake 

occurred and that the debris avalanche arrived to this 

point 4–5 minutes afterward (Moore and Rice, 1984). 

Thus, debris-avalanche deposits should overlie the 

primary blast deposit. However, some blast deposit in 

this area overlies the debris-avalanche deposit. These 

observations can be explained by the occurrence 

of more than one blast-generating explosion of hot 

cryptodome material, separated by a minute or more, 

as the landslide blocks were sliding downhill and 

exposing different parts of the cryptodome (Sparks and 

others, 1986; Criswell, 1987; Hoblitt, 2000).

The North Fork Toutle River has carved some 

magnificent exposures of the debris-avalanche deposit 

in this area. South of the Boundary Trail, elk trails 

provide access to lower terraces along the North 

Fork Toutle River where deeper parts of the debrisavalanche deposit are exposed, locally capped by 

debris-flow and fluvial deposits. Higher bluffs expose 

hummock sections as high as ~50 m, some of which 

reveal original edifice stratigraphy transported intact 

(figs. 41B, C, D, E). Large blocks of black and reddish 

andesitic and basaltic rock layers derive from parts of 

the edifice formed during the Castle Creek and Kalama 

eruptive periods (fig. 1). Lighter-colored dacitic blocks 

Figure 40. Map of the Coldwater Lake area. Day 1 planned (numbers) and optional (capital letters) stops and detail of area covered 

during the hike at Stop 5 (lower case letters) are shown. Stop 5a represents the junction of Hummocks Trail and Boundary Trail; Stop 5b 

marks the location of tree blowdown and exposure of the lateral blast deposit; Stop 5c indicates where the north bank of the North Fork 

Toutle River exposes a variety of textures and structures in the debris-avalanche deposit; Stop 5d shows the area along Hummocks 

Trail where the chaotic topography of the debris-avalanche deposit can be experienced.

men17-7185_fig 40

0 1

0.5

2 KILOMETERS

0 1 MILE

N

504

504

504

504

Coldwater Lake

Coldwater Lake

North Fork Toutle RiverNorth Fork Toutle River

Castle 

 Lake

3

C

5

d

a

b

c

D

E

4

F

3

C

5

d

a

b

c

D

E

4

F

5 Field-trip stop—Number is

planned stop; letter is

optional stop

Base from U.S. Geological Survey (USGS) 30-meter Landsat and National EXPLANATION

 Agricultural Imagery Program imagery

Shaded relief from USGS 1-meter light detection and ranging (lidar) data

Roads from USGS’s The National Map
46 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Johnston

Ridge

P

Toutle River

Coldwater Creek

Castle

Creek

Coldwater Lake

Modern dacite unit

Andesite and basalt unit

Older dacite unit

Andesite and basalt unit and

debris-avalanche matrix

Debris-avalanche matrix

Basement

Parking lot

Minor road or trail

Major road

P

EXPLANATION

A B

C D

E

4 m

N

Figure 41. Photographs of the 1980

debris-avalanche deposit near Coldwater 

Lake. A. Margin of debris-avalanche 

deposit where its flat surface is mantled 

by a thin layer of the North Fork Toutle 

River lahar deposit (flat green area in 

foreground). Logs from the 1980 tree 

blowdown are present on a flank of 

Johnston Ridge facing away from 

the volcano—swept by the blast but 

untouched by the avalanche here. B. 

Debris-avalanche hummock showing 

tilted and variously oriented basaltic and 

andesitic Kalama- and Castle Creek-age 

rocks overlying a sheared fragment 

of older light-colored dacitic rock of 

Pine Creek age, consistent with the 

stratigraphy in the volcano edifice. Note 

person at lower left for scale. C. Debrisavalanche hummock with fractured 

blocks, color mottling, and shear zones (diagonal dark lineations). D. Fractured jigsaw-fit block. E. Simplified map of debris-avalanche hummocks near 

Coldwater Lake, about 10 kilometers downstream of the source crater. Map shows lithologic units from the pre-failure edifice preserved in hummocks. 

Modified from Takarada (2008). Smithsonian Institution photographs A, B, C by L. Siebert; USGS photograph D by T.C. Pierson. All photographs from 2016.
Road Log and Description of Field-Trip Stops 47

derive from the Pine Creek eruptive period. The 

debris-avalanche deposit at this location was part of 

slide block I (Glicken, 1996).

The debris-avalanche deposit near Coldwater 

Lake shows the complex interactions of debris from 

the different slide blocks that slid sequentially from 

Mount St. Helens’ crater (fig. 33). Debris from slide 

block I traveled as far as Coldwater Lake at the front 

of the advancing avalanche. Slide block I material 

was then shouldered aside and overridden by the more 

disaggregated and fluidized material from slide blocks 

II and then III—both of which traveled another 10km

down the valley. Glicken (1996) calls the material 

of slide block I the block facies, because more than 

70 percent of the material is composed of enormous 

(10–100 m in diameter) avalanche blocks. Glicken 

(1996) uses mixed block and matrix (or intermediate) 

facies to describe material containing 10–70 percent 

avalanche blocks and matrix facies to describe the 

highly mixed, homogeneous, and distal mixtures of 

gravel, sand, and fines (less than 10 percent avalanche 

blocks) (fig. 42). Juvenile clasts of cryptodome dacite 

in the more disaggregated intermediate and matrix 

Figure 42. Schematic longitudinal cross section through the middle part of the debris-avalanche 

deposit located within the North Fork Toutle River valley, about 17 kilometers downstream from Mount 

St. Helens. Cross section shows a proportionately greater amount of matrix facies downstream of the 

abrupt change in surface slope caused by a constriction in the North Fork Toutle River valley. Sketch 

modified from Glicken (1996).

facies suggest that blast explosions may have added 

momentum and enhanced the mobility of slide blocks 

II and III as they slid off the edifice (Glicken, 1996).

Long-term channel cross section surveys document 

the extent of post-1980 depositional and erosional 

modification to the North Fork Toutle River drainage 

at this and other locations (Mosbrucker and others, 

2015) (fig. 43). Fluvial incision has widened the river 

nearly 90 m since initial measurements in 1983 and 

bank retreat of more than 20 m into the interior of 

debris-avalanche hummocks has occurred since 2007. 

The narrow terrace just above river level consists of 

debris-flow deposits emplaced November 3–8, 2006, by 

a lahar from Loowit Canyon. The lahar was the result of 

a prolonged “atmospheric river” rainstorm that dropped 

60 cm of rain on Mount St. Helens—including 37 cm 

on November 7 alone (fig. 44). 

Continue backtracking on S.R. 504. We will drive 

1.25 hours to Kelso, Wash.

106.6 Turn left to merge onto I–5 south toward Portland.

116.6 Overnight stay at motel in Kelso, Wash.

Matrix facies—Consists of mixed debris-avalanche blocks and juvenile

dacite-rich material from blast explosions in slide block III

Block facies—Consists of debris-avalanche blocks from slide blocks

II and III

Debris-avalanche block facies grades into matrix facies

EXPLANATION

Flow of

debris-avalanche

 blocks

Break in slope at constriction

(17 kilometers from source)

Explosively generated current

WEST EAST

~100 METERS

~50 METERS
48 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

100 200 300 400

Distance from south bank, in meters

760

765

770

775

780

785

790

795

800

Elevation, in meters above NAVD88

1983

1987

1997

2007

2016

EXPLANATION

80 meters bank erosion

7 meters

bed erosion

Figure 43. Cross-sectional profiles of the North Fork Toutle River at station NF 120, the site of Day 1 

Stop 5. Profiles show the evolution of the river from 1983 to 2012, documenting 80 meters of riverbank 

erosion during this time (Mosbrucker and others, 2015). View is downstream. Arrow shows location 

of terrace in figure 44. NAVD88, North American Vertical Datum of 1988.

Figure 44. Photograph of a low terrace (approximately 3 meters high) in the North Fork Toutle 

River valley. Section is located on the north side of cross section NF 120 at Stop 5 (marked by 

arrow in fig. 43). Vertical section includes the multi-hued 1980 debris-avalanche deposit at base; an 

erosional surface (disconformity); about 2 meters of lahar deposits, emplaced in 2006; and 30–40 

centimeters of coarse fluvial gravel on top. Smithsonian Institution photograph by L. Siebert, 2016.
Road Log and Description of Field-Trip Stops 49

Day 2: Pyroclastic density current, pyroclasticfall, lahar, and volcano-fluvial deposits in Lewis 

River drainage, Mount St. Helens

Field trip stops for Day 2 are shown in figure 45; note that the 

sequence of numbered and lettered field trip stops continues from 

Day 1. Descriptions for each stop are largely extracted from an 

unpublished field-trip guide written by Michael Clynne and others 

in 2003 and revised in 2012.

Figure 45. Locations of Day 2 planned stops (numbers) and optional stops (letters) on the south and southeast

sides of Mount St. Helens. CB: Cave Basalt; CF: Cedar Flats fan; MM: Marble Mountain. 

Day 2 begins by driving south on I–5 from Kelso to 

Woodland. Cumulative mileage for Day 2 begins in Woodland, 

Washington. Take Exit 21 off I–5. Traveling from the north, the 

exit ramp becomes Pacific Avenue, which continues for about 

one mile to a traffic light at S.R. 503 (Lewis River Road); turn 

left and continue under the freeway. Cumulative mileage starts 

at the second traffic signal—travelling east on S.R. 503 at the 

intersection with East CC Street. S.R. 503 parallels the Lewis 

River. Travel time is about 1.25 hours to the town of Cougar, 

where Stop 6 in located.

0 2 4 KILOMETERS

0 2 MILES

N

503

Swift Creek Swift Creek

Swift Reservoir

Pine CreekPine Creek

Muddy River Muddy River

6

H

J K

7 L

9

M

8

G

6

H

J K

7 L

9

M

8

G

8 Field-trip stop—Number is

planned stop; letter is

optional stop

EXPLANATION

Cave

Basalt

Marble

Mountain

Cedar

Flats

fan

Mount

St. Helens

Base from U.S. Geological Survey (USGS) 30-meter Landsat Imagery

Shaded relief and roads from USGS’s The National Map
50 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

0.0 Head east on S.R. 503 at the intersection with East CC

 Street.

3.5 Missoula Flood terraces and modern flooding

The catastrophic Missoula Floods emplaced 

deposits that are preserved as terraces on both sides 

of the Lewis River for the first 3.5 mi from Woodland 

(fig. 46). The sediments consist of sand, silt, and gravel 

deposited by repeated catastrophic breaching of glacial 

Missoula Lake, which was dammed by an advancing 

lobe of a continental glacier in western Montana dozens 

of times between about 20,000 and 15,000 calendar 

years ago (O’Connor and Benito, 2009). Lower 

elevation terraces are cut into interstratified Lewis River 

flood deposits and Holocene lahar and lahar-runout 

deposits from Mount St. Helens. In 1996, Woodland 

suffered significant damage from a flood on the Lewis 

River. Many of the homes you see along the road were 

inundated with as much as 1.5 m of standing water.

15.0 Tertiary volcanic rocks

Outcrops on the left side of the road are 

composed of Tertiary volcanic rocks of the western 

Cascades. Lake Merwin, on the right side of the road, 

is one of three major reservoirs maintained for power 

generation and recreation along the Lewis River.

22.9 Intersection of S.R. 503 with Yale Bridge Road. 

 Continue straight through the intersection on S.R. 503.

25.0 Yale Reservoir

The lake on the right side of the road is Yale 

Reservoir, the second of three reservoirs on the Lewis 

River. Note that 4.7 mi before the town of Cougar, S.R. 

503 turns south and follows Yale Bridge Road toward 

Amboy—do not turn here. Continue east on Lewis 

River Road toward Cougar, Wash.

28.0 Cougar, Washington

At Cougar, Lewis River Road becomes U.S. 

Forest Service Road 90 (F.R. 90), and the route 

continues through the town on this road. Cougar is 

only 18 km in a straight line from Mount St. Helens, 

making it the closest community to the volcano. 

The town is also the last place between here and the 

volcano to refuel, except for a remote convenience 

store at Eagle Cliff located at the upstream end of 

Swift Reservoir. Remnants of Amboy Drift (glacial 

deposits of ~170–130 ka (Hayden Creek age) are 

found in roadcuts east of Cougar. Deposits on the 

south side of Mount St. Helens are dominated by those 

of the Ape Canyon, Cougar, and Swift Creek eruptive 

stages (fig. 1).

30.5 Swift Reservoir hydroelectric power canal dam

At this point, the road curves to the southeast 

and you will pass an earthen dam on the left side of 

the road at the end of a hydroelectric power canal. 

The canal, operated by PacifiCorp, routes Lewis River 

water with minimal head loss from Swift Dam to an 

auxilliary power house located here on the right side of 

the road. On April 21, 2002, the dam catastrophically 

failed, destroying parts of the power-generating facility 

and scouring the canal down to bedrock (Cave Basalt 

lava flow) (Regan and others, 2008). 

Failure of the 25-m-high dam was caused by an 

unusual piping failure (fig. 47). The 30-ft-thick clay seal 

on the canal bed was compromised, allowing canal water 

to enter a lava tube in the basalt beneath the canal. The 

hydrostatic head created within the lava tube caused a 

catastrophic blowout at the base of the dam. Leakage 

began between 3:35 a.m. and 3:50 a.m. and the blowout 

occurred at 6:55 a.m., creating a 3-m-deep high-velocity 

torrent aimed directly at the power-generating building. 

Just past the building, the road enters a forested flat 

area that is the surface of the Cave Basalt. Tumuli, pushups, and pahoehoe surfaces can be seen from the road.

32.2 Bridge over the power canal and Cougar-age debris-

 avalanche deposits

The route crosses over the power canal 1.5 mi 

upstream of the downstream end of the canal. From 

the bridge, the steep slope ahead on the north side of 

the power canal is composed of Cougar-age debrisavalanche deposits from Mount St. Helens that are 

mostly obscured by vegetation. The Cave Basalt 

Figure 46. Schematic cross section of Lewis River valley, located 

about 1 mile east of Woodland, Washington. Diagram shows terraces 

at about 40 meters (140 feet) elevation above sea level, capped by late 

Pleistocene Missoula Flood deposits (river elevation is about 6 meters 

[20 feet]). This is the highest level achieved by more than 80 floods that 

came down the Columbia River and backed up into tributary valleys. 

Modified from Pringle (2002).

Lewis River

FEET

200

100

-100

-200

0 0.5 1 1.5 2 MILES

Sea level

NNW SSE S.R. 503

Tertiary

bedrock

Post-Missoula flood alluvium,

including ancient lahar deposits

from Mount St. Helens

? ?

Missoula flood

deposits

Missoula flood

deposits

VERTICAL SCALE GREATLY EXAGGERATED

Distance, in miles
Road Log and Description of Field-Trip Stops 51

Figure 47. Failure of the Swift Reservoir power canal dam. A. Photograph showing the blowout of the power canal dam and flooding of Forest 

Road 90 at about 6:55 a.m. on April 21, 2002. PacifiCorp photograph by T. Low, 2002. B. Aerial photograph of dam failure and the damage to 

power-generating facility. Note the sinkhole into the Cave Basalt lava flow at top center of frame. USGS photograph by C.D. Miller, 2002.

A B

flowed through a small notch to the left, near the west 

end of the power canal, and cascaded downslope into 

the Lewis River valley to form the surface you just 

drove over. Just before crossing the bridge, the route 

passed from Cowlitz County into Skamania County. 

Continue along F.R. 90 up the hill.

34.0 Optional Stop G—Swift Reservoir overlook 

 (lat 46.0663° N, long 122.1998° W) Pull out at the 

 overlook on the right side of the road, just before the 

 sharp turn to the left. This is a somewhat overgrown 

 viewpoint that offers an upstream view of Swift 

 Reservoir. The overlook lies on a Pleistocene terrace 

 that is a remnant of a Cougar-age debris fan. A vertical

 exposure of the fan deposits is visible (but extremely 

 difficult to access) downstream on the south Lewis 

 River valley wall, about 70 m lower than the overlook 

 elevation (fig. 8). Terrace stratigraphy (upward from 

 base of section) consists of two Ape Canyon-age 

 lahars, a large Cougar-age debris-avalanche deposit 

 that once blocked the Lewis River valley, a Cougar-

 age (24.4 ka) two-pumice dacite PDC deposit, and 

 a younger Cougar-age lahar deposit and tephra 

 (Clynne and others, 2005, 2008). The 100-m-thick 

 two-pumice PDC deposit, also visible in a borrow 

 pit across the road to the west, was emplaced as 

 multiple flow units following the debris avalanche 

 after traveling more than 15 km from the volcano. 

 A minimum volume estimate for this PDC is 0.5 km3

. 

 Lithic blocks of dome rock in the two-pumice PDC 

 deposit suggest that intrusion of magma into a large dome

 complex (a proto Mount St.Helens—seefigure1B) 

 probably destabilized the edifice and triggered the 

 debris avalanche. It appears to have been the largest 

 edifice-collapse event in the volcano’s history prior to 

 1980. 

From here, continue east and watch for a sign 

marking the turnoff for F.R. 83 directing you towards 

Ape Cave, Climbers Bivouac, and Lava Canyon.

34.9 Turn left onto F.R. 83. Stay on F.R. 83 until Stop 6.

36.6 Turn left onto F.R. 8303 to go to Optional Stops H and

 I (sign for Ape Cave), otherwise continue straight on

 F.R. 83.

36.8 Optional Stop H—Trail of Two Forests 

 (lat 46.0993° N, long 122.2131° W) After 0.2 mi on 

 F.R. 8303, pull in to the parking lot on the left side of 

 the road. To get to Optional Stop I, turn left out of the 

 parking lot and continue along F.R. 8303. Otherwise, 

 turn right to return to F.R. 83.

A short loop trail, the Trail of Two Forests leads 

to spectacular lava casts (vertical and horizontal) of 

large trees that formed when the 1.9 ka Cave Basalt 

lava flowed through the forest. Some of the casts form 

narrow-diameter tunnel mazes that can be explored by 

adventurous children and non-claustrophobic adults 

(bring flashlights).

37.5 Optional Stop I—Ape Cave (lat 46.1079° N,

 long 122.2118° W) After 0.7 mi on F.R. 8303, pull

 into the well-marked parking area on the right side 

 of the road. At the end of the parking lot is Ape Cave, 

 a large-diameter and unusually long lava tube that 

 formed in the Cave Basalt lava flow. The lava tube 

 is nearly 4 km long and one of the longest lava tubes 

 in the conterminous United States (Tucker, 2015). 
52 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

 It can be explored individually or with guided 

 walks during summer, but reliable light sources 

 (lanterns or flashlights), sturdy shoes, and warm 

 clothes are needed. The temperature in the cave is 

 about 6 °C (42 °F) year-round with a steady light 

 breeze owing to cold-air drainage.

After finishing with Optional Stops H and I, return 

to the intersection with F.R. 83. Turn left and continue 

north along F.R. 83. Note that the mileage from Stops H 

and I does not contribute to the cumulative mileage. 

37.9 Intersection of F.R. 83 with F.R. 8100. Continue 

 straight (north) on F.R. 83.

38.1 Stop 6—Swift Creek PDC deposits (lat 46.1198° N, 

 long 122.2061° W) Watch for a large roadcut on the 

 left side of the road. There is a place to park on the 

 right side of the road just as you arrive at the outcrop. 

PDC deposits of the younger part of the Swift 

Creek volcanic assemblage (fig. 48) (Hyde, 1975) 

are exposed in a road cut along the west side of the 

West Fork Swift Creek valley on F.R.83 (fig. 45). The 

outcrop consists of three PDC deposits erupted during 

the Swift Creek eruptive stage (fig. 1) overlying a 

“flowage deposit”, which in turn rests directly on till 

emplaced during the Last Glacial Maximum. Clasts in 

400

500

600

700

800

900

1,000

1,100

1,200

300

200

350

250

150

FEET METERS

NORTH SOUTH

Tephra

Pyroclastic-flow deposit

Flowage deposit (lahar?)

Pyroclastic-flow deposit

Flowage deposit

Gravel deposited by

 streams and lahars(?)

Pyroclastic-flow deposit,

 18,550 years old, 

 overlain by silt and clay

Stream gravel overlain by 

 sand, silt, and clay 

 containing tephra couplet

Pyroclastic-flow deposit, 

 20,000 years old

Flowage deposit

Pyroclasticflow deposit 

and lahars

Older part of Swift

Creek assemblage

Older part of Swift

Creek assemblage

Younger part of 

Swift Creek

 assemblage

Lewis River

Configuration of paleovalley

 not known

the lowest PDC deposit consist of 73 percent pumice 

and 27 percent nonvesicular andesite and dacite, but 

only 23 percent of the fragments in the overlying 

middle PDC deposit are pumice. Tephra set S separates 

these two deposits from an upper PDC deposit in 

which pumice is absent. This third PDC deposit is only 

about 1 m thick and contains angular, radially jointed 

rock fragments with diameters of 10–20 cm. 

When finished with this outcrop, continue in the 

same direction along F.R. 83. Drive 20 minutes to Stop7.

38.4 Swift Creek lava flow

Just after a sharp curve to the right, F.R. 83 begins 

to contour around the edge of the largest lava flow 

in the history of Mount St. Helens. You will begin to 

see outcrops of this lava on the left side of the road. 

The 200-m-thick Cougar-age (20–18 ka) andesite 

flow originated from a vent that was then at or near 

the summit at 1,830 m elevation, and traveled about 

6 km to the south (Clynne and others, 2008). The 

flow is overlain by PDC deposits of the same age. 

Climbers Bivouac—the access point for the Monitor 

Ridge climbing route to the summit—lies atop the 

flow. Summit climbs during the summer months are by 

permit only; daily allotments of 100 permits often sell 

out well in advance.

Figure 48. Cross-sectional view of deposits of the Swift Creek eruptive stage exposed in the Lewis 

River valley near the mouth of Swift Creek. The younger part of the Swift Creek assemblage is 

exposed at Stop 6. Stippled pattern marks areas of limited exposure. Modified from Hyde (1975).
Road Log and Description of Field-Trip Stops 53

40.4 Swift Creek

The roadcut on the east side of Swift Creek exposes 

an early Kalama-age lahar deposit underlain by Pine 

Creek-age dacitic lithic PDC deposits. Over the next few 

miles, the road crosses forested areas underlain by Pine 

Creek lithic PDC deposits, early Kalama-age lahar and 

PDC deposits, and an older Castle Creek-age lahar.

41.8 June Lake trailhead

The Worm Flows lava complex is visible to the 

north from this point (fig. 49). This sinuous complex of 

fresh andesitic lava flows, with prominent flow levees, 

was emplaced in the early- to mid-1500s during the 

Kalama eruptive period. We will see better views of 

these flows at the Lahar Viewpoint (Optional Stop K) 

and Stratigraphy Viewpoint (Stop 7).

43.5 Tertiary bedrock knoll and tephra-fall deposits

F.R. 83 passes through a small knoll of Tertiary 

volcanic rocks that is almost completely buried by Mount 

St. Helens deposits. The entire set of Spirit Lake stage 

tephra-fall deposits is present on the Tertiary bedrock, 

including 1980, X, W, B, P, and Y tephra sets. The next 

bridge that you reach is the location of Optional Stop J.

44.8 Optional Stop J—West Pine Creek bridge 

 (lat 46.1535° N, long 122.1048° W) Pull off the road

 in a safe location. Lithic PDC deposits of Pine Creek 

 age are exposed in the channel banks. This channel 

 conveyed part of the Pine Creek lahar on May 18, 

 1980, which eroded the channel and destroyed the

 pre-eruption bridge at this location. From here, 

 continue in the same direction on F.R. 83.

0 1 2 KILOMETERS

0 1 2 MILES

N

Muddy RiverMuddy Riv

Mudd

er

y River

Muddy River

Pine CreekPine Creek

Smith Cre ekSmith Cre ek

Swift CreekSwift Creek

J

K

L

7

Field-trip stop—Number is

planned stop; letter is

optional stop

EXPLANATION

7

Worm

Flows

Swift Creek

Lava Flow

Mount

St. Helens

Base from Google Maps, 2017 

National Forest Road 83National Forest Road 83

Figure 49. Topographic map of the southeast quadrant of Mount St. Helens showing Day 2 stops. This map shows the Pine Creek-Muddy River 

debris fan at center and surrounding terrain, with planned field-trip stops (numbers) and optional stops (letters) labeled. 
54 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

45.3 Pine Creek-Muddy River fan

The road passes onto the Pine Creek-Muddy 

River debris fan—a broad surface underlain by a very 

thick (approximately 200–400 m) sequence of lahar 

and PDC deposits that has nearly filled the paleovalley 

of upper Pine Creek (Crandell and Mullineaux, 1973). 

The deposits were emplaced mainly during the Pine 

Creek and Kalama eruptive periods. Much of the fan 

surface was swept clear of vegetation by a large lahar 

in 1980 (Pierson, 1985). 

45.6 Optional Stop K—Lahar Viewpoint (lat 46.1593° N, 

 long 122.0956° W) Pull into the Lahar Viewpoint 

 parking lot on the right side of the road. 

Streams on the Pine Creek-Muddy River debris fan 

now form a mainly parallel drainage pattern. Pine Creek 

stays within the Pine Creek valley, but Muddy River, 

having captured some of the Pine Creek drainage, now 

spills through a low divide at the head of Lava Canyon 

on the ridge between the Pine Creek and Muddy River 

valleys and descends the flank of the ridge that once 

separated the two fluvial systems (fig. 49). 

The 1980 Pine Creek-Muddy River lahar was 

triggered during the first minutes of the May 18 eruption 

by the action of a lithic PDC scouring, entraining, 

and melting snow and ice from the outer flanks of 

Mount St. Helens (Brugman and Meier, 1981; Pierson, 

1985; Waitt, 1989) (fig. 50). The complex lahar path 

is shown in figure 51. An average of 6 m of snow and 

ice (as much as 10 m locally) were eroded from the 

volcano’s southeast flank. The initiating PDC, generated 

by “boiling over” of hot fragmental material during 

the first minutes of the eruption, was partly funneled 

through Shoestring Notch, a low point on the crater rim, 

accelerated down the outer flank of the volcano as fast 

as 50 m/s (Moore and Rice, 1984), and transformed to a 

high-concentration, high-velocity (40 m/s) lahar by the 

time it reached the upper Pine Creek-Muddy River fan. 

Pumiceous PDCs formed by column collapse during 

the afternoon that day also swept through this area and 

triggered a lahar composed almost purely of pumice in 

Muddy River (Pierson, 1985). 

Lahar Viewpoint is located where the 1-km-wide 

lahar split into two flows—one continuing down Pine 

Creek, and the other descending Lava Canyon to the 

Muddy River, where it joined a lahar from Smith Creek 

traveling down that channel (Pierson, 1985). At Lahar 

Viewpoint, the lahar was only a few meters deep but 

moving at about 20 m/s. The Pine Creek lahar reached 

Figure 50. Photographic view of the south flank of Mount St. Helens between 8:33 and 8:34 a.m. on May 18, 1980 

(eruption began at 08:32 a.m.). As the lateral blast moved northward (away from camera), lithic pyroclastic density 

currents “boiled over” onto the west, south, and east flanks. The front of a major pyroclastic density current 

(labeled surge front) moved down the southeast flank at 50 meters per second. Photograph by K. Seibert, 1980. 

Lateral blast

Surge front
Road Log and Description of Field-Trip Stops 55

Figure 51. Map of volcanic slopes and river channels on west, south, and east flanks of Mount St. Helens that were impacted 

by May 18, 1980, lahars (see recent aerial view in fig. 45). Topographic contours (gray) in meters above sea level. Modified from 

Pierson (1985). See figure 54 for descriptions of A and B.

Ape Canyon

MuddyRiver

Gorge

Area devastated by lateral blast

Areas swept by PDCs and lahars in Pine

Creek-Muddy River basin

Zone of highly inflated PDC flow

Zone of PDC snow scour and wetting

Zone of water-saturated lahar flow

Lahar overflow into standing forest

Other areas swept by pyroclastic density 

current (PDC)-generated lahars

Topographic contour—Contour interval

750 meters. Datum is sea level

Boundary between net erosion and net

deposition

Channel knick point

Approximate boundary

Gaging station

Location of figure 54A 

Location of figure 54B 

1,250

A

B

B

EXPLANATION

A

Lahar delta 

in reservoir

Ape Cave geologic site

Muddy River fan

Pine Creek–

122°15’ 122°00’

46°15’00’’

46°07’30’’

7’30’’

1,250

2,000

0 1 2 3

0 1 2

MILES

3 KILOMETERS

Spirit

 Lake

Swift Reservoir

Smith Creek

Pine Creek

Muddy

River

Swift Creek

Mount

St. Helens

Swift Reservoir 23 km downstream from the crater only 

about 20 minutes after the beginning of the eruption; 

the Muddy River lahar arrived shortly thereafter. The 

combined lahars deposited 14 million cubic meters of 

muddy debris in a delta at the head of Swift Reservoir 

(Pierson, 1985).

Most trees encountered by the high-velocity lahar 

were sheared off a few inches above ground level, and 

a levee of tangled tree trunks can still been seen along 

the western flow margin just upstream of F.R. 83. Post1980 tree growth here has reduced visibility of the lahar 

deposits, which are now better exposed near Stop 7 

(Stratigraphy Viewpoint). 

From here, turn right out of the parking lot and 

continue east on F.R. 83 to Stop 7.

46.0 Stop 7—Stratigraphy Viewpoint (lat 46.1633° N,

 long 122.0922° W) Pull into the pullout along the left 

 side of the road just before crossing the bridge over the

 Muddy River (a small stream here). 

Much of the eruptive history of Mount St. 

Helens is recorded by bedded pyroclastic-fall deposits 

(Mullineaux, 1986, 1996). At this stop, a short walk 

across the Muddy River channel leads to streambank 

outcrops that reveal an impressive stratigraphic section 

of tephra and interbedded lahar deposits of the Spirit 

Lake eruptive stage (3.9 ka to present) and older 

deposits of Mount St. Helens (fig. 52). This outcrop 

exposure is occasionally modified by slumping and 
56 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

A

B May 18, 1980, lahar

Lahar

Tephra set W

Tephra set B

Tephra set P

Tephra set Y

Tephra set J

Figure 52. Pyroclastic-fall, pyroclastic 

density current, and lahar deposits at 

Stratigraphy Viewpoint along the Pine 

Creek-Muddy River debris fan margin (Stop 

7). A. Photograph of the Pine Creek-Muddy 

River debris fan and Stratigraphy Viewpoint 

(light-colored bluff) along its margin. 

Shoestring Notch in Mount St. Helens’ 

crater rim can be seen in the background. 

Smithsonian Institution photograph by 

L. Siebert, 2016. B. Photograph of the 

stratigraphic section at bluff outcrop 

(stream cutbank) showing major tephra sets 

labeled by letters; intervening units are lahar 

and pyroclastic density current deposits 

(see fig. 1 for approximate ages). Note longhandled shovel for scale. USGS photograph 

by R. Waitt, 1981. C. Map of the tephra fall 

distribution from three major Holocene 

explosive eruptions at Mount St. Helens. 

Modified from Clynne and others (2005). 

D. Photograph of the stratigraphic section 

in A, as seen today. Scale bar marked in 

red and white intervals of 10 centimeters. 

Smithsonian Institution photograph by L. 

Siebert, 2016.
Road Log and Description of Field-Trip Stops 57

C

D

0

0

200

200 400 KILOMETERS

400 MILES

N

CANADA

IDAHO

MONTANA

WASHINGTON

OREGON

UNITED STATES

Mount 

St. Helens

1850 B.C.E.

(Yn tephra)

1980 tephra

1482 C.E.

(We tephra)

PACIFIC

OCEAN

Ye tephra

We tephra

Tephra set X

Tephra set B

Tephra set P

Tephra set J

lahar

lahar

surge

surge

surge

lahar

Spirit Lake

eruptive stage

Swift Creek

eruptive stage

erosion and the stratigraphic section varies along the 

cutbank.

The exposed sequence includes the thick pumice 

tephra set S (about 16 ka) at the base, the highly 

oxidized set J (about 13 ka) of the Swift Creek stage 

(fig. 53), and tephra layers Ye (3.3 ka) and We (1482 

C.E.) of the Spirit Lake stage. The section also 

contains several intercalated lahar and PDC deposits 

and is capped by tephra set X (about 1500 C.E.) and 

the 1980 lahar deposit (fig. 53). Sets We and X are 

precisely dated by anomalously narrow growth rings 

on adversely affected trees (Yamaguchi, 1983). Tephra 

set J may now be the lowest pyroclastic-fall unit 

exposed; most of the basal tephra set S has been buried 

by the aggrading stream channel. A more detailed 

section of tephra set S, which comprises at least three 

major pumiceous air-fall tephra layers, can be seen at 

nearby exposures to the northeast.

If you wish to go to Optional Stop L, continue on 

from the pullout for Stop 7 and continue on F.R. 83 for 

another 0.5 mi to the end of the road. If you are not going 

to Optional Stop L, turn right out of the parking lot and 

backtrack on F.R. 83. Drive 45 minutes to Stop 8.
58 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

46.5 Optional Stop L—Lava Canyon Trail (lat 46.1658° N,

 long 122.0882° W) The Lava Canyon trailhead is at the 

 end of F.R. 83. The Lava Canyon trail traverses 

 deceptively hazardous terrain. Make sure everyone in 

 your party stays on the trail. The fast-moving water can 

 sweep a person over the waterfalls if he or she slips in. 

The trail descends into Lava Canyon, an incision 

cut by the Muddy River into an andesitic lava flow that 

flowed down the canyon 1,900 years ago. In places the 

canyon cuts into weaker Tertiary rocks lying beneath 

the lava flow. Several waterfalls drop over cliffs in the 

Kalama-age lava flow, which is about 16 m thick here. 

It displays two common features of columnar jointing: 

an upper 8-m-thick set of small, irregular, ill-formed 

columns (“entablature”), and a lower set of wellformed columns (“colonnade”) of equal thickness. A 

0.4-mi wheelchair-accessible section of the trail leads 

to views of Lava Canyon at the uppermost waterfall. 

Here, a large boulder from the 1980 lahar is perched 

on top of the scoured lava flow surface. An extended 

0.6-mi loop that crosses the Muddy River on a 

swinging suspension bridge provides good views 

of the young lava flow overlying oxidized lower 

Oligocene rocks. The trim line cut by part of the 1980 

Pine Creek-Muddy River lahar can be seen in the 

prominent change in canyon-side vegetation from 

post-lahar alder trees to old-growth trees above. 

When you are finished, turn around and backtrack 

on F.R. 83, passing Stops 7 and 6, until you reach the 

intersection with F.R. 90.

56.9 Turn left onto F.R. 90 towards Windy Ridge.

60.7 The third of the large reservoirs on the Lewis River,

 Swift Reservoir, is on the right. The road parallels the

Figure 53. Close-up photographs of two volcaniclastic units at Stratigraphy Viewpoint (Stop 7). A, Coarse air-fall pumice unit of tephra set J

(upper part). B, Low-concentration pyroclastic surge deposit (Smith Creek eruptive period) sandwiched between two air-fall pumice units. 

Deposit displays the low-angle crossbedding characteristic of many surge deposits. USGS photographs by T. Pierson, 2017.

 reservoir until you are nearly to Stop 8, about 10 mi 

 ahead.

68.7 U.S. Forest Service Pine Creek Information Center is

 on the left side of the road. The information center is 

 usually open in summer during normal business hours.

 The next stop is about 500 ft ahead.

68.8 Intersection with F.R. 25

Continue straight on F.R. 25 to follow the fieldtrip route. If you need gas or diesel, turn right on F.R. 

90 and drive 0.7 mi to a convenience store with fuel 

pumps on the left. If you are continuing northward to 

Windy Ridge Viewpoint or Randle, Wash., this station 

provides the last fuel available before Randle.

69.5 Stop 8—Pine Creek Boulder and inactive quarry

 (lat 46.0725° N, long 122.0182° W) Pull into the 

 parking area located on the right side of the road. 

 Stop 8 (fig. 45) consists of two adjacent localities 

 highlighting volcaniclastic sediment that has been 

 transported down Pine Creek, (1) a massive boulder 

 transported by the 1980 Pine Creek lahar (fig. 54A), 

 located approximately at location C in figure 52; and 

 (2) a small inactive quarry showing a sequence of Pine

 Creek-age flow units (fig. 54B). After parking, walk 

 behind the boulder and carefully make your way down 

 the informal trail leading to a beach along the Lewis 

 River. The informal trail is steep, descending about 

 20 m in 60 m. Please proceed carefully. We will eat 

 lunch here, on the Lewis River beach, while viewing

 pre-1980 lahar deposits in riverside bluffs. 

At this location, the Pine Creek lahar had a peak 

flow velocity of about 12 m/s, a peak flow depth of 

about 6 m, and a peak flow width of about 100 m; its 
Road Log and Description of Field-Trip Stops 59

A B

peak discharge was about 7,300 m3/s (Pierson, 1985). 

Such lahars can transport massive clasts (fig. 54A).

The 37-ton (short ton) Pine Creek Boulder 

(approximate dimensions 8×5×5 m) was deposited on 

the road 10m above the creek bed by the 1980 Pine

Creek lahar, making use of bed aggradation during 

lahar passage to reach that height. It was later moved 

off the road to its present location. The largest known 

boulder transported by 1980 lahars (10.4×6.0×4.6 m) 

was found resting on a terrace of 1980 lahar deposits 

along the Muddy River near location A in figure 51. It 

must have rolled there as bedload, because the lahar 

depth at that location is only 4 m—too shallow to 

have transported such a large clast as suspended load 

(Pierson, 1985). Older deposits of Pine Creek age are 

seen in the cutbank along the Lewis River below Pine 

Creek Boulder, where we plan to have lunch. As you 

walk to and from the outcrops along the Lewis River 

you pass through a young forest that has grown on 

the surface of the 1980 lahar deposits. In contrast, the 

large tree stumps here are rooted into the underlying 

Kalama-age deposits that formed the pre-1980 surface. 

Note that where lahar deposits are thin, trees growing 

on underlying surfaces can survive inundation and 

abrasion injury (fig. 54B). They also record the impact 

date by producing tree rings for a decade or two that 

are much thinner than average.

From the Pine Creek Boulder parking area, 

continue about 100 m on F.R. 25 across the bridge 

over Pine Creek. Just after the bridge, a small dirt 

road enters a quarry on the right side of the road 

(lat 46.0743° N, long 122.0164° W). Note this is a 

privately owned quarry and we recommend that you 

get permission from the landowners (Three Rivers 

Recreational Area, LLC) prior to entering. The quarry 

provides excellent exposures of Pine Creek-age lahars 

emplaced within a paleochannel that cuts into older 

Swift Creek-age deposits (fig. 55). These lahars overlie 

a PDC deposit of Pine Creek age that is exposed in the 

bank of the Lewis River. 

From the quarry, continue in the direction you 

were traveling on F.R. 25 for about 10 minutes. Watch 

for the next bridge, located just before the parking 

place for the next site.

Figure 54. Signs of passage left by 1980 lahars other than major deposits. A. Photograph of a boulder weighing 37 short tons that was 

transported to this location by the Pine Creek lahar. Note mud marks on background trees indicating that the boulder was never more than 

half submerged by the lahar that emplaced it. USGS photograph, September 1980. B. Photograph of mud coatings on trees that mark the peak 

inundation depth of the 1980 Muddy River lahar. Note how much less material was emplaced by the lahar in comparison to this maximum depth. 

Person in yellow for scale. USGS photograph by L. Topinka, October 1980. These photograph locations can be viewed in figure 51 of this report. 

Figure 55. Cross-sectional photographic view of Pine Creekage lahar deposits and volcano-fluvial layers in quarry at Stop 

8. The view is nearly parallel to flow direction (from left to right), 

with a coarse gravel bar at center right. Smithsonian Institution 

photograph by L. Siebert, 2016.
60 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

73.0 Optional Stop M—Cedar Flats Natural Area 

 (lat 46.1117° N, long 122.0186° W) Driving back 

 toward Cougar, watch on the left side of the road for 

 an unsigned pullout next to a split-rail log fence that 

 marks a trailhead. Pull off the road and park at the 

 fence. This is the start of a 1 mi loop trail through a 

 spectacular remnant of lowland Pacific Northwest 

 old-growth forest. Rare examples of fully mature 

 Douglas fir (Pseudotsuga menziesii), western hemlock

 (Tsuga heterophylla), and western redcedar (Thuja 

 plicata) are present. Cedar Flats Natural Area, on 

 which the forest grows, occupies a large debris fan 

 at the mouth of Pine Creek. The debris fan is 

 underlain by a Cougar-age debris-avalanche deposit, 

 which likely transformed to a lahar by the time it was 

 deposited here (fig. 56). Its exposure at Cedar Flats 

 suggests that the debris avalanche diverted around 

 the Marble Mountain massif and traveled down both 

 the Swift Creek and Pine Creek valleys. 

The debris-avalanche deposit at Cedar Flats 

is overlain by Swift Creek-age lahars and lithic 

PDC deposits and Kalama-age deposits from 

hyperconcentrated flows and muddy streamflows (M. 

Clynne, oral commun., 2016). The Swift Creek deposits 

are mostly oligomictic and dominated by porphyritic 

hornblende-hypersthene dacite blocks, which were 

derived from lava domes. Clasts of the Kalama-age 

deposits are mostly weakly vesiculated bread-crusted 

andesite with rare blocks of highly vesiculated andesite 

as much as several meters in diameter. As many as five 

beds of the Kalama-age deposits are visible here and are 

related to eruption of the middle Kalama PDCs, which 

are identical in lithology and composition to the Worm 

Flows lava complex. About half way through the loop, 

the trail passes along a steep embankment that overlooks 

the Muddy River. The Cougar-stage debris-avalanche 

and younger deposits crop out along this embankment, 

but off-trail access is steep and difficult. 

73.4 Stop 9—1980 Muddy River lahar deposits 

 (lat 46.1223° N, long 122.0145° W) Cross the Muddy

 River bridge and park on the right side of the road or in

 the parking area on the left a short distance away. You

 are on the northeast bank of the Muddy River. As you

 look downstream from the bridge, the cutbank on the

 northeast side of the river is composed of 1980,

 Kalama-age, and older (unknown age) lahar deposits.

 From the road, walk down the embankment on the 

 downstream side of the bridge and proceed along the

 river. Note that the vertical cutbank can be unstable,

 owing to undercutting by the river. However, the

 continuous erosion also exposes excellent outcrops

 along the cutbank in the few hundred meters

 downstream from the bridge. Please locate an outcrop

 that can be safely accessed to view the stratigraphic

 sequence.

Figure 56. Photograph of a Cougar-age debris-avalanche deposit 

at Cedar Flats, which had already transformed to a lahar by the 

time it reached this location. This exposure shows an extremely 

compact and poorly sorted deposit in a cutbank of the Muddy 

River. Scraped outcrop surface shows abundant vesicles and small 

polymictic angular to subrounded clasts with characteristic mottled 

pastel-colored texture of hydrothermally altered material. Head of 

military trenching tool for scale; looser material above is colluvium. 

Smithsonian Institution photograph by L. Siebert, 2016. 

men17-7185_fig 56

As was noted at Stop 8, young trees are growing 

on the surface of the 1980 lahar deposits, and mixed 

among them are stumps of trees that were growing on 

the underlying Kalama-age deposit and killed by lahar 

inundation in 1980. 

Two lahars flowed down Muddy River on May 

18, 1980 (fig. 57): (1) a large lahar in the morning 

that was composed dominantly of lithic clasts and 

(2) a much smaller lahar in the afternoon composed 

entirely of rounded pumice clasts (Pierson, 1985). 

Deposits of both can be seen along the northeast 

bank about 100 m downstream of the Muddy River 

bridge (fig. 58A), although the upper unit is only 

locally preserved. Evolution of the morning lahar 

is described in detail at Optional Stop K. In this 

area, the morning lahar was at least 4.2 m deep and 

capable of inflicting severe damage to trees growing 

on low river terraces (fig. 58B). The afternoon lahar 

derived water from rapid snowmelt generated by 

pumiceous PDCs descending Mount St. Helens’ 

southeast flank during the main Plinian phase of the 

eruption (Pierson, 1985).

In its lower reach, the Muddy River channel has 

a flatter gradient than the lower Pine Creek channel. 

The morning lahar was probably partly hydraulically 

dammed by a sharp bend in the river just downstream 

of Stop 9. In fact, the back-calculated flow velocity 

here was less than 4 m/s—one of the slowest velocities 

measured (Pierson, 1985). The Muddy River lahar 
Road Log and Description of Field-Trip Stops 61

Figure 57. Lake-level record for Swift Reservoir on May 18, 1980, 

and reconstructed hydrographs for the Pine Creek and Muddy 

River lahars as they entered Swift Reservoir. The Pine Creek 

lahar arrived about 34 minutes before the Muddy River lahar and 

30 minutes after being triggered on the flank of the volcano. The 

total sediment volume deposited underwater was 13,431,000 cubic 

meters (m3

). A second, smaller Muddy River lahar, composed of 

pumice fragments, arrived in the late afternoon with a volume of 

about 520,000 m3

. Modified from Pierson (1985).

8 a.m. 10 a.m. 12 p.m. 2 p.m. 4 p.m. 6 p.m.

Pacific Daylight Time

2,000

0

4,000

8,000

6,000

Lahar infilling rate,

in cubic meters per second

23

22

24

Reservoir stage,

in meters

entered the Lewis River with a peak flow rate of about 

5,000 m3/s and reached the head of Swift Reservoir 

about 34 minutes behind the 1980 Pine Creek lahar.

Particle-size distributions for the Pine Creek and 

Muddy River lahars are broadly similar and typical of 

granular debris-flow lahars (fig. 58C). Slurry matrix 

samples of both of these east flank deposits are coarser 

than those of the North and South Fork Toutle River 

deposits. Deposits are, in places, as thick as 2.5 m, but, 

in general, they are less than 0.5 m and thin relative 

to the depth of the flows that deposited them. Thin 

coatings of muddy sand adhered to steep channel 

banks and tree trunks, providing estimates of flow 

depth (fig. 58B). 

This is the field trip’s last site at Mount St. 

Helens. After finishing at this stop, backtrack on F.R. 

25 to the junction with F.R. 90. From here there are 

two possible routes (detailed below) to the starting 

point for Day 3 in Hood River, Oregon. Both routes 

have optional stops, but alternative route 1 is about 

75 mi shorter than route 2. Alternative route 1 follows 

narrow and winding paved rural roads without services 

to the town of Carson, Wash., crosses the Columbia 

River at the Bridge of the Gods, and proceeds to Hood 

River on I–84. This route passes through the Indian 

Heaven volcanic field (Scott and others, 1995), a 

field with numerous small Quaternary monogenetic 

volcanoes between Mount St. Helens and Mount 

Adams. However, the volcanic field is heavily forested 

and no notable geologic features are visible along this 

route. Alternative route 2 returns to I–5 at Woodland, 

connects with I–205 just north of Vancouver, and joins 

I–84 in northeast Portland before continuing east to 

Hood River.

Day 2: Alternative Route 1

73.4 Backtrack toward Cougar on F.R. 25.

77.9 Turn left onto F.R. 90 at the intersection with F.R. 25.

78.3 Eagle Cliff bridge

You are now crossing the Eagle Cliff bridge, 

which replaces a bridge destroyed by lahars during 

the May 18, 1980, eruption. You will soon pass the 

Eagle Cliff store and campground, which typically has 

gas available during the summer months. If your car 

is running low on gas, this will be your last chance to 

refuel for the next 35 miles. Continue on F.R. 90 and 

watch for an intersecting paved road on the right.

82.1 Turn right onto Curly Creek Road. When the road 

 begins to climb a low hill with a series of curves, 

 watch for your next turn.

84.9 Optional Stop N1—McClellan overlook

 (lat 46.0403° N, long 122.9194° W) Turn right into 

 the parking lot for McClellan overlook, which provides 

 an excellent view of the south side of Mount St. 

 Helens. When finished, turn right out of the parking 

 lot and continue in the direction you were traveling 

 along Curly Creek Road.

87.1 Curly Creek Road ends in a T-intersection with 

 Meadow Creek Road; turn right. Meadow Creek Road 

 eventually turns into Wind River Road, also known as 

 F.R. 30. 

111.0 Wind River bridge

The relatively flat valley that you have been 

traveling along is underlain by a late Pleistocene mafic 

lava flow from the Trout Creek Hill volcanic vent. 

Erosion by the river has exposed the lava flow in a 

deep canyon below the bridge. Continue along Wind 

River Road, also commonly referred to as the Wind 

River highway.
62 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

112.0 You are now entering the community of Carson, Wash.

 Continue through Carson on Wind River Road.

114.5 Turn right onto S.R. 14. You are now in the Columbia

 Gorge.

117.3 Pass through the town of Stevenson, Wash. 

118.1 Rock Cove

As you leave Stevenson, the road is on a causeway 

that divides Rock Cove on the right from the mainstream 

of the Columbia River on the left. The islands in Rock 

May 18, 1980

afternoon lahar deposit

May 18, 1980

morning lahar deposit

Kalama-age (?)

lahar deposit

Paleosol

Pine Creek lahar

Muddy River lahar

99.5

99

98

95

90

80

70

60

50

40

30

20

10

5

2

1

0.5

Cumulative weight percent (percent coarser)

-8 -6 -4 -2 0 2 4 6 8 10

Grain diameter, φ scale

100 10 1 0.1 0.01 0.001

Grain diameter, in millimeters

A B

C

Figure 58. Evidence of lahars in the lower Muddy River. 

A. Photograph of the sequence of lahar deposits at Stop 9. 

Bottom unit was emplaced by a probable Kalama-age lahar. 

A moderately mature paleosol (with A and Box horizons) is 

developed on the top of this unit (the pre-1980 forest floor). 

Shovel blade (20 centimeters long) at top of paleosol. Overlying 

that is the May 18 morning lahar deposit, containing mostly 

pebble-size lithic clasts. At the top of the section is the 

discontinuous deposit emplaced by the May 18 afternoon 

pumice lahar. USGS photograph by T.C. Pierson, 2016. B. 

Photograph of a tree near the lahar flow margin abraded by 

the May 18 Muddy River morning lahar. Note the removal of 

tree bark and the thin coating of mud on the tree trunk as well 

as the burial of the trunk base in the lahar deposit (thickness 

here unknown). USGS photograph by G. Beach, 1981. C. Plot of 

grain-size distribution of Pine Creek and morning Muddy River 

lahar deposits. Phi (j) scale represents grain diameter as the 

negative logarithm (base 2) of grain diameter in millimeters (at 

top). Modified from Pierson (1985).

Cove are hummocks in the Red Bluff landslide, derived 

from Greenleaf Peak, 3.7 mi (6 km) away on the ridge to 

your right. Continue traveling along S.R. 14.

120.5 Bridge of the Gods

Turn left onto the narrow, two-lane Bridge of the 

Gods, and cross the Columbia River. The bridge toll is 

currently $2.00 for cars. You are now entering Oregon. 

At the end of the bridge, follow the road as it loops 

around. Turn right at the bottom of the ramp onto Wa 

Na Pa Street (Cascade Locks Highway). Proceed east 

through the town of Cascade Locks. 
Road Log and Description of Field-Trip Stops 63

Rocky Butte, the intrusive core of a 285±16 ka basaltic 

andesite volcano of the Boring Volcanic Field just west 

of this junction.

150.3 Bridge over the Sandy River

On the last day of the field trip (Day 4 Stop 9), we 

will visit Timberline-age lahar deposits from Mount 

Hood on the Sandy River delta, visible here on the 

north side of the I–84 bridge.

164.0 Optional Stop O2—Multnomah Falls

 (lat 45.5788° N, long 122.1185° W) To visit the falls,

 take Exit 31 (left exit) to Multnomah Falls parking. 

 Hide or take valuables and lock your car; this is a 

 high-theft area. Walk along the pathway, crossing

 through a pedestrian underpass, to Multnomah Falls 

 Lodge (restaurant and concessions); continue past 

 these to the waterfall. This 620-ft (189 m) two-tiered 

 waterfall is the highest of many along “Waterfall 

 Alley” in the western part of the Columbia Gorge and

 the most visited natural recreation site in the Pacific 

 Northwest. Multnomah Creek plunges off vertical 

 cliffs composed of Miocene-age Columbia River 

 Basalt lava flows. A short trail leads to the iconic 

 Benson footbridge between the upper and lower falls, 

 and a longer trail leads to an overlook above the upper

 falls. To the west, the loop trail to Wahkeena Falls 

 leads to additional waterfalls. Multnomah Falls Lodge 

 is on the National Register of Historical Places; a gift

 shop and restrooms are available on the ground level. 

From the parking lot, merge back onto I–84 

eastbound. Note, for those with more time, Multnomah 

Falls is also accessible by a scenic loop road between 

Exit 22 (Corbett) and Exit 35 (Ainsworth State 

Park). This is one of several remnants of the Historic 

Columbia River Highway—the first road across the 

Cascades—which celebrated its centennial in 2016. 

The narrow highway also passes by Vista House at 

Crown Point, with spectacular views of the Columbia 

Gorge and several other major waterfalls.

166.0 Dodson debris fan

You are passing the westernmost of several 

boulder-rich debris-flow deposits near Dodson, Oreg., 

that were emplaced during an extreme rainstorm in 

early February 1996. The deposits are on the south side 

of the road, but are heavily vegetated. As a result of the 

debris flow, I–84 was closed for five days and Union 

Pacific railroad tracks were severely damaged. The 

debris flow also inundated a house, whose residents 

escaped only moments before the flow arrived.

169.6 Beacon Rock

The monolith visible on the Washington side of 

the Columbia River is Beacon Rock, a volcanic plug 

composed of basaltic andesite that stands 848 ft (258 m) 

Cascade Locks takes its name from an early 

navigational lock, finished in 1896, that was 

constructed to help river boats pass the notorious 

Cascade Rapids (from which the Cascade Range 

gets its name). This became a moot issue after 1937, 

when the rapids (and the lock) were drowned by the 

backwater pool from Bonneville Dam (elevation 

around 21 m above sea level, a.s.l.). If you are going 

to Optional Stop P, proceed 0.5 mi from the end of the 

bridge ramp, and turn left on SW Portage Road into 

Marine Park. If continuing directly to Hood River, 

drive 1.0 mi farther and merge onto I–84. At this point, 

alternative routes 1 and 2 follow the same path to 

Hood River, Oreg. Skip ahead to Optional Stop P (at 

mile marker 176.0 on alternative route 2) to continue 

with the road log from this point.

Day 2: Alternative Route 2 

73.4 Begin the return to Woodland, Wash. by heading south

 on F.R. 90 from the F.R. 25 and F.R. 90 junction. 

96.2 Pass through the town of Cougar, Wash. Continue on 

 Lewis River Road.

101.5 Continue straight on S.R. 503.

124.6 Woodland, Wash. Turn left onto I–5 southbound 

 towards Portland, Oreg.

137.6 Merge onto I–205 southbound towards Salem, Oreg.

148.1 Glenn L. Jackson Memorial Bridge

The Glenn L. Jackson Memorial Bridge on 

I–205 over the Columbia River has support piers on 

Government Island, a large mid-channel bar composed 

in part of volcaniclastic sediment from Mount Hood. 

In clear weather you can see a spectacular view of 

Mount Hood from the bridge. 

The Columbia River forms the border between 

Washington and Oregon. The low hills on the Oregon 

side of the river are volcanoes of the Pliocene–

Pleistocene Boring Volcanic Field (fig. 59). The 

volcanic field is named for the town of Boring, 

Oregon (not for being uninteresting). Boring Volcanic 

Field vents extend from the Portland area eastward, 

nearly parallel to the Columbia Gorge. Other vents 

probably related to the Boring Volcanic Field and 

of similar age extend eastward nearly to the Hood 

River. More than 80 vents, located on both sides of 

the Columbia River, were active from about 3.3 Ma 

to 57 ka. 

141.4 Take Exit 22 for I–84 eastbound towards The Dalles.

The junction of I–205 and I–84 crosses channels 

scoured by the Missoula floods that diverged around 
64 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

tall. Emplaced 57 ka, it is the youngest dated vent of the 

Boring Volcanic Field. The plug is the congealed magma 

in the conduit of a cinder cone, which was exposed when 

the Missoula floods stripped away the volcaniclastic 

ejecta that formed the surrounding cone.

173.3 Bonneville Dam

Bonneville Dam is a large hydroelectric dam 

that was the first one built on the Columbia River, 

completed in 1937 to bring ample electric power to the 

Pacific Northwest. Part of Bonneville Dam is built on a 

massive landslide deposit emplaced by the ~600-yearold Bonneville landslide. A navigational lock at 

Bonneville Dam was completed in 1938—the highest 

Figure 59. Colorized digital elevation model of the Portland area. The upper limit of Missoula flooding here was approximately 125 meters

above sea level, corresponding to the transition between blue and green. Higher elevation areas of roughly circular shape in darker brown are 

volcanoes of the Boring Volcanic Field, including Rocky Butte and Mount Tabor. Modified from O’Connor and Burns (2009).

0 1 2 3 4 5 KILOMETERS

0 1 2 3 MILES

5

5

205

405

84

5

205

84

405

5

W

Columbia River

illamette River

Portland

Rocky

Butte

Mount 

Tabor Sand y River

Clackamas River

N

Elevation, in meters 

 above sea level

EXPLANATION

100

0

200

400

 Boring 

Volcanic 

 Field

single-lift lock in the world at that time. A second 

powerhouse was added to the dam, finished in 1981. 

A larger lock was constructed and opened to ship and 

barge traffic in 1993. Bonneville Dam produces more 

than 1,000 megawatts of electric power today.

Just after the exit for Bonneville Dam and east of 

the Eagle Creek fish hatchery, roadcuts on the south 

side of the freeway expose lower Miocene Eagle Creek 

Formation lahar deposits, which underlie the Columbia 

River Basalt Group.

176.1 Alternate routes 1 and 2 merge together here at the 

 town of Cascade Locks. Cascade Locks takes its

 name from an early navigational lock, finished in 
Road Log and Description of Field-Trip Stops 65

 1896, that was constructed to help river boats pass 

 the notorious Cascade Rapids (from which the Cascade

 Range gets its name). This became a moot issue after 

 1937, when the rapids (and the lock) were drowned

 by the backwater pool from Bonneville Dam (pool

 elevation around 21 m a.s.l.).

176.1 Optional Stop P—Bonneville landslide

 (lat 45.6710° N, long 121.8926° W) Take Exit 44 

 off I–84 to Cascade Locks. The exit ramp turns into 

 Wa Na Pa Street. Follow Wa Na Pa Street east into 

 the town and drive 0.5 mi. Turn left on SW Portage 

 Road into Marine Park, passing through the railroad 

 underpass and staying left. Park by the old lock, then 

 walk across the lock on the pedestrian bridge and 

 continue north to the river’s edge for a view of the 

 Bonneville landslide. 

Weak, tilted layers of volcaniclastic sediments 

are responsible for a large number of landslides on 

the Washington side of the Columbia Gorge (Palmer, 

1977; O’Connor and Burns, 2009; Pierson and others, 

2016). The town of Cascade Locks provides several 

viewpoints of what remains of the natural Bridge of 

the Gods—a “bridge” across the Columbia River 

recorded in the legends of local native people. This 

“bridge” has been inferred to be the natural dam that 

held back the Columbia River for months to possibly 

years, forming when the Bonneville landslide broke 

loose from the northern side of the Columbia Gorge in 

the early 15th century and slid completely across the 

river to the southern shore (fig. 60A). Stumps of trees 

killed by submersion in the landslide-dammed lake 

were preserved until they were resubmerged in the 

backwater pool behind Bonneville Dam (fig. 60B). The 

0 1 2 3 4 KILOMETERS

0 1 2 MILES

WASHINGTON

OREGON

Columbia River

RUCKEL LANDSLIDE

BONNEVILLE

LANDSLIDE

Greenleaf Peak

Table

Mountain

Stevenson

Cascade Locks

Bridge of

the Gods

CASCADE

LANDSLIDE COMPLEX

CASCADE

LANDSLIDE COMPLEX

BONNEVILLE DAMBONNEVILLE DAM

N

Shaded relief from 2005 Puget Sound Lidar Consortium

 6-foot light detection and ranging (lidar) data

Shaded-relief outside of map area from Esri and its

 licensors © 2016. 

Landslide scarp

Landslide deposit body

Submerged landslide deposit

Active landslide—Shows movement in 

last 20 years

Maximum shoreline of a lake dammed 

 by the Bonneville landslide

City boundary

EXPLANATION 

A

Figure 60. Large landslides in Columbia Gorge. A. Map of the Bonneville and surrounding landslides on the Washington side of the western Columbia 

Gorge. The blue line shows the approximate location of the maximum shoreline of a lake dammed by the Bonneville landslide (O’Connor and Burns, 

2009). Modified from Pierson and others (2016). B. Photograph of remnants of the submerged forest of the Columbia Gorge drowned by the temporary 

lake formed behind the Bonneville landslide in the early 15th century. Archival photograph 3931 by D.B. Lawrence, Oregon Historical Society, 1933.
66 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

lake-forming blockage was partly breached sometime 

after the landslide occurred, allowing formation of a new 

river channel about 1 km south of the former channel. 

This new channel was quite narrow (140 m) and steep 

(dropping 12 m over 3.2 km), and it was filled with 

large landslide blocks. A formidable set of rapids—first 

described by Lewis and Clark as the Cascade Rapids—

prompted construction of a canal and locks that were 

completed in 1896. Operation of the canal and locks, and 

bypassing of the Cascade Rapids, permitted steamboat 

navigation up the Columbia River and, thus, largescale commerce between the west and east sides of the 

Cascade Range. Marine Park in Cascade Locks provides 

a clear view of the eroded toe of the landslide blockage, 

but the rapids here are now submerged beneath the 

backwater pool of Bonneville Dam (fig. 61).

Bonneville landslide is one of several large 

landslides making up the Cascade landslide complex, 

several of which can be seen from Marine Park in 

Cascade Locks (fig. 61) (Pierson and others, 2016). 

Both the Bonneville and Red Bluffs landslide, each 

about a cubic kilometer in volume, traveled between 5 

and 6 km from their 400- to 500-m-high headscarps to 

their toes. Both are less than 600 years old (Pierson and 

others, 2016). A reactivated, 8 km2

 portion of the Red 

Bluffs landslide, the Crescent Lake landslide (fig. 60A), 

currently creeps during the wet winter months at an 

average rate of about 15–20 centimeters per year.

Breaching of the lake dammed by the Bonneville 

landslide occurred in stages (O’Connor, 2004; 

O’Connor and Burns, 2009). Prominent deltaic surfaces 

at both 75–80 and 40 m a.s.l. indicate that the lake 

B

Figure 60.—Continued 

surface was at or near maximum capacity for some time, 

followed by a prolonged interim stable lower lake level. 

Additional incision into the blockage lowered the lake 

to its pre-1937 historical elevation of 13 m a.s.l. At least 

one episode of lowering the lake level was cataclysmic, 

leaving behind boulder-rich flood bars and bedded sands 

just downstream of the landslide blockage. Distinctive 

beds of relatively coarse sediment in floodplain and 

estuary deposits farther downstream suggest a large 

dam-breach flood occurred. These beds underlie 1479–

1482 C.E. tephra from Mount St. Helens (O’Connor and 

others, 1996; Atwater, 1994). This suggests that a large 

flood affected the lower Columbia River within a few 

decades of landslide emplacement. The most plausible 

scenario to create the downstream flood features is 

partial breaching of the stable 75–80 m a.s.l. lake level 

and release of water downstream, lowering lake level to 

40 m a.s.l. decades after emplacement of the landslide 

deposit. The timing and nature of further incision to 

attain historical lake levels is uncertain but may have 

occurred just before Lewis and Clark’s visit (1805–06), 

based on the degree of preservation of drowned forest 

trees that were exposed to air after the lake level 

dropped to 13 m a.s.l. 

Once finished with this stop, turn left on Wa Na 

Pa Street. Merge onto I–84 eastbound ahead.

186.9 Wind Mountain

Across the Columbia River to the north, Wind 

Mountain is a ~6 Ma quartz diorite intrusion (Korosec, 

1987). Another of the great Columbia Gorge landslides, 

the Collins Point landslide was active a few decades ago 
Road Log and Description of Field-Trip Stops 67

Figure 61. Photograph of

headscarps of the Bonneville 

landslide (left) and Red Bluffs 

landslide (right) in the background, 

with the truncated toe of the 

Bonneville landslide deposit in 

the left foreground (covered in 

vegetation). The water surface here 

is directly above the submerged 

Cascade Rapids. The mountain 

peak on the left is Table Mountain; 

on the right is Greenleaf Peak. 

The view is from the Oregon shore 

at Cascade Locks, looking north. 

Smithsonian Institution photograph 

by L. Siebert, 2014.

men17-7185_fig 61

and is sourced from the north. It wraps around both sides 

of Wind Mountain and its eastern arm protrudes about 

0.25 mi out into the Columbia River channel.

187.7 Shellrock Mountain

To the south, you are passing Shellrock 

Mountain, which has been dated to 5.7±0.6 Ma and is 

the southern continuation of the quartz diorite intrusion 

at Wind Mountain (Korosec, 1987).

199.0 Take Exit 63 off I–84. Turn left at the end of the ramp.

 Once across the freeway overpass, take the next 

 available right turn, onto Riverside Drive. Turn right 

 onto Nichols Parkway. Proceed a few hundred feet to 

 Hampton Inn and Suites hotel, our stop for the night.

Day 3: Hyaloclastites, glacial tills, and other 

diamicts in the Hood River and Sandy River 

basins, Mount Hood 

Numbered and lettered stops restart at 1 and A, respectively, 

for field trip Days 3 and 4 here at Mount Hood. Day 3 includes 

visits to exposures of various volcaniclastic sediments on three 

sides of Mount Hood (fig. 62). The route starts south of Hood 

River, Oreg., on the north side of the volcano, traverses the east 

flank on Oregon Route 35 (O.R. 35), and then turns west on U.S. 

Highway 26 (U.S. 26) across the volcano’s south flank.

0.0 Cumulative mileage for Day 3 begins at the Hampton

 Inn and Suites hotel in Hood River. We begin by 

 backtracking a short distance on I–84. From the hotel, 

 turn left onto Nichols Parkway, left onto Riverside 

 Drive, left onto North 2nd Street, and then merge onto 

 I–84 westbound. Drive 3.4 mi west to a high roadcut 

 of orange-yellow deposits on the right with a wide 

 pullout area.

3.4 Stop 1—Hyaloclastite deposits along I–84

 (lat 45.7072° N, long 121.5791° W) Park along the 

 shoulder of the road just past the intersection with 

 Morton Road. 

The hyaloclastites at this stop formed when 

basaltic lava flows rapidly quenched and shattered as 

they flowed into the cold water of the Columbia River. 

These roadcuts expose pillow lavas and orange-colored 

palagonitized hyaloclastite debris—including glassy, 

chemically altered, coarse sand and fine gravel (fig. 63). 

These units are part of a pillow lava-palagonite sequence 

that outcrops westward along the base of Pliocene lava 

flows in the Columbia Gorge. The pillows commonly 

are aligned and the clastic beds dip steeply owing to 

emplacement as submerged delta-front foreset beds. 

A K-Ar age of 3.53±0.08 Ma was obtained from the 

base of this sequence at Perham Creek, about 5.5 km 

southwest of this site (Conrey and others, 1996). The 

lava source is uncertain.

Once finished at the outcrop, merge back onto 

I–84 westbound. Stay in the right lane because you 

will be using the next exit to make a U-turn. The drive 

to Stop 2 takes about 10 minutes.

3.9 Take the next right exit, marked as a service road, to 

 make a U-turn onto I–84 eastbound. Follow the service 
68 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 62. Locations of planned stops (numbers) and optional stops (letters) for field trip Days 3 and 4 at Mount Hood. 

0 10 20 KILOMETERS

0 10 20 MILES

N

224

500

281

282

141

211

212

14

14

26 35

84

84

9

8

7

6

5

4 D

C

3

B

2

1

A

9

8

7

6

5

4 D

C

3

B

2

1

A

Columbia River

Sandy River

Hood River

Sandy

TroutdaleTroutdale

Cascade

Locks

MOUNT HOOD

5 Field-trip stop—Number is

planned stop; letter is

optional stop

Base from U.S. Geological Survey (USGS) 30-meter Landsat imagery EXPLANATION

Shaded relief and roads from USGS’s The National Map

 road to the left as it passes under I–84 and curves back

 the right. At the intersection with Mitchell Point Drive, 

 turn right and merge onto I–84 eastbound.

7.8 Mount Adams view

Mount Adams is briefly visible across the 

Columbia River to the north-northeast up the White 

Salmon River valley. Hood River lies about halfway 

between Mount Hood and Mount Adams, and volcanic 

mass flows from both volcanoes have reached as far as 

the Columbia River (Vallance, 1999).

8.7 Back in the town of Hood River, take Exit 64 with

 signs for O.R. 35, leading to White Salmon and 

 Government Camp. Turn right at the end of the ramp

 and follow O.R. 35 southward towards Mount Hood 

 and Government Camp. At highway mile marker 100,

 you are nearing Stop 2.

11.4	 Stop 2—Mount Hood flank-collapse lahar deposit

 (lat 45.6804° N, long 121.5085° W) Just after mile 

 marker 100, watch for a railroad bridge over the road. 

 Pull off into a parking area located on the right side of 

 the road just after passing under the railroad bridge. 

 From the parking area, walk down to the railroad 

 tracks and continue to the south along the tracks for 

 about 200 m to lat 45.6785° N, long 121.5097° W. 

 The outcrop is located in the railroad cut on the east 
Road Log and Description of Field-Trip Stops 69

Figure 63. Deposits of a late Pliocene basaltic hyaloclastite delta, located along Interstate 84, just west of Hood River, Oregon (field 

trip Day 3 Stop 1). The deposits formed when basaltic lava flows entered standing water. A. Photograph of delta-front foreset beds 

dipping to the right (west). Highway barriers are about one meter tall. View is to the south, taken from the north side of Interstate 84. 

USGS photograph by T. Pierson, 2017. B. Photograph of a fractured pillow lava, about 1 meter wide, surrounded by a palagonitized 

matrix. Smithsonian Institution photograph by L. Siebert, 2016.

A B

 side of the tracks. Be extremely careful because this is

 an active rail line with trains passing several times a 

 day, albeit slowly. 

The Hood River lahar originated from edifice 

collapse on the north flank of Mount Hood and 

transformed from a debris avalanche to a large debrisflow lahar that flowed down the Hood River valley, 

crossed the Columbia River valley (filling it ~100m

deep), and then flowed at least 4 km up the White 

Salmon River valley on the Washington shore—a total 

distance of about 50 km (Vallance, 1999). The lahar 

deposit has been estimated to be ~100,000 years old 

(Scott and Gardner, 2017), but it could be younger. 

It contains wood too old for radiocarbon dating 

(implying the lahar must be older than 38,000 years) 

and the probable failure scar on the north flank of 

Mount Hood has filled in with lavas as old as 50 ka, 

providing an older minimum age (W.E. Scott, 2016, 

oral commun.). Additionally, the deposit contains 

an andesite clast that has a K-Ar age of 61±21 ka, 

suggesting the lahar must be younger than ~80 ka to 

have entrained this clast (Scott and others, 2003). An 

arcuate scarp at Cooper Spur on the northeast flank 

of Mount Hood may represent the east edge of the 

in-filled edifice-collapse scarp. At least two other large 

clay-rich lahar deposits are found in the Hood River 

valley, suggesting a history of edifice instability at 

Mount Hood. One of these lies between an andesite 

lava flow dated at 475±14 ka and the 424-ka basaltic 

andesite of Cloud Cap; the other remains undated. 

At this outcrop along the railroad tracks, 38 km 

downstream of the volcano, the top of the deposit is 

about 60 m above the valley floor and is locally as 

thick as 40 m, although at this location it is about 

20 m thick (fig. 64A). The massive, matrix-supported 

diamict is extremely well indurated. The matrix 

is a highly cohesive, yellowish- to brownish-gray 

muddy sand that contains 7–9 percent clay, including 

expandable smectite-vermiculite, and, locally, kaolinite 

(Vallance, 1999; Cook, 2002). The unit overlies local 

alluvium from the Hood River that contains clasts 

that were not sourced from Mount Hood. It is, in turn, 

overlain by poorly exposed sandy deposits emplaced 

by the Missoula floods—catastrophic late Pleistocene 

glacial outburst floods that swept down the Columbia 

Gorge dozens of times (O’Connor and Benito, 2009; 

O’Connor and Burns, 2009). 

After finishing at the outcrop, turn right out of the 

parking area and continue driving south on O.R. 35. 

The route takes you over Middle Mountain, a bedrock 

knob in the middle of the Hood River valley. Watch for 

signs marking U.S. 281 to Parkdale and Cooper Spur. 

The drive to Stop 3 takes about 30 minutes. 

21.3 Optional Stop A—View of Mount Hood 

 (lat 45.5539° N, long 121.5682° W) After going over

 Middle Mountain, Mount Hood comes into full view. 

 Pull onto the shoulder of the road to take photographs

 of the volcano from one of its best vantage points. In 

 contrast to the smooth, broad fans of volcaniclastic 

 sediments of latest Pleistocene and Holocene age that 

 form much of the south flank of Mount Hood (fig.9),

 the north flank is steep and rugged, formed more by

 mass wasting and lava flows than by accumulation of
70 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

A

B

C

 volcaniclastic sediment. Because the summit vent at

 Crater Rock is located south of the summit ridge, the 

 north volcano flank and Hood River valley have not

 received primary eruptive products (except minor 

 tephra) since late Pleistocene time.

Continue south on O.R. 35. The Hood River valley 

is one of the major pear-, cherry-, and apple-producing 

regions in Oregon. Mount Hood is an essential ingredient 

in this agricultural enterprise beyond merely providing 

Figure 64. Photographs of edifice-collapse deposits from the Hood River lahar at Mount Hood. A. Deposit in lower Hood River valley at Day 3 

Stop 2, about 38 kilometers from the volcano. The deposit contains large angular clasts, supported by a muddy sand matrix. A fragile clast about 

3 meters long and composed of unconsolidated gray diamict is visible at the lower right. Smithsonian Institution photograph by L. Siebert, 2014. 

B. Massive, clay-rich lahar deposit in the town of Hood River, approximately 42 kilometers downstream of Mount Hood. Large andesite clasts were 

cut (rather than plucked) by an excavator at this building site, revealing the degree of induration of the matrix material. The notebook at center 

right is 20 centimeters tall. USGS photograph by T.C. Pierson, 2003. C. Hood River lahar deposit along the White Salmon River at Underwood, 

Washington. The lahar traveled north from Mount Hood, up the Hood River valley, and across the Columbia River to the White Salmon River valley, 

46 kilometers from source. The deposit here is about 12 meters thick and 100 meters above the current river level. Note that the size of the largest 

clasts (small boulder) has not diminished with distance—a characteristic of clay-rich debris-flow lahars. USGS photograph by W. Scott.

great logos for fruit crates and a dramatic backdrop for 

the “Fruit Loop” scenic drive of the Hood River valley, 

which for several weeks in April is awash in blossoms. 

The soils in the upper valley, the Parkdale Series and 

its relatives, form chiefly in thick ash-cloud deposits 

of Polallie age (Harris, 1973). The fertile soils are 

typically well draining but have good moisture-holding 

capacity and allow deep rooting. Of equal importance, 

the perennial snow and ice on Mount Hood provide an 
Road Log and Description of Field-Trip Stops 71

27.5 Just after the curve on Evans Creek Drive, make a

 hard-left turn onto Laurance Lake Drive. Drive 2.5 mi

 (past a large pond on the left) until a bouldery debris 

 field is reached.

30.0	 Stop 3—Eliot Branch debris-flow deposit

 (lat 45.4604° N, long 121.6329° W) The Eliot Branch

 lahar was triggered in the early morning of November 

 7, 2006, by intense and prolonged rainfall from an

 “atmospheric river” rainstorm. Approximately 30 cm 

 (12 inches) of rain fell at the 4,000-ft elevation level on

 the mountain over 5 days. The lahar was triggered by

 coalescing of a series of relatively small morainal-

 debris landslides on buried stagnant ice below the toe

 of the Eliot Glacier (fig. 66A). This granular lahar and

ample and relatively low-cost supply of irrigation water 

during the summer. 

Proximal stops at Mount Hood begin with a look 

at recent debris-flow deposits at Eliot Branch on the 

north flank, west of O.R. 35 (fig. 65).

22.4 Turn right onto U.S. 281 (Cooper Spur Road) toward 

 Parkdale. 

24.2 After a short distance, you will come to the

 intersection with Baseline Drive (Parkdale is to the 

 right). Continue straight on Cooper Spur Road.

27.0 Turn right onto Evans Creek Drive and follow it as it 

 curves to the right.

Figure 65. Location of planned stops (numbers) and optional stops (letters) on Days 3 and 4 that are proximal to Mount Hood. 

0 5 10 KILOMETERS

0 5 10 MILES

N

Field-trip stop—Number is

planned stop; letter is

optional stop

EXPLANATION

6

5

4 D

C

3

6

6

5

4 D

C

3

MOUNT

HOOD

N

Sandy RiverSandy River

Zigzag RiverZigzag River

East Fork Hood

River

East Fork Hood

River

Eliot CreekEliot Creek

Polallie CreekPolallie Creek

White RiverWhite River

35

26

35

281

Parkdale

Lava Flow

Base from U.S. Geological Survey (USGS) 30-meter Landsat imagery

Shaded relief and roads from USGS’s The National Map
72 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

 its derivative phases flowed a total of 47 km down the

 Middle Fork Hood River, then down the main stem of

 the Hood River, and all the way to the Columbia River.

 The flow path included 28 km as a debris flow, which 

 was in the process of transforming to hyperconcentrated

 flow, and 19 km as a muddy water flood following 

 wholesale dilution by a major tributary. Our stop is on

 the debris field that was the primary depositional area 

 for the debris-flow phase of the flow (fig. 66B–D). The

 debris flow transported clasts as large as about 5 m in 

 diameter (fig. 66B) and spread debris as much as 

 several meters thick across an area almost 2 km long 

 and about 500 m wide (figs. 66C, D). Deposition of the

 debris flow was promoted by both a decrease in 

 channel slope and an abrupt narrowing of the channel, 

Figure 66. Photographs of the Eliot Branch lahar source and deposit. A. Bank failure of debris from Eliot Glacier moraine—one of 

many streambank failures contributing debris to the Eliot Branch lahar. USGS photograph by J. Griswold, 2007. B. A large boulder, 

approximately 5 meters in diameter, that was transported by the debris-flow phase of the Eliot Branch lahar. C. View of the debris-flow 

depositional area, looking upstream (to the south). Damage to trees that are still standing indicates the flow depth at this point. Person 

to the right of the boulder for scale. D. Log jam at the downstream end of the debris field; view is looking downstream (northeast). The 

vertical tree fragment in the center foreground is approximately 2 meters tall. USGS photographs B, C, D by T.C. Pierson, 2006.

A B

C D

 caused by the presence of the 7.7 ka basaltic andesite 

 Parkdale lava flow (fig. 67). 

Once finished with this stop, return toward O.R. 35 

on Laurance Lake Drive. You will not follow the same 

route to the highway; instead travel south on Cooper 

Spur Road.

32.4 At the intersection, turn right onto Evans Creek Road.

32.9 Turn right onto Cooper Spur Road and continue 

 following this road until it ends at a T-intersection with

 O.R. 35 at Polallie Creek.

40.5 Turn right at the O.R. 35 intersection. Just after the 

 intersection, the road crosses a bridge over Polallie 
Road Log and Description of Field-Trip Stops 73

Figure 67. Eliot Branch channel 

before and after deposition of the Eliot 

Branch debris flow, with the Parkdale 

lava flow largely blocking the channel. A. Composite oblique aerial view of 

the 7,700-year-old Parkdale lava flow 

with vent and Eliot Branch at lower 

left. View is to the north-northeast, 

looking downstream. The town of 

Parkdale, Oregon, is to the upper 

right. Darker portion of the image on 

the left is compiled from photographs 

taken on June 28, 2005, and the lighter 

portion at right from photographs 

taken on June 16, 2006—both prior 

to the November 7, 2006, Eliot Branch 

debris flow. Image data from Google 

Earth, U.S. Department of Agriculture 

Farm Service Agency, and the State of 

Oregon, 2006. B. Oblique aerial view 

of the 2006 Eliot Branch debris-flow 

deposits at lower left. The debris flow 

primarily deflected around the Parkdale 

vent, forming the light-colored area 

along the West Fork Hood River on the 

west side of the Parkdale lava flow, 

with very little material deflecting down 

the east side of the lava flow. Image 

from Google Earth, 2010.

AB
74 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

 Creek. Turn left into the parking area for a restroom

 stop.

40.6 Optional Stop B—Polallie Creek debris fan 

 (lat 45.4177° N, long 121.5702° W) On Christmas 

 night in 1980, an intense winter storm triggered a 

 ~5,000 m3 landslide of water-saturated volcaniclastic 

 debris. The landslide quickly liquefied and flowed 

 down Polallie Creek on the east flank of Mount Hood 

 at velocities as much as 15 m/s (Gallino and Pierson, 

 1985). The debris flow—15 m deep and moving at 

 least 12 m/s—buried a Forest Service campground on 

 a debris fan at the mouth of the creek with about 

 100,000 m3 of debris (fig. 68A). One person staying at

 the campground in a pickup truck camper was killed.

 This deposit, as much as 10 m thick, dammed the East 

 Fork Hood River for 10–20 minutes with a debris

 blockage more than 200 m long. The blockage was 

 subsequently breached by rising water impounded 

 upstream, releasing a dam-break flood that destroyed 

 about 8 km of O.R. 35 (fig. 68B). Similar events have 

 swept down most of the valleys draining Mount Hood

 during the past century, but flows on White River, 

 Newton Creek, Eliot Branch, and Ladd Creek have 

 done the most damage to roads and bridges. 

From here, continue south on O.R. 35 and watch 

for signs to Mount Hood Meadows Ski Resort.

52.1 Take the exit for Mount Hood Meadows Ski Resort 

 and Bennett Pass Sno-Park. At the end of the offramp, 

 turn left on Bennett Pass Road and cross the bridge 

 over O.R. 35. Pull into the parking lot for the Bennett 

 Pass Sno-Park on the left side of the road after crossing

 the bridge.

Figure 68. Impacts of Polallie Creek debris flow on December 25, 1980, on the east side of Mount Hood. A. Aerial photograph of the 

debris fan crossing Oregon Route 35 (O.R. 35), taken two days after the event. The view is to the north. Helicopter at lower center for 

scale. B. Photograph of damage to a section of O.R. 35 along the East Fork Hood River. The dam-break flood released from blockage of 

the river caused extensive erosion along the riverbank, undercutting the highway here. U.S. Forest Service photographs, 1980.

A B

52.5 Stop 4—Glacial till at Bennett Pass (lat 45.3102° N,

 long 121.6421° W) Walk back over the bridge and

 at the far end climb down to the outcrop on the left

 side (lat 45.3106° N, long 121.6443° W). There is a 

 wide margin along the road in front of the outcrop. 

 However, vehicles travel at high speed along O.R. 35 

 and this is on the inside of a blind curve. Be extremely 

 cautious about vehicle traffic at this site.

Mount Hood was extensively glaciated during the 

Last Glacial Maximum (known locally as the Evans 

Creek glaciation, table 3) and previous Pleistocene 

glaciations, and glacial deposits composed of 

volcaniclastic rock fragments are widespread on the 

volcano’s flanks. However, glaciers on the volcano 

today are small in comparison to what they once were 

(fig. 69). The roadcut on O.R. 35 at Bennett Pass here 

on the south flank of Mount Hood originally exposed 

three diamicts, interpreted by Crandell (1980) as 

glacial tills separated by buried soils (fig. 70) (Scott, 

Gardner, and others, 1997; Scott and Gardner, 2017). 

Today, the lowest unit is obscured by talus eroded from 

the outcrop face.

The tills mantle the crest of a ridge of Miocene 

volcanic rock, which is buried farther upslope 

by Mount Hood lavas. The tills display several 

characteristics that can be used to distinguish 

them from similar-looking volcaniclastic deposits: 

emplacement on a ridge instead of a valley floor, 

striated and faceted clasts, unusually high silt content 

in the matrix compared to lahar and PDC deposits in 

the area, and a high degree of compaction locally.

In general, these glacial units are unstratified, 

but the middle unit locally displays crude bedding, 

suggesting a flow-till origin (Scott, Gardner, and 
Road Log and Description of Field-Trip Stops 75

A

B

men17-7185_fig 69

Figure 69. Photograph of small glaciers 

remaining on Mount Hood. The glacier at 

the head of this valley (the Sandy River) 

is the Sandy Glacier on the west flank 

of Mount Hood. View is to the east. This 

glacier, as part of a Pleistocene summit 

ice cap, once extended as far as 27 

kilometers down the Sandy River. Reid 

Glacier is to the right. USGS photograph 

by T.C. Pierson, 2005.

Figure 70. Photographs of glacial till deposits at Bennett Pass. A. Outcrop 

of till units exposed at Bennett Pass, 11 kilometers southeast of Mount Hood 

summit. View is to the northwest, with Mount Hood in the background. Most 

of the roadcut exposure consists of the middle of three till units (compact 

pebbly diamicts). The lowermost till is covered by talus. Smithsonian 

Institution photograph by L. Siebert, 2016. B. Closeup view of the middle till unit 

texture—an extremely poorly sorted pebbly diamict. Shovel blade is about 20 

centimeters long. USGS photograph by T.C. Pierson, 2016.
76 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

others, 1997). The upper two units appear to date from 

the Last Glacial Maximum (Evans Creek glaciation) 

and the lowest till likely dates from either 150 ka or 

75 ka, estimated by the degree of soil development 

on its top and the probable timing of pre-Evans Creek 

glaciations in this area (Scott, Gardner, and others, 

1997; Scott and Gardner, 2017).

Clasts in the upper till correlate this unit to the 

Polallie eruptive period. About half of the clasts are 

highly porphyritic and vesicular, similar to clasts 

derived from collapse of Polallie-age lava domes on 

Mount Hood. A terminal moraine of Evans Creek 

age east of this location contains a high proportion of 

similar clasts, many of them showing weak prismatic 

jointing. This suggests dome growth and collapse were 

occurring during this glacial advance and shedding 

debris onto glacial ice. The glacier then transported the 

material to moraines as supraglacial drift (Scott and 

Gardner, 2017). 

From the Bennett Pass Sno-Park turn right onto 

Bennett Pass Road to cross back over the bridge. Turn 

left just past the bridge and merge back onto O.R. 35 

westbound. Continue about 5 minutes to Stop 5.

54.8	 Stop 5—White River PDC and debris-flow deposits

 (long 45.3032° N, long 121.6740° W) Just after 

 crossing the White River, pull into the large parking lot 

 for the White River West Sno-Park on the right side of

 the road. Park at the far end of the lot. We will eat 

 lunch at this stop. 

Lithic PDC and lahar deposits from the latest 

eruption of Mount Hood funneled down White River 

canyon. The PDCs originated as collapses of a lava 

dome growing at the Crater Rock vent between 1781 

and 1793 C.E. (Old Maid eruptive period) (Crandell, 

1980; Cameron and Pringle, 1987; Scott, Gardner, and 

others, 1997). Crater Rock is visible from this vantage 

point as well as the ski lifts of the Timberline Lodge 

and Ski Area along the axis of the debris fan below the 

lava dome.

Leave your car and walk upstream for about 0.6 

mi. A bluff on the southwest bank of the river provides 

good exposure of one Old Maid-age PDC deposit, 

located at 45.3087° N, 121.6804° W. The PDC deposit 

varies in thickness from 5 to 20 m (Scott and Gardner, 

2017) (fig. 71). Farther upstream, several overlying 

lahar deposits can be found. This PDC deposit 

contains several identifying features that are useful in 

distinguishing PDC units from other volcaniclastics: 

(1) an abundance of slightly to moderately vesicular 

clasts, many displaying prismatic jointing; a friable 

sandy matrix; entrained charcoal fragments; pinkish 

coloration near the top of the deposit; and gasescape pipes. In addition, uniform thermoremanent 

magnetization was determined from numerous clasts; 

it matches that of other PDC deposits of Old Maid age 

and at least part of the Crater Rock lava dome (Scott 

and Gardner, 2017).

Lahar and PDC deposits on the northeast 

riverbank opposite the White River West Sno-Park 

were emplaced during the Polallie eruptive period. 

Modern deposits mantling the wide valley floor are 

from repeated floods and debris flows that continue 

to be triggered by rainstorms in the steep upper 

canyon. The previous O.R. 35 bridge at this location 

was closed 20 times since 1907 owing to washouts, 

including five closures since 1998 and most recently 

in 2006 (fig. 72) (Anderson and others, 2006). That 

bridge was replaced in 2012 with a new bridge that is 

4 m higher and considerably longer, providing a larger 

cross-sectional area for material to pass under. 

Once finished with this outcrop, return to the cars. 

Turn right onto O.R. 35 heading west. Stop 6 is about 

45 minutes away.

58.6 Turn onto U.S. 26 west toward Portland.

58.7 “The Barlow Road”

U.S. 26 roughly follows the historic Barlow 

Road—part of the Oregon Trail. We will follow this 

route for the rest of today and part of tomorrow. Early 

settlers traveling the Barlow Road passed through the 

area about 50 years after Mount Hood’s last eruption.

61.1 Turn right onto Timberline Highway (sign for 

 Timberline Lodge). 

66.2 Stop 6—Timberline Lodge (lat 45.3303° N, 

 long 121.7078° W) Timberline Lodge is both the last

 stop for the day as well as the overnight accommodation.

 The historic Timberline Lodge (fig. 73) is situated 

 on the southwest flank of Mount Hood on the 

 volcaniclastic debris fan downslope of Crater Rock 

 (fig. 12). The lodge, one of the iconic mountain lodges

 of the western United States, was constructed from

 1936 to 1938 by the depression-era Works Progress 

 Administration and lavishly furnished by local artisans.

 This National Historical Landmark is one of 

 Oregon’s most visited sites and provides year-round

 skiing and snowboarding on the Palmer snowfield, 

 as well as expansive views of the Mount Hood summit 

 to the north and Mount Jefferson and other central 

 Oregon volcanoes to the south. The Pacific Crest 

 National Scenic Trail, extending from Mexico to 

 Canada, passes by 0.2 mi uphill of the lodge and 

 provides good views of volcaniclastic deposits in the 

 upper Zigzag River (3 mi west of the lodge) and White 

 River valleys (0.7 mi east of the lodge) (fig. 17).
Road Log and Description of Field-Trip Stops 77

ABC

D

Figure 71. Photographs of lithic

pyroclastic density current (PDC) and 

secondary lahar deposits in the upper 

White River valley. A. Old Maid-age PDC 

(block-and-ash flow) deposit exposed 

in vertical face of a low terrace on left 

side of photograph. View is upstream to 

the north, toward Mount Hood summit. 

Old Maid deposits are inset within the 

canyon that has been cut into lightcolored and thicker Polallie-age deposits 

at far right. Smithsonian Institution 

photograph by L. Siebert, 2013. B. Closeup 

view of PDC deposit in A, showing the 

massive structure, uniform lithology of 

clasts, and poorly sorted nature of these 

deposits. Note that the larger clasts are 

angular to subangular (the largest here 

is about 0.5 meters in diameter). Lahar 

deposits typically show crude rounding 

of coarse clasts. This deposit lacks a 

well-developed pinkish oxidized zone at 

the top of the deposit, possibly due to a 

relative cool emplacement temperature. C. Closeup view of a finer grained part 

of the PDC deposit in A and B, with 

small pieces of black charred wood 

at top center and left. Red pocketknife 

is approximately 6 centimeters long. 

Orange color of some clasts is probably 

due to degassing alteration products 

along cracks in the dome prior to 

collapse. D. Modern debris-flow and 

hyperconcentrated-flow deposits in low 

terrace within the active White River 

channel near the Oregon Route 35 bridge. 

These deposits are composed of eroded 

and reworked material from the PDC 

deposits upstream. Black pencil for scale 

is 15 centimeters long. Note that coarse 

clasts are more rounded than in the PDC 

deposit upstream. USGS photographs B, 

C, D by T.C. Pierson, 2009.
78 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Aug. 1907

Aug., Oct. 1926

Oct. 1927

Oct. 1930

Oct. 1935

Oct. 1947

Oct. 1949

Sept., Oct. 1960

Oct. 1961

Jan. 1966

Jan. 1967

Jan. 1968

Sept. 1981

Sept. 1998

Oct. 2000

Oct. 2002

Oct., Nov. 2006

2010

Washout event

November 7, 2006

September 8, 1998

TIMELINE, IN YEARS

EXPLANATION

Figure 72. History of debris-flow damage to the Oregon Route 35 bridge crossing the White River. Washouts have 

caused the bridge to close 20 times since 1907. Modified from Anderson and others (2006). Mount Hood National Forest 

photographs by D. Jones (top) and T. DeRoo (bottom).
Road Log and Description of Field-Trip Stops 79

Day 4: Distal lahar and volcano-fluvial deposits 

downstream of Mount Hood 

Stops on Day 4 examine deposits along the Sandy River, 

including lahar and fluvial aggradational valley-fill deposits of 

the Timberline and Old Maid eruptive periods. Stop numbers 

and letters continue from Day 3.

0.0 Timberline Lodge. Return to U.S. 26 by way of 

 Timberline Highway. 

5.3 Turn right onto U.S. 26 and continue west past the 

 small community of Government Camp, which 

 originally provided housing for workers building 

 Timberline Lodge. It is about a 50-minute drive to 

 Stop 7.

6.7 Tom Dick and Harry Mountain cirques

If a good view of glacial cirques is desired, turn 

left into the parking lot for Mount Hood Ski Bowl. Look 

south to see Pleistocene-age glacial cirques on Tom Dick 

and Harry Mountain (1,544 m a.s.l.). The mountain is 

composed of Pliocene-Pleistocene andesite and basalticandesite lava flows emplaced on top of the Mioceneage Laurel Hill pluton and Rhododendron Formation 

volcaniclastic rocks (Sherrod and Scott, 1995). The 

upper portion of the ski area is located within a glacial 

cirque carved into the mountain. When you are finished, 

turn left out of the parking lot to rejoin U.S. 26 and 

continue westward.

14.9 Town of Rhododendron built on lahar deposits

U.S. 26 soon begins a long curving downgrade. 

Recent widening of the highway has produced fresh 

Figure 73. Photograph of 

the Timberline Lodge on the 

southwest flank of Mount 

Hood in winter. Arcuate scarp 

of edifice collapse during the 

Timberline eruptive period can 

be seen behind the lodge. The 

post-collapse Crater Rock lava 

dome casts a shadow on the 

left side of the scarp. Timberline 

Lodge and Ski Area photograph.

men17-7185_fig 73

exposures of the Laurel Hill pluton in this location. 

At the bottom of the grade, the highway flattens out 

onto a forested low-gradient surface in the valley 

floor. This is a broad lahar deposit consisting mostly 

of Timberline-age lahar units and locally includes 

Old Maid-age deposits. The large boulders scattered 

among the trees were emplaced as part of these 

lahars.

17.0 When entering the small community of Zigzag, watch

 for the Mount Hood National Forest Zigzag Ranger 

 Station on the left side of the road. Just after passing 

 the ranger station, turn right onto the East Lolo Pass 

 Road. Continue on the East Lolo Pass Road, which 

 parallels the Sandy River.

18.4 The Sandy River comes very close to the right side of

 the East Lolo Pass Road here. Look for signs of 

 significant bank erosion and damage to some of the 

 houses and buildings along the river, which were 

 caused by a flood with a high sediment load in January

 2011.

21.2 After passing the sign indicating entrance into Mount

 Hood National Forest, take a slight right at the fork 

 in road onto Muddy Fork Road. This turn is not 

 marked well, but is located at the point where East 

 Lolo Pass Road begins to climb uphill out of the 

 valley. Muddy Fork Road follows the valley floor and 

 parallels the Sandy River.

21.9 At the next intersection, turn right to continue on

 Muddy Fork Road. In a short distance, the road crosses 

 a bridge over the Sandy River.
80 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

22.1 Stop 7—Old Maid Flat (lat 45.3837° N,

 long 121.8681° W) At the entrance to McNeil 

 Recreation Site (the next left turn), park on the 

 shoulder of the road and walk into the campground. 

 There are only a few places to park in the campground

 that are not fee-site campsites. Walk to campsite 16 

 (lat 45.3868° N, long 121.8676° W), and from here 

 proceed about 400 ft northwest between campsites 

 into the forest to the edge of the cutbank overlooking 

 the Sandy River. Find a place to safely descend to the

 river where you can view the cutbank.

Old Maid Flat was a primary depositional 

site for lahar and volcano-fluvial sediments during 

the Timberline and Old Maid eruptive periods. It 

also was, and continues to be, a major sediment 

source, supplying coarse volcaniclastic sediment to 

channel reaches farther downstream the Sandy River 

during major rainstorms. This stop, about 15 km 

downstream of the Crater Rock vent, provides the 

opportunity to examine a variety of syneruptive and 

posteruptive depositional units, including debris-flow, 

hyperconcentrated-flow, and volcano-fluvial deposits 

related to Old Maid volcanic activity. Timberlineage deposits are buried at this location, but exposed 

farther upstream. The Old Maid units show a variety of 

sedimentary textures and features (fig. 74). The Sandy 

River channel in this steep reach is very active during 

winter high flows, transporting large volumes of coarse 

volcanic sediment. Modern sedimentary features and 

structures can also be seen here. Tree wells on the 

surface of Old Maid Flat and tree snags exposed in the 

channel bed indicate that old-growth forest trees were 

rapidly buried in growth position by these sedimentrich flows. Note that these banks are unstable. Please 

be careful. 

Dendrochronologic evidence indicates that the 

first major lahar during the Old Maid eruptive period 

occurred in late summer or early autumn of 1781—the 

year the dome-building eruption started. Other significant 

flows include an outbreak flood from the formerly 

dammed Muddy Fork tributary in 1787, a lahar in 1791, 

and another lahar in 1800 (Pringle and others, 2002, 

2010). The 1781 lahar was the largest generated during 

the Old Maid eruptive period (Pierson and others, 2011). 

An unknown number of smaller lahars that did not injure 

trees along the valley sides also contributed sediment 

between 1781 and 1805, when sedimentary infill of Old 

Maid Flat was complete and forest vegetation became 

established over much of that surface (Cameron and 

Pringle, 1986; Pierson and others, 2011). Following 

infill, parts of the original depositional surface at Old 

Maid Flat were carved 2–15 m deep and as much as 

~50 m wide by erosional channels. Based on the maturity 

of trees growing within these channels today, much of 

the erosion on the surface of Old Maid Flat appears to 

have occurred shortly after lahar deposition and before 

widespread establishment of forest vegetation. Stops 8 

and 9 explore downstream deposits in the depositional 

reach at Oxbow Regional Park and at the Sandy River 

delta.

When you are finished at this stop, backtrack the 

way you came to U.S. 26 westbound.

22.3 Turn left onto Muddy Fork Road.

23.0 Turn left onto East Lolo Pass Road.

27.2 Turn right onto U.S. 26 westbound.

43.9 Sandy, Oregon. Note that the speed limit is strictly 

 enforced in town. The city of Sandy was founded as a 

 trading post for pioneers traveling on the Barlow Road 

 segment of the Oregon Trail (now U.S. 26 here). 

 Established in 1845, the Barlow Road connected The 

 Dalles, Oreg., at the eastern end of the Columbia 

 Gorge, to the Willamette River near Oregon City. 

 This overland route was an alternative to the faster, 

 but more dangerous, river route that rafted wagons 

 through the treacherous Cascade Rapids on the 

 Columbia River. 

44.7 At far end of town, turn right onto Bluff Road. The 

 next optional stop is just ahead.

45.6 Optional Stop C—Jonsrud viewpoint (lat 45.4099° N,

 long 122.2736° W) Pull into the parking lot of the 

 Jonsrud viewpoint on the right side of the road. The 

 view here looks up the Sandy River valley, incised 

 about 150 m deep at this location, with Mount Hood in 

 the distance, about 45 km away (fig. 75). From here, 

 turn right out of the parking lot onto Bluff Road. 

 Continue traveling on this road as it curves to the left.

49.8 Turn right onto 347th Avenue, which becomes SE 

 Cottrell Road.

51.1 Turn left onto SE Lusted Road.

51.3 Turn right onto SE Hosner Road. Continue on SE

 Hosner Road, continuing straight through next 4-way 

 intersection. Road begins steep descent into Sandy 

 River canyon and becomes SE Oxbow Parkway.

52.8 Where the road makes a sharp turn to the right along

 the steep curving descent, you will pass a small wedge

 of Missoula flood deposits, largely overgrown with 

 vegetation, on the inside of the curve.

53.3 At the bottom of the steep grade, continue straight to

 enter Oxbow Regional Park, which is a fee site. Enter

 the park and follow SE Oxbow Parkway through the

 park.
Road Log and Description of Field-Trip Stops 81

Figure 74. Photographs of volcaniclastic deposits of mainly Old Maid age exposed at Old Maid Flat, about 15 kilometers downstream 

of Mount Hood. A. Poorly sorted bouldery debris-flow deposit. Wooden shovel handle is 3.5 centimeters (cm) in diameter. B. Sequence 

of crudely stratified hyperconcentrated-flow deposits. Trenching shovel at lower right for scale, handle 55 cm long. C. Vertical and 

lateral transitions from gravelly to sandy facies in hyperconcentrated-flow deposits. D. Modern volcano-fluvial deposit emplaced by 

a major flood in November 2006, composed of moderately sorted medium- to fine-grained sand. E. Granular (noncohesive) debris-flow 

deposit overlain by a hyperconcentrated-flow deposit. Trenching shovel at lower center for scale. F. Outsized clast of nonvesiculated 

dacite in a hyperconcentrated-flow deposit. Pencil for scale. USGS photographs by T.C. Pierson, 2009.

A B

C D

E F
82 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 75. Photographic view of Sandy 

River from Jonsrud viewpoint outside 

of Sandy, Oregon. The grassy terrace in 

the center of the photograph is underlain 

by Polallie-age lahar and lahar-runout 

deposits. Mount Hood is in the background. 

Smithsonian Institution photograph by L. 

Siebert, 2016.

men17-7185_fig 75

53.7 Stop 8—Oxbow Regional Park (lat 45.4972° N,

 long 122.2908° W) Drive most of the length of the 

 park and turn into the parking area for the boat ramp

 on the left side of the road. If you enter the one-way 

 campground loop, you have gone too far. From the 

 boat ramp, find the trailhead on the east right 

 (upstream) side of the parking lot and walk the trail 

 along the river, heading upstream. 

Oxbow Regional Park lies along the lowgradient lower reach of the Sandy River, about 70 km 

downstream of Mount Hood and about 15 km upstream 

of the river mouth (fig. 76). It showcases past rapid 

syneruptive channel aggradation in a river system 

with high rates of sediment transport (Pierson and 

others, 2009, 2011). An episode of profound channel 

aggradation occurred in this lower reach around 1.5 ka 

during the Timberline eruptive period, when excessive 

sedimentation rates caused the channel bed to be raised 

~30 m. After the sediment supply waned, the channel 

re-incised to near its original level. Again, between 

1781 and 1793 during the Old Maid eruptive period, the 

channel bed rose ~23 m due to high rates of eruptionrelated sedimentation and thereafter re-incised to its 

present level. Both aggradation episodes engulfed 

standing old-growth forests in sediment. Those trees 

are now gradually being re-exposed by bank erosion, 

revealing “ghost forests” (figs. 15, 16).

Posteruption channel incision occurred following 

periods of aggradation associated with past eruptions. 

Incision occurred in a stepwise fashion, leaving flights 

of fill terraces along the riverbanks (figs. 15, 16, 77, 

78). Primarily only terraces of Old Maid age remain 

visible today; older flights are mostly buried. Exposed 

terraces are commonly capped by fine-grained fluvial 

sand emplaced by major floods. A radiocarbon date 

from one sand unit suggests it is a backwater deposit 

from the outbreak flood that was caused by initial 

breaching of the Bonneville landslide dam in the 15th 

century. This flood surge would have had to back-flood 

about 15 km up the Sandy River from the Columbia 

River to reach this location. Dendrochronologic 

evidence permits reconstruction of the timing of the 

Old Maid aggradation-degradation cycle, and suggests 

the aggradation phase lasted about a decade and the 

degradation phase 50–100 years (Pierson and others, 

2011).

Sedimentary textures and lithofacies of the 

aggradational valley-fill deposits demonstrate that 

most of the rapid deposition was accomplished by 

normal fluvial processes. Though lahars did occur and 

emplace deposits near the base of both the Timberline 

and Old Maid sedimentary sequences, they were not 

the dominant contributors of sediment, as is common 

in other more proximal volcanic settings (for example, 

Smith, 1987a,b, 1991; Pierson and others, 1996). 

Lithofacies and historical records also indicate that 

the Sandy River channel was braided during most 

of the two aggradation-degradation cycles (Smith, 

1987a,b; 1991; Moulton and Dunlay, 1990; Miall, 

1996; Manville and others, 2005). Width of the active 

braidplain (250–600 m) was constrained by the walls 

of the incised valley and may partly explain the 

unusually thick sedimentary fill accumulations in this 

river reach.
Road Log and Description of Field-Trip Stops 83

Figure 76. Shaded-relief map of the lower Sandy River with geology of the Sandy River delta shown. Stop 8 examines 

exposures between river miles 11 and 12 at Oxbow Regional Park and Stop 9 is located at river mile 2.2, west bank, at the apex 

of the Sandy River delta. Inset map at lower left is a sketched map drawn by Lewis and Clark in 1805–1806, showing the same 

area with the Sandy River labeled “Quick Sand River” (O’Connor, 2004). The eastern delta distributary channel shown on Lewis 

and Clark’s map, active into the 20th century, was closed off by an engineering diversion in the 1930s, but was restored in 2013. 

Figure from Pierson and others (2011).

0 1 2 3 KILOMETERS

0 1 2 3 MILES

6

14

13

12

11

10

9 8 5 7 4 32 1

Sandy

River

Columbia River

Sandy River delta

OXBOW

REGIONAL

PARK

DABNEY

STATE

PARK

Camp 

Angelos

N

Columbia River overbank flood deposits

(thick accumulation)

Sandy River sediments deposited during and

following the Old Maid eruptive period

(beneath thin mantle of recent Columbia

River deposits)

Sandy River sediments deposited during and

following the Timberline eruptive period

(beneath thin mantle of recent Columbia

River deposits)

Approximate location of River Mile (RM) 

points, measured upstream from mouth

EXPLANATION

5

Lewis and Clark map

Sediments composing the valley-fill aggradational 

deposits in this Sandy River reach are dark gray 

immature volcanic sands and gravels (Pierson and 

others, 2011). Most of the rock fragments within the 

deposits appear fresh and have similar compositions 

to those that were erupted during the Timberline 

and Old Maid eruptive periods. Small amounts of 

epiclastic particles are also found. Sediments deposited 

during aggradation are dominantly crossbedded 

medium to coarse fluvial sands with minor gravel 

lenses. Sediments in the top few meters of the lower 

degradation terraces, that is, sediments deposited 

during the degradation phase of the cycle, are more 

gravel rich and crudely bedded.

Four lithofacies associations (FAs) comprise the 

Timberline and Old Maid aggradation-degradation 

sequences (fig. 79), originally defined and categorized 

by Smith (1987a), and described below.

FA–1—Fluvial channel deposits. Poorly to 

moderately sorted medium to coarse sand, locally 

including fine gravel lenses and beds. Sand beds 

display mainly trough crossbedding and planar 

crossbedding, with local ripple cross-laminations. 

FA–1 reflects dune and ripple migration and braid 

bar formation at conditions of dominantly subcritical 

flow in shallow sand-dominated channels at moderate 

to high rates of sediment transport (Miall, 1996). It 

is common in aggradational braided river systems 

(Manville and others, 2005) and was directly observed 

by the Lewis and Clark Expeditions in 1805 and 1806 

at the mouth of the Sandy River (Moulton and Dunlay, 

1990).
84 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Figure 77. Simplified cross sections along the low-gradient depositional reach of the Sandy River in Oxbow Regional Park. 

Section A is located at river mile 12.8; section B is located at river mile 11.6 (see fig. 76). Terrace levels L1 through L4/5 (a level 

encompassing two closely spaced surfaces) were cut during the decades following the end of the Old Maid eruption. Two 

generations of paleoterrace surfaces and forests (undifferentiated in figure) were buried by subsequent channel-fill deposits 

during the Timberline and Old Maid eruptive periods. Modified from Pierson and others (2011).

20

15

10

5

0

-5

Elevation above reference stage, in meters

20

15

10

5

0

-5

Elevation above reference stage, in meters

August 2006

river level Gravel bar

Alluvial terrace surfaces prior to 1781 C.E.,

with overbank flood deposits on surfaces locally

Lahar deposit August 2006

river level

L 4/5

L 3

L 2

L 1

L 4/5

Syneruption aggradational alluvial fill

(about 1781–1805 C.E.)

Late Pleistocene to late Holocene lahar

deposits and alluvium

Coarse alluvium (sandy gravel/gravelly sand),

including modern deposits

Fine-grained overbank flood deposits

with coarse woody debris locally

Lahar deposit

EXPLANATION

Section A

Section B

FA–2—Overbank floodplain deposits. Moderately 

to well-sorted medium to very fine sand and silt. Bedding 

mainly consists of ripple-drift cross-laminations or bedparallel laminations. Sand beds commonly capped by a 

layer of silt less than 1 cm thick. 

FA–2 is observed at the base of the Old Maid 

stratigraphic section wherever it rests on a pre-Old Maid 

paleoterrace surface as well as on some degradation 

terrace surfaces. It is interpreted as the result of one 

or more episodes of overbank flooding that occurred 

as the aggrading channel bed brought paleoterrace 

surfaces within the vertical range of peak flood flows. 

On degradation terraces these sands represent floodplains 

that were soon abandoned above the range of floods as 

the channel incised. FA–2 deposits beneath the basal 

lahar deposit could have been formed by a sedimentcharged wave of river water pushed ahead of the 1781 

lahar (see Cronin and others, 1999). These overbank 

sediments were laid down by shallow low-velocity 

overbank flows, which also stranded woody debris 

on paleoterrace surfaces and gently buried layers of 

undisturbed forest duff.

FA–3—High-energy braided-channel deposits.

Poorly sorted coarse to very coarse sand, gravelly 

sand, and sandy gravel. Strata are horizontally bedded, 

low-angle crossbedded, or massive (with coarse gravel 
Road Log and Description of Field-Trip Stops 85

Figure 78. Digital elevation model with 2-meter resolution of the Sandy River at Oxbow Regional Park, approximately between 

river miles 8.8 and 13.8 (see fig. 76). Image derived from 2009 Oregon Department of Geology and Mineral Industries bare-earth light 

detection and ranging (lidar) data. This river reach is about 66–75 kilometers downstream of Mount Hood, with the river flowing from 

right to left. The broad, flat terraces in the river valley were cut by incision of aggradational fill deposits following various eruptive 

periods. The L4/5 and lower terraces were formed following the Old Maid eruptive period and the L6 terrace was formed at the 

end of the Timberline eruptive period. The high terrace at the center of the right oxbow (labeled “E”) is a late Pleistocene surface 

composed of the Estacada Formation—an older volcaniclastic unit from Mount Hood (Trimble, 1963).

0 0.25 0.5 KILOMETER

0 0.25 0.5 MILE

Base from 2009 Oregon Department of Geology and Mineral Industries

2-meter light detection and ranging (lidar) data

L2

L1 L3

L6

L3

L2

L4/5

L4/5

L2

L1 L3

L6

L3

L2

L4/5

L4/5

E

L3

E

River terrace level

Terrace composed of 

Estacada Formation 

EXPLANATION

N

lenses). Also labeled sheetflood deposits in Smith 

(1987a). 

FA–3 is typical of high sediment transport rates 

and high bed shear stress at the transition between 

subcritical and supercritical flow (Miall, 1996). It 

probably reflects narrowing and deepening of active 

channels as degradation begins, but while sediment 

transport rates are still high. These are conditions 

common in the deeper parts of braided rivers (Smith, 

1987a; Miall, 1996).

FA–4—Debris-flow and hyperconcentratedflow lahar deposits. Massive, extremely poorly 

sorted, indurated beds of muddy sandy gravel or 

muddy gravelly sand, having both matrix-supported 

and clast-supported textures.

FA–4 represents lahar deposition on paleoterrace 

surfaces. Both the Timberline and Old Maid 

aggradational sequences begin with lahars—two 

closely spaced in time at the beginning of Timberline 

deposition and one at the beginning of Old Maid 

deposition. All of these early lahars were in excess 

of 3 m flow depth, inferred from the presence of 

flood-intolerant conifers growing on the inundated 

paleoterrace surfaces (Pierson and others, 2011).

Once finished at this stop, we will eat lunch in 

the picnic area prior to departure. The drive to the 
86 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

next stop takes about 30 minutes. From the boat 

ramp, backtrack to exit the park. 

54.1 At the park exit, continue straight uphill on SE Oxbow

 Parkway (becoming SE Hosner Road).

55.5 Turn right at the stop sign onto SE Oxbow Drive. 

57.7 The road intersects and continues as SE Division

 Drive.

59.2 The road forks as it approaches a blinking-light,

 four-way stop. Take the fork to the right onto SE 

 Troutdale Road. 

61.9 At the traffic light intersection with SW Cherry Park

 Road and Buxton Road, veer right through the 

 intersection onto Buxton Road. 

62.4 At the T-intersection with Historic Columbia River 

 Highway, turn left and stay in the right lane. 

62.5 Take the first right at the traffic light onto Graham 

 Road. Follow Graham Road as it passes over the 

 railroad tracks and under I–84, staying in the right 

 lane. After passing under I–84 you will see the 

 Troutdale Airport ahead; two lanes of traffic curve to 

 the left parallel to the freeway, but keep straight instead.

63.1 Turn right onto NE Harlow Road.

63.4 Stop 9— Timberline lahar deposits at the Sandy 

 River delta (lat 45.5457° N, long 122.3863° W) 

 Once you pass the storage facility on the right, you 

 will notice several pullouts along the left side of the 

 road. Pull into one of these and park. Short steep

 trails descend to outcrop exposures; vegetation may

 need to be cleared.

The Sandy River delta is a depositional structure 

as much as ~90 m thick that is predominantly built of 

Mount Hood volcaniclastic sediment (fig. 76) (Rapp, 

2005). Drill cores reveal that delta deposits rest on the 

Troutdale Formation (indurated Columbia River sediment 

of Pliocene age) and largely predate the Timberline 

eruptive period (fig. 80). Interbedded Columbia River 

overbank flood deposits appear in the uppermost 15 m of 

the section but are apparently absent below that, although 

well-logging methods may not detect thin layers at depth. 

The full age range of the delta is not known, but organic 

material from 12–14 m depth (5 m below the base of the 

Timberline deposits) in a drill core from the delta center 

dates to about 5 ka (Rapp, 2005).

The 8.5-m section along the bluff on the west 

side of the Sandy River here consists of lahar and 

volcano-fluvial deposits of Timberline age that overlie 

pre-Timberline Columbia River flood deposits. Old 

A

B
Road Log and Description of Field-Trip Stops 87

Figure 79. Photographs of sediments emplaced during the Old Maid eruptive period at Oxbow Regional Park. Photographs A, B, C, E, 

and F were taken at outcrops located between river miles 11 and 12 (fig. 76). A. A recently exposed face of the highest Old Maid-age 

alluvial terrace (L4/5) at Oxbow Regional Park. View looks downstream, showing trees that were rapidly buried in growth position. 

The surface of the Timberline-age paleoterrace on which the trees were growing is at the contact just below the man’s waist. USGS 

photograph by T.C. Pierson, 2009. B. Closeup cross-sectional view of a Timberline-age paleoterrace surface soil overlain by fine to 

very fine overbank flood sands of lithofacies association 2 (FA–2). Paleoflow direction was approximately left to right. Note the infilled 

macropores in the soil and the thin, light-colored silt partings in the overbank flood sands (probably separating individual flood events). 

Scraper shaft is 0.7 millimeters wide. USGS photograph by T.C. Pierson, 2005. C. Crossbedded medium to coarse sand (FA–1) in lower 

third of the 20-meter-high L4/5 terrace. Paleoflow was from left to right. Pencil at center right for scale. USGS photograph by T.C. 

Pierson, 2006. D. Horizontal plane-bed stratification with localized low-angle crossbedding in alternating coarse sand and fine gravel 

layers (FA–3). Paleoflow was approximately from right to left. Pencil at right for scale. E. Normally graded top of a lahar deposit (FA–4) 

at the base of the Old Maid section beneath terrace L4/5. Paleoflow was approximately from left to right. Note the thin, white silty layer 

at the top of lahar unit, which probably formed during dewatering of the deposit. FA–2 deposits cap the lahar unit. Scraper at right for 

scale. F. Inversely graded base of the same lahar deposit in E. The lahar unit rests on a thin silty layer with groundwater iron staining, 

which, in turn, overlies a thin overbank flood deposit resting on a Timberline-age paleoterrace surface. Pencil at bottom right for scale. 

USGS photographs D, E, F by T.C. Pierson, 2009.

C D

E F
88 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

gravel sand silt clay

2 1 0.1 0.062 0.01 0.002

Particle size, in millimeters

-2 0 2 4 6 8 9

Horizontal distance, in meters

0 10 20 30 40 50 60 70 75

Phi (φ)

100

80

60

40

20

0

11

12

13

10

9

8

7

6

5

4

3

2

Elevation above sea level, in meters Percentage by weight

B

A

280–10

1,720–1,540

1,710–1,540

1,700–1,520

1,520–1,330

1540–1350

1740–1540

Tephra

280–10

Columbia River silt

Lahar 1

Lahar 2

Crossbedded sand

Lahar 3

EXPLANATION

NORTH

EXPLANATION

Fill

Silt

Silty sand/sandy silt

Sand

Crossbedded sand

Poorly sorted sand

and gravel

Old Maid-age deposits

Timberline-age deposits

Columbia River deposits

Sandy River deposits

Section covered or not exposed

Grain size sample location

Surface

14C sample calibrated age—

Age ranges are reported in

calibrated years before

present, where present is

1950 C.E. 

?

*

?

? ?

?

?

?

?

?

?

?

?

?

? ?

*

*

*

*

*

Low flow level of Sandy River

Section

10 Section

9 Section 8

Section 4

Section 5

Section 6

Section 7

Section 2

Section 1 Section 3

Lahar 2

Lahar 1

Lahar 3

Crossbedded

sand

Figure 80. Holocene stratigraphy and chronology at Stop 9, as 

exposed in a riverbank outcrop along the west bank of the Sandy 

River delta at river mile 2.5. A, Plot of grain size distribution in 

select sediment samples from section 2. B, Correlated stratigraphy 

in sections 1–10 of the Sandy River delta. Age ranges are reported 

in calibrated years before present (cal. yr B.P.), where present is 

1950 C.E. Present-day downstream is to the right. Modified from 

Rapp (2005), in which the stratigraphy is described in more detail.
Road Log and Description of Field-Trip Stops 89

Figure 81. Photographs of lithofacies within the uppermost Timberline-age lahar deposit exposed at the mouth of the Sandy River at the delta 

apex at Stop 9. A. Sandy gravel unit within the youngest Timberline-age lahar deposit showing poorly developed stratification. B. Faint horizontally 

bedded coarse sand with fine sandy silt partings between sand beds. The thin silty “cap layers” form quickly between surges as depositional 

units consolidate and the expelled pore water transports fines to the surface. These repeating units typically are deposited by individual surges 

within a lahar; bedded units underlie a more massive layer. Shovel for scale. C. Dish structures and soft-sediment deformation within the same 

unit as in B caused by localized upward expulsion of pore water during consolidation, which disrupts silty partings between depositional units. D. 

Outsized clasts in a coarser lahar lithofacies. E. Coarsest lithofacies within the uppermost lahar deposit of the Timberline section here, with texture 

probably reflecting the relatively high concentration of the flow at the time of deposition. Faint layering at bottom of photograph suggests increasing 

concentration, from hyperconcentrated flow to debris flow, during progressive deposition. F. Large fragile clast of the semi-consolidated Pliocene 

Sandy River Mudstone, probably entrained by bank collapse from farther upstream and carried by the uppermost lahar. The material below the clast 

is talus. Vertical scale is 2 meters. USGS photographs A and C–E by T.C. Pierson, 2005; Smithsonian Institution photograph B and F by L. Siebert, 2016.

A B

C D

E F
90 Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-Capped Volcanoes

Maid-age deposits are present nearby but inset within 

the Timberline-age deposits at a slightly lower elevation. 

They can be seen from this location across the channel 

on the east side of the delta.

Stratigraphy of Timberline-age Section

This bluff exposes a compressed version of the Timberline 

aggradation sequence (fig. 81) seen at Stop 8 farther upstream—

6 m thick here but about 30 m thick at Oxbow Regional Park, 

located 16 km upriver (fig. 79). The two lahar units at the base of 

the section (together about 1.3 m thick) were emplaced as debris 

flows and are massive, very compact, extremely poorly sorted 

pebbly diamicts. Above them are about 2 meters of crossbedded 

fluvial sand, which grades upward to a third lahar deposit at the 

top of the section that is nearly 3 m thick. This top lahar unit 

shows evidence of emplacement by a flow at the concentration 

boundary between debris flow and hyperconcentrated flow. It has 

faint horizontal bedding, isolated outsized clasts, and thin silty 

partings that locally break up into dish structures caused by rapid 

vertical dewatering—all characteristics of hyperconcentrated-flow 

deposits. The Timberline volcaniclastic units overlie a micaceous 

clayey silt deposit at least 2.5 m thick. Abundant mica implies 

a Columbia River provenance (little mica found in the Sandy 

River basin), and the texture suggests deposition by overbank 

flooding. Radiocarbon dates for this section (fig. 80) confirm that 

the volcaniclastic units in this outcrop are from the Timberline 

eruptive period.

Historical accounts of the Sandy River delta 

during and following the Old Maid eruptive 

period

Historical evidence suggests that high rates of sediment 

transport, perhaps accompanied by channel aggradation, were 

occurring in the Sandy River delta in 1792 (Cameron and Pringle, 

1987). Direct observations were made at the river mouth by Lt. 

William Broughton of the Vancouver expedition. Broughton was 

the first European to cross the notorious Columbia River bar, 

enter the estuary, and verify that the mighty Columbia River—

long hypothesized by cartographers and explorers—actually 

existed. He left his ship anchored in the estuary and continued 

up the Columbia River in a small boat, reaching as far as Point 

Vancouver, just upriver from the Sandy River mouth, and claimed 

the river’s drainage basin for Great Britain. Broughton noted a 

shallow sand bar actively forming at the river’s mouth—a location 

where the Columbia River’s current is particularly swift and 

where a sand bar could not persist without a large and continuous 

sediment supply (Cameron and Pringle, 1987).

Members of the Lewis and Clark expedition described 

and mapped the mouth and lower ~5 km of the Sandy River 

on November 3, 1805, and explored it further on their return 

on April 1, 1806 (fig. 76, inset) (Cameron and Pringle, 1986, 

1987; Moulton and Dunlay, 1990; O’Connor, 2004). Based 

on these descriptions, the Sandy River was a wide braided 

channel, transporting predominantly sand-size sediment at a 

high rate on an unstable and unarmored sandy bed, where flow 

depth was nowhere more than about 10 cm. The expedition 

members named it the Quick Sand River, because they could 

not wade across it owing to the quick condition of the sandy 

bed. In their notes, they compared this channel to the shallow 

sediment-choked Platte River they had visited in Nebraska at 

the beginning of the expedition. 

On the return trip in spring of 1806, an exploring party 

noted the river had low banks easily overtopped by high 

water—a location where today there is a terrace surface 18 m

above the channel. The expedition must have observed the 

lower Sandy River near maximum aggradation. Based on other 

evidence, maximum aggradation level was reached at Oxbow 

Regional Park by 1793, more than a decade earlier (Pierson and 

others, 2011). In 1805, the Sandy River channel was noted to be 

about 110–275 m wide with numerous mid-channel sand islands 

and very turbid flow. Today, the main active channel across 

most of the delta is 80–140 m wide and several meters deep, 

with only a few large mid-channel islands. The channel widens 

to 250 m at the river mouth.

Acknowledgments

This guidebook draws on earlier field-trip guides to 

Mount St. Helens (Waitt and others, 1989; Doukas, 1990; 

Waitt and Pierson, 1994; Pringle, 2002; Major and others, 

2009) and Mount Hood (Scott, Gardner, and others, 1997; 

Pierson and others, 2009; Scott and Gardner, 2017), as 

well as unpublished field-trip guides by Michael Clynne, 

John Pallister, William E. Scott, and colleagues. The 

material presented is a distillation of myriad contributions 

illuminating the history of these two volcanoes from many 

more researchers than can be referenced here. Dave Ramsey 

facilitated preparation of location map graphics.
References Cited 91

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Menlo Park Publishing Service Center, California

Manuscript approved January 26, 2018

Edited by Monica Erdman

Layout and design by Cory Hurd
Pierson and others—Geologic Field-Trip Guide of Volcaniclastic Sediments from Snow- and Ice-capped Volcanoes—Scientific Investigations Report 2017–5022–F

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20175022F