U.S. Department of the Interior

U.S. Geological Survey

Data Series 746

Prepared in cooperation with the National Park Service

Historical Rock Falls in Yosemite National Park, California 

(1857–2011)
COVER

Photograph of the October 11, 2010, rock fall from the southeastern face of El Capitan. Photograph by Luke Lydiard, 

used with permission.
Historical Rock Falls in Yosemite National 

Park, California (1857–2011)

By Greg M. Stock, Brian D. Collins, David J. Santaniello, Valerie L. Zimmer, 

Gerald F. Wieczorek, and James B. Snyder

Prepared in cooperation with the National Park Service

Data Series 746

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS

Any use of trade, firm, or product names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

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

Suggested citation:

Stock, G.M., Collins, B.D., Santaniello, D.J., Zimmer, V.L., Wieczorek, G.F., and Snyder, J.B., 2013, Historical rock 

falls in Yosemite National Park, California (1857–2011): U.S. Geological Survey Data Series 746, 17 p. and data files, 

http://pubs.usgs.gov/ds/746/.
 iii

Contents

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

Historical Rock Fall Records........................................................................................................................1

Methods...........................................................................................................................................................3

Data Files.........................................................................................................................................................4

Identification Numbers ........................................................................................................................5

Event Location.......................................................................................................................................6

Type of Slope Movement.....................................................................................................................8

Season....................................................................................................................................................8

Date and Time........................................................................................................................................8

Estimated Volume .................................................................................................................................9

Rock Units ............................................................................................................................................10

Probable Triggering Condition(s)......................................................................................................10

Earthquakes (EQ) .......................................................................................................................11

Precipitation (P), Rain-on-Snow (R/S), Snow Melt (SM).....................................................11

Freeze-Thaw (F/T)......................................................................................................................11

Snow Avalanche (SA) ...............................................................................................................11

Wind (W)......................................................................................................................................11

Lightning (L).................................................................................................................................11

Root Wedging (RW)...................................................................................................................12

Thermal Stress (TS)...................................................................................................................12

Crack Propagation (CP) ............................................................................................................12

Blasting (BL), Construction (C) ................................................................................................12

Ground Vibration (GV) ...............................................................................................................12

Unrecognized (UNR) and Unknown (UKN) ............................................................................12

Impacts to Humans.............................................................................................................................12

Published References........................................................................................................................13

Narratives.............................................................................................................................................13

Environmental Conditions..................................................................................................................13

Summary........................................................................................................................................................14

Acknowledgments.......................................................................................................................................14

References Cited..........................................................................................................................................15

Figures

1. Shaded relief map of Yosemite National Park showing general locations referred to in this 

report ..............................................................................................................................................2

2. Photograph of Yosemite Valley looking east, showing the steep granitic cliffs that flank the 

valley...............................................................................................................................................3

3. Shaded relief map of Yosemite Valley derived from 1-m airborne lidar data showing general 

locations and additional location information where documented rock falls occurred ..4

4. Example format of “Master” sheet from the rock fall database ....................................................5

5. Example format of “Environmental Conditions” sheet from the rock fall database ...................5
iv 

Datum Information

Horizontal coordinate information is referenced to the North American Datum of 1983 

(NAD 83) and projected to Universal Transverse Mercator (UTM) Zone 11S coordinates in 

metric units.

Vertical coordinate information is referenced in metric units to the North American 

Vertical Datum of 1988 (NAVD 88). Elevation, as used in this report, refers to distance above 

the vertical datum.

6. Example format of “References” sheet from the rock fall database ............................................6

7. Example format of the rock fall database narrative document ....................................................14

Tables

1. Geographic names and labels of select locations in Yosemite National Park ............................7

2. Relative size categories of rock falls in Yosemite National Park. ................................................10

3. Rock units comprising rock falls in Yosemite National Park.........................................................10
Historical Rock Falls in Yosemite National Park, California 

(1857–2011)

By Greg M. Stock1, Brian D. Collins2, David J. Santaniello1, Valerie L. Zimmer1, Gerald F. Wieczorek3, and 

James B. Snyder4

Introduction

Inventories of rock falls and other types of landslides 

are valuable tools for improving understanding of these 

events. For example, detailed information on rock falls is 

critical for identifying mechanisms that trigger rock falls, 

for quantifying the susceptibility of different cliffs to rock 

falls, and for developing magnitude-frequency relations (for 

example, Wieczorek and Jäger, 1996; Dussauge-Peisser and 

others, 2002; Chau and others, 2003; Dussauge and others, 

2003; Guzzetti and others, 2003; Malamud and others, 2004; 

Brunnetti and others, 2009; Tatard and others, 2010; Hergarten 

2012). Further, inventories can assist in quantifying the 

relative hazard and risk posed by these events over both short 

and long time scales (for example, Hungr and others, 1999; 

Guzzetti and others, 2003; Wieczorek and Snyder, 2009). 

This report describes the accompanying rock fall inventory 

database for Yosemite National Park, California (fig.1). The 

inventory database documents 925 events spanning the period 

1857–2011.

Rock falls, rock slides, and other forms of slope 

movement represent a serious natural hazard in Yosemite 

National Park. Rock-fall hazard and risk are particularly 

relevant in Yosemite Valley, where glacially steepened granitic 

cliffs approach 1 km in height (fig. 2) and where the majority 

of the approximately 4 million yearly visitors to the park 

congregate. In addition to damaging roads, trails, and other 

facilities, rock falls and other slope movement events have 

killed 15 people and injured at least 85 people in the park 

since the first documented rock fall in 1857.

Herein we use the term “rock fall” as a generic, collective 

term for all slope-movement processes (that is, landslides) 

in Yosemite, including rock falls, rock slides, debris slides, 

debris flows, debris slumps, and earth slumps, as described by 

the slope movement classification system of Varnes (1978). 

Although the database contains examples of all types of slope 

movements, rock falls are the dominant mode due to the very 

steep and relatively unfractured granitic cliffs in many areas 

of Yosemite National Park, particularly in Yosemite Valley 

(fig. 3). Most other documented rock falls in the park occurred 

in near proximity to Yosemite Valley, primarily in adjacent 

1

National Park Service, Yosemite National Park, El Portal, California

2

U.S. Geological Survey, Menlo Park, California

3

U.S. Geological Survey, Reston, Virginia (retired)

4

National Park Service, Yosemite National Park, El Portal, California (retired)

Tenaya Canyon and along the El Portal and Big Oak Flat 

Roads in the Merced River gorge (fig. 1).

This report begins with a brief summary of previous and 

current work on documenting rock falls in Yosemite National 

Park. It then describes each of the organizational categories 

in the inventory database, including event location, type of 

slope movement, date, volume, relative size, probable trigger, 

impact to humans, narrative description, references, and 

environmental conditions. The inventory database itself is 

contained in a Microsoft Excel spreadsheet (Yosemite_rock_

fall_database_1857-2011.xlsx). Narrative descriptions of 

events are contained in the database, but are also provided in 

a more readable Adobe portable document format (pdf) file 

(Yosemite_rock_fall_database_narratives_1857-2011.pdf).

Historical Rock Fall Records

Rock falls have occurred in Yosemite National Park 

since deglaciation of the region exposed most of the cliffs 

between about 15,000 and 19,000 years ago (Matthes, 1930; 

Huber, 1987; Wieczorek and Jäger, 1996). Although native 

people inhabited Yosemite Valley for millennia, there are no 

formally recorded rock falls during this period because of a 

lack of a written history. Following the modern discovery of 

Yosemite Valley in 1851, larger rock falls were mentioned 

in the writings of many visitors, including Josiah Whitney 

(State Geologist of California), John Muir (noted naturalist) 

and Joseph LeConte (professor of geology at the University 

of California). More systematic documentation of both small 

and large rock falls affecting facilities began after 1916 and 

was recorded in the monthly National Park Service (NPS) 

Superintendent’s Reports, because repairs were necessary 

to maintain damaged trails and roads. Following a multiplefatality rock fall onto the Yosemite Falls Trail in 1980 

(Snyder, 1981), the U.S. Geological Survey (USGS) assumed 

a more prominent role in landslide studies in Yosemite, 

collaborating with the NPS to document rock-fall events in 

detail (Wieczorek, 1981; Wieczorek and others, 1989). An 

initial collection of historical landslide information compiled 

by the NPS became the basis of the first rock-fall inventory 

(Wieczorek and others, 1992), which was subsequently 

updated and expanded by Wieczorek and Snyder (2004). The 

report and inventory database presented here represents a 

further update to and expansion of these earlier works.
2 Historical Rock Falls in Yosemite National Park, California (1857–2011)

260000 280000 300000

4220000

4200000

Base map created from 2006 airborne lidar.

California

Yosemite

 National

 Park

4160000

4180000 YYV

CFL

GIN

MPG

STR

CLARK RANGE

CATHEDRAL

Tenaya RANGE

Lake

Smedberg

Lake

May Lake

Tuolumne Meadows

Little Yosemite Valley

Hetch Hetchy

Wawona

WAWONA ROAD

Grand Canyon of

 The Tuolumne

Jack Main

 Canyon

Rancheria

Mountain

Rodgers

Canyon

HETCH HETCHY

ROAD

CLARK RANGE

CATHEDRAL

Tenaya RANGE

Lake

Smedberg

Lake

May Lake

Tuolumne Meadows

TIOGA ROAD

GLACIER POINT

ROAD

TIOGA ROAD

(CA HWY 120) (CA HWY 120)

Little Yosemite Valley

Hetch Hetchy

BIG OAK 

FLAT ROAD

Wawona

Tioga Pass

WAWONA ROAD

Grand Canyon of

 The Tuolumne

EL PORTAL

ROAD

GLACIER POINT

ROAD

Mount Conness

Jack Main

 Canyon

Rancheria

Mountain

Rodgers

Canyon

HETCH HETCHY

ROAD

Merced River

Gorge

BE

MG

BP

PJ RP

CC

FR

EP

ELTD

CL

AR

WP

DR

PL

LV

TP

TF

PV

PC

HM

GC

FM

CQ

CP

MD BU

TU TC

RS

CF

BS

BL

LB

RT

WF

WC

ES GD

MO

OC

RC

LR

RD

GV

DA

CD

BE

MG

BP

PJ RP

CC

FR

EP

ELTD

CL

AR

WP

DR

PL

LV

TP

TF

PV

PC

HM

GC

FM

CQ

CP

MD BU

TU TC

RS

CF

BS

BL

LB

RT

WF

WC

ES GD

MO

OC

RC

LR

RD

GV

DA

CD

0 5 10 15 20 KILOMETERS

0 5 10 15 MILES
Methods 3

Figure 1. Shaded relief map of Yosemite National Park showing general locations 

referred to in this report. Additional geographic features referenced in the database 

are indicated by white two-letter labels (refer to table 1 for key). The two-letter labels 

represent geographic locations only—they should not be interpreted as marking the 

precise locations of rock falls. Black lines mark major roadways. Green circles mark 

meteorological stations used to compile environmental conditions that existed during and 

prior to rock falls; GIN = Gin Flat, CFL = Crane Flat, YYV = Yosemite Valley, STR = Ostrander, 

MPG = Mariposa Grove. Red polygon indicates the Yosemite Valley area shown in figure 3. 

Figure 2. Photograph of Yosemite 

Valley looking east, showing the steep 

granitic cliffs that flank the valley. The 

cliff of El Capitan on the left side of 

the photo is approximately 915 m in 

height. Bridalveil Fall is in the centerright of the photo. Photo by Christine 

Loberg, used with permission.

Methods

The information reported in the inventory database was 

collected from review of published and unpublished historical 

accounts, archival records, scientific reports and papers, 

field studies, collection and interpretation of photographs 

and remote sensing data, eyewitness observations, and other 

reports from park visitors, employees, and residents. Since 

the early 20th century, many complementary investigations 

and mapping of prehistorical, historical, and recent rock falls 

and associated hazard and risk have also been conducted in 

Yosemite (for example, Matthes, 1930; Snyder, 1981, 1986a, 

1986b, 1996; Wieczorek and others, 1995; 1998, 1999, 

2000, 2008; Wieczorek and Jäger, 1996; Uhrhammer, 1996; 

Wieczorek and Snyder, 1999; Wieczorek, 2002; Guzzetti and 

others, 2003; Huber and others, 2007; Wieczorek and others, 

2008; Collins and Stock, 2010; Stock and Uhrhammer, 2010; 

Stock and others, 2011, 2012; Zimmer and others, 2012). 

Many of the narrative descriptions provided in this database 

draw on these works. Additional information about some of 

the data columns (for example, environmental conditions) are 

provided in the “Data Files” section of this report.

Many more than the 925 rock falls reported here 

undoubtedly occurred during the period of record investigated 

herein (1857–2011), but these events went unnoticed or 

unreported because of the small size of individual rock falls 

or the lack of impact to trails, roads, structures or utilities. In 

addition, some larger, historical rock falls are not included, 

despite the fact that at least some information on these events 

is known. During preparation of this and earlier versions of 

the inventory database, examination of historical photographs 

identified some rock falls that had never been reported. For 
4 Historical Rock Falls in Yosemite National Park, California (1857–2011)

Figure 3. Shaded relief map of Yosemite Valley derived from 1 m airborne lidar (light detection and ranging) data showing general 

locations (numbered labels) and additional location information (white two-letter labels—refer to table 1 for location key) where 

documented rock falls occurred. The 21 delineated areas represent large areas of Yosemite Valley, within which additional information 

on particular rock fall locations is reported in the database. The two-letter labels represent geographic locations only—they should not 

be interpreted as marking the precise locations of rock falls. 

2

3

4

5

6

7

8

9

10

12

13

15 14

16

17

18

19 20 21

1

11

4180000

265000 270000 275000

4175000

11: Half Dome

12: Grizzly Peak

13: Giant Staircase

14: Panorama Cliff

15: Illilouette Gorge

16: Glacier Point

17: Union Point

18: Sentinel Rock

19: Taft Point

20: Cathedral Rocks

21: Crocker Point

 1: Merced River Gorge

 2: Rockslides

 3: El Capitan

 4: Three Brothers

 5: Yosemite Falls 

 6: Indian Canyon

 7: Royal Arches

 8: North Dome/

 Washington Column

 9: Basket Dome

10: Tenaya Canyon

2

3

4

5

6

7

8

9

10

12

13

15 14

16

17

18

19 20 21

1

11

NO

NW

GP GA

SP

GZ

SC

RW

PW

PP

RF

RX

4W

AS

CA SB

UF

LF

MP

MB

LC

GS MC

4M

WT PB

PR

OB

SF

CR

FT RA

LG LT

HI

CV

CB

TA

SD

WS

PF

OI

HT

HK

IC

LE

FW

SE EB

CK

BF

CW

CS

ER

FB

ST MT

MM

EG

AP

YP

QD

ND

ML

LI

BR

MW

BD HF

NF

SN

VF

CG

SW

WA

NO

NW

GP GA

SP

GZ

SC

RW

PW

PP

RF

RX

4W

AS

CA SB

UF

LF

MP

MB

LC

GS MC

4M

WT PB

PR

OB

SF

CR

FT RA

LG LT

HI

CV

CB

TA

SD

WS

PF

OI

HT

HK

IC

LE

FW

SE EB

CK

BF

CW

CS

ER

FB

ST MT

MM

EG

AP

YP

QD

ND

ML

LI

BR

MW

BD HF

NF

SN

VF

CG

SW

WA

2

3

4

5

6

7

8

9

10

12

13

15 14

16

17

18

19 20 21

1

11

2

3

4

5

6

7

8

9

10

12

13

15 14

16

17

18

19 20 21

1

11

0 1 2 3 KILOMETERS

0 1 2 MILES

example, historical photographs of the eastern portion of 

Glacier Point document the occurrence of unreported rock 

falls in the 1850s, around 1900, and around 1950. However, 

the timing of these events can only be widely bracketed by the 

irregular time interval (years to decades) between available 

photographs and the limited areas covered by the photos. 

Because some event attributes can only be assigned if the 

timing of events is reasonably well known (for example, 

season, probable trigger), events that could not be located in 

time within a 5-year period are excluded from this database.

Data Files

The rock fall inventory data are contained in three 

separate sheets within the accompanying Excel spreadsheet 

file and within a pdf document of the narrative descriptions 

of events. The first Excel sheet (see example in fig. 4) is titled 

“Master” and contains an unique number assigned to each 

event (ID), the location, type of slope movement, season, date 

and time of occurrence, estimated volume, volume range and 

relative size of event, probable trigger, rock type, impact to 

humans (fatalities, injuries and infrastructure impacts) if any, 

citation of any published sources of information pertaining to 

the event, and a narrative description of the event. The second 

Excel sheet (see example in fig. 5) is titled “Environmental 

Conditions” and adds temporally corresponding environmental 

data, including temperature and precipitation conditions, 

when it exists, for each rock-fall event. This allows users to 

independently evaluate the environmental conditions leading 

up to and during the time of occurrence. Also included in the 

“Environmental Conditions” sheets are the event ID, location, 

failure type, and probable trigger. The last sheet, titled 

“References” (see example in fig. 6), contains a complete 

list of references corresponding to the database entries in the 

“Master” sheet.

In addition to updating the earlier database of Wieczorek 

and Snyder (2004), by reporting an additional 406 rock falls 

and other slope-movement events, this version also makes 
Data Files 5

Figure 4. Example format of “Master” sheet from the rock fall database. Only a small selection of the rock fall events and 

organizational categories are shown.

Figure 5. Example format of “Environmental Conditions” sheet from the rock fall database. Only a small selection of the rock fall events 

and organizational categories are shown.

several modifications to the digital files of the data. These 

modifications include adding columns to the database file, 

a restructuring of column formatting, and a new separate 

document for the narrative descriptions. This reformatting 

should provide improved usability of the database and 

more consistent reporting of event attributes (for example, 

location, date and time, and volume range). Details of these 

modifications are provided below as a reference for those 

users already familiar with previous versions of this database.

Identification Numbers

Each rock fall or other slope movement in the database 

has been assigned an unique identification (ID) number that is 

assigned based on the year (YYYY), month (MM), and day 

(DD) of the rock fall in a YYYYMMDDXX format where 

XX allows for multiple rock falls occurring on the same day. 

For example, ID 2003122602 refers to the second rock fall 

that occurred on December 26, 2003. When the exact date is 
6 Historical Rock Falls in Yosemite National Park, California (1857–2011)

Figure 6. Example format of “References” sheet from the rock fall database. Only a selection of the references are shown.

unknown, the earliest day in the range is chosen for the ID; 

if no range is known, the month and day are set to zeros. The 

ID numbers are unique to this database; they do not correlate 

directly with ID numbers used in previous versions of the 

database (that is, Wieczorek and others, 1992; Wieczorek and 

Snyder, 2004). The ID numbers in previous versions were 

assigned in chronological order, and changed in subsequent 

versions of the database due to the discovery of previously 

unlisted historical records of rock falls. We discontinued the 

practice of changing ID numbers to maintain chronological 

order in favor of the new system, where recently discovered 

historical rockfalls are assigned a chronologically sequential 

number referenced to the date of the event. There are therefore 

three sets of ID numbers, one each for the 1992, 2004, and 

present inventories. Although only the present ID numbers are 

referenced in this document, previous version ID numbers are 

included in the master sheet of the database to assist crossreferencing with previous versions.

In many cases, smaller rock falls occurred for periods 

of days, weeks, and sometimes months following a large 

rock fall, making accurate reporting of all associated events 

difficult. For example, numerous small rock falls continued 

at Middle Brother for at least one month following two large 

rock falls on March 10, 1987 (Wieczorek and others, 1995; 

Wieczorek, 2002). In general, the descriptions of large rock 

falls (with volumes between 500 and 5000 m3

) that occurred 

within a short time period from the same release point are 

listed as separate events. Information on smaller rock falls is 

particularly difficult to collect from events occurring at night, 

or from events that were only heard without being observed, 

because reports of location and volume can vary widely. In 

winter, distinctions between ice fall and rock fall are also 

problematic. Therefore, this inventory lists only those rock 

falls for which sufficient and reliable descriptive information 

or physical evidence are available. 

Event Location

Two data columns, labeled “General location” 

and “Additional location information,” provide spatial 

identification information for events in the inventory database. 

The general location refers either to one of 21 areas in 

Yosemite Valley (fig. 3) that are delineated by major natural 

topographic breaks or cliff features, or to other areas outside 

Yosemite Valley but still within Yosemite National Park (for 

example, Hetch Hetchy, Tuolumne Meadows, Tioga Road, and 

others; fig. 1) . The additional location information refers to a 

more specific area within each general location and typically 

references geographic feature names, well-known developed 

infrastructure, or rock climbing areas (for example, Middle 

Brother, Yosemite Falls Trail, and Chapel Wall, which are 

located within the Three Brothers, Yosemite Falls, and Union 

Point general locations, respectively). These more specific 

locations are marked with two-letter character designations in 

figures 1 and 3 and are referenced to both formal and informal 

geographic names in table 1. A third column, labeled “Figure 

location reference,” is included in the database to direct users 

to the appropriate figure in which the general location of these 

features can be found.

Where available, additional detailed location information 

is provided in the narratives. Despite the identification of 

specific locations of features on the maps, it is imperative 

that users recognize the limitations of these data in precisely 

locating the rock fall events—whereas in some cases the exact 

location of the rock fall is known to within several meters, in 

many cases, the only spatial location information known about 

an event is that it occurred somewhere in the vicinity of a trail 

or other landmark. In some of these cases, particularly for those 

occurring early in the database history, the spatial location 

can only be identified to within an area of several square 

kilometers.
Data Files 7

Table 1. Geographic names1 and labels of select locations in Yosemite National Park referenced2 in figures 1 and 3.

Label Geographic name Label Geographic name Label Geographic name

4M Four Mile Trail (F3-17) GC Grouse Creek (F1) PJ Pat and Jack Pinnacle (F1)

4W Camp Four Wall (F3-4) GD Gardiner Camp (F1) PL Parkline (F1)

AP Ahwiyah Point (F3-11) GP Glacier Point (F3-16) PP Panorama Point (F3-14)

AR Arch Rock (F1) GS Gunsight (F3-20) PR Pulpit Rock (F3-1)

AS Arrowhead Spire (F3-6) GV Grapevine (F1) PV Pate Valley (F1)

BD Basket Dome (F3-9) GZ Grizzly Peak (F3-12) PW Porcelain Wall (F3-11)

BE Benson Pass (F1) HF Hidden Falls (F3-10) QD Quarter Domes (F3-10)

BF Bridalveil Fall (F3-20) HI Happy Isles (F3-16) RA Royal Arches Cascade (F3-7)

BL Bernice Lake (F1) HK Housekeeping Camp (F3-16) RC Rail Creek (F1)

BP Badger Pass (F1) HM Hodgdon Meadow (F1) RD Red Peak Pass Trail (F1)

BR Mount Broderick (F3-13) HT Horse Tail Falls (F3-3) RF Ribbon Falls (F3-2)

BS Beehive Switchbacks (F1) IC Illilouette Creek (F3-15) RP Reed’s Pinnacle (F1)

BU Bunnell Point (F1) LB Little Blue Slide (F1) RS Rostrum (F1)

CA Castle Cliffs (F3-6) LC Lower Cathedral Rock (F3-20) RT Rattlesnake (F1)

CB Church Bowl (F3-7) LE Lehamite Falls (F3-6) RW Rhombus Wall (F3-7)

CC Cookie Cliff (F1) LF Lower Yosemite Falls (F3-5) RX Rixon’s Pinnacle (F3-4)

CD Cascades (F1) LG LeConte Gully (F3-16) SB Sunnyside Bench (F3-5)

CF Chilnualna Falls (F1) LI Liberty Cap (F3-13) SC Sentinel Creek (F3-18)

CG Cathedral Gully (F3-20) LR Lake Eleanor Road (F1) SD Stanford Point (F3-21)

CK Clark Point (F3-13) LT Ledge Trail (F3-16) SE Southeast Face (F3-3)

CL Cloud’s Rest (F1) LV Lee Vining Canyon (F1) SF Staircase Falls (F3-16)

CP Cathedral Peak (F1) MB Middle Brother (F3-4) SN Snow Creek Trail (F3-9)

CQ Chinquapin (F1) MC Middle Cathedral Rock (F3-20) SP Sierra Point (F3-12)

CR Columbia Rock (F3-5) MD Moraine Dome (F1) ST Sierra Point Trail (F3-12)

CS Cathedral Spires (F3-20) MG Muir Gorge (F1) SW Southwest Face (F3-3)

CV Curry Village (F3-16) ML Mirror Lake Trail (F3-11) TA Taft Point (F3-19)

CW Chapel Wall (F3-17) MM Medial Moraine (F3-11) TC Tiltill Creek (F1)

DA Mount Dana (F1) MO Mono Grade (F1) TD Turtleback Dome (F1)

DR Dog Rock (F1) MP Manure Pile Buttress (F3-3) TF Tamarack Flat (F1)

EB East Buttress (F3-3) MT Mist Trail (F3-13) TP Tenaya Peak (F1)

EG Eagle Peak (F3-4) MW Mount Watkins (F3-10) TU Tueeulala Falls (F1)

EL Elephant Rock (F1) ND North Dome (F3-8) UF Upper Yosemite Falls (F3-5)

EP El Portal (F1) NF Nevada Fall (F3-13) VF Vernal Fall (F3-13)

ER Eagle Rock (F3-17) NO Nose (F3-3) WA Washington Column (F3-8)

ES Eye in the Sky (F1) NW Northwest Face (F3-11) WC Wawona Campground (F1)

FB Fifi Buttress (F3-21) OB Old Big Oak Flat Rd (F3-2) WF Wapama Falls (F1)

FM Fort Monroe (F1) OC Old Coulterville Road (F1) WP Windy Point (F1)

FR Foresta Road (F1) OI Old Inspiration Point (F3-21) WS Washburn Slide (F3-21)

FT Yosemite Falls Trail (F3-5) PB Pohono Bridge (F3-21) WT Wawona Tunnel (F3-1)

FW Forbidden Wall (F3-5) PC Piute Creek (F1) YP Yosemite Point (F3-5)

GA Glacier Point Apron (F3-16) PF Profile Cliff (F3-19)

1

Both formal and informal geographic names are used to describe locations.

2

Text in parentheses refers to the figure number (for example, F1 for fig. 1) in which the location is identified by the 2-letter label. For locations within 

Yosemite Valley, the number following the figure number identifies the subarea delineated in figure 3.
8 Historical Rock Falls in Yosemite National Park, California (1857–2011)

The dual-location format differs from previous versions 

of the database that listed only detailed locations for rock 

falls. Whereas the previous method had the advantage of 

providing detailed spatial information within the “Location” 

column, it proved to be a disadvantage for those not intimately 

familiar with the many place names of Yosemite National 

Park. Listing of only detailed locations also often made spatial 

analyses of general areas difficult; for example, rock falls from 

the Glacier Point area were variously listed as occurring at 

“Washburn Point,” “Happy Isles,” “Curry Village,” “Staircase 

Falls,” “LeConte Gully,” and so on, complicating analyses of 

rock falls from the Glacier Point area. The new general and 

additional location information columns therefore provide 

more clarity on the locations of rock falls, and allows users 

to sort and view events by both general and more specific 

areas of interest. For completeness, any spatial information 

previously included in the “location” column remains 

preserved in the narrative descriptions.

Type of Slope Movement

Types of slope movement are classified using the system 

of Varnes (1978), which is based on the type of material 

involved in movement (rock, soil, or debris) and the dominant 

type of movement (falling, sliding, toppling, flowing, or a 

combination thereof). Most historical reports either identify 

the type of slope movement or are sufficiently descriptive 

so that the type of slope movement can be inferred based on 

examination of the form and character of the source area or 

resulting deposits. Mass movements involving rock falls or 

rock slides are the most numerous types of slope movement 

recorded in Yosemite National Park (89 percent). Debris 

flows, debris slides, and earth or debris slumps have been less 

frequently reported. This may be due in part to inconsistent 

recording of these events prior to 1990, but more consistent 

recording after 1990 indicates that these events do in fact 

occur less frequently than do rock falls and rock slides.

Season

Evaluating the seasonality of rock falls can improve 

understanding of rock-fall triggering mechanisms (see, for 

example, Luckman, 1976). Accordingly, the database lists 

the season of each rock fall or other slope movement event, 

where known, under a column titled “Season.” This provides 

a measure of time for each event in the database, even if the 

exact date is not known (for example, for instances when rock 

debris is encountered on a trail and only reported some time 

after the rock fall has occurred). Winter is defined as the period 

between the winter solstice and spring equinox (December 

21–22 through March 20–21), spring as the period between 

the spring equinox and summer solstice (March 20–21 through 

June 20–21), summer as the period between the summer 

solstice and fall equinox (June 20–21 through September 

22–23), and fall as the period between the fall equinox and 

winter solstice (September 22–23 through December 21–22). 

Due to planetary physics (that is, the tilt of the Earth with 

respect to the sun), the exact dates of equinoxes and solstices 

vary from year to year; for consistency we define all transitions 

as occurring at 12:01 PST on the second day listed (for 

example, 12:01 a.m. on December 22). Listing rock falls by 

season allows users to perform seasonal correlations of other 

rock fall parameters using a standardized data set and reduces 

the possibility of conflicting interpretations of the data.

Date and Time

Dates and times of occurrence of historical rock falls are 

known with varying degrees of accuracy. The larger and more 

damaging events typically attract more attention and are often 

noted with precision to the day, hour, and sometimes minute. 

Recent events documented with digital cameras (whose clocks 

are set correctly) may even record timing with subminute 

precision. However, some events, such as those observed 

after the winter or spring seasons, can only be associated with 

several months within a particular season. As Yosemite Valley 

visitation has increased and as park visitors, employees, and 

residents have become more aware of rock falls, the precise 

timing of rock falls has been more regularly reported. At 

present, the timing of relatively large rock falls occurring in 

Yosemite Valley is usually reported to the minute, but smaller 

rock falls occurring in more remote areas outside of Yosemite 

Valley may only be reported to the day or month. 

To allow for easy sorting of the database, the dates for 

each event are subdivided into three separate columns of year, 

month, and day. Because some events in the database do not 

report an exact day, month, or even year, this division allows 

users to conduct general temporal analyses of rock falls. As 

noted previously, events that could not be located in time 

within a 5-year period are excluded from this database.

All times for rock fall events are reported in 24-hour time 

and standardized to Pacific Standard Time (PST) rather than 

to local time (for example, Pacific Daylight Time, PDT). In 

earlier editions of the database, event times were recorded in the 

local time of Yosemite National Park with no distinction made 

between PST and PDT. However, this earlier convention can 

pose problems when events occur near the switch from PST to 

PDT, in that they might be assigned the same exact time. Local 

time is still commonly reported in the narrative descriptions, but 

the official (PST) time is reported in the “Time” column.

To standardize all events in the inventory, we reviewed 

the changes in policy regarding PST and PDT in California 

beginning in 1918, when PST became the official time of 

California (Douma, 2008; State of California, 1949, 1962, 

2013). We accounted for all changes in dates and times 

marking the start and end of PDT throughout this time period. 

Because of the conversion from local PDT to PST, three 

of the events (Event IDs 1943031801, 1980011302, and 

1982102502) also changed dates and are now reported as 

occurring one day prior to the dates reported in the previous 
Data Files 9

(Wieczorek and Snyder, 2004) version of this database. In 

addition, one new event not included in prior databases and 

that was reported to have occurred on August 5, 1957, at 

midnight is reported as Event ID 1957080401 with a time of 

23:00 PST on August 4, 1957. The only specific times reported 

for events prior to 1918 are by John Muir for earthquaketriggered rock falls on March 26, 1872 (IDs 1872032601 and 

1872032606; Muir, 1912). These times cannot be standardized 

to PST and are reported directly as John Muir recorded them.

Estimated Volume

Rock fall volume is an important component of many 

types of hazard assessments, and particularly those that involve 

computer simulations and probabilistic assessments based on 

volume-frequency relations (for example, Guzzetti and others, 

2003, Dussauge-Peisser and others, 2002; Dussauge and others, 

2003, Stock and others, 2012) as it can strongly influence 

calculated rock-fall runout distances. Rock-fall volumes are 

reported here in cubic meters. We intentionally use the term 

“Estimated volume” to reflect the uncertainty associated with 

the majority of volumes reported in the database. The volume or 

weight of a rock fall is sometimes mentioned in historical reports, 

often in terms of cubic yards or pounds, but more frequently is 

reported in only relative size terms such as “small” or “large.”

Several factors complicate the determination of accurate 

volume estimates from rock-fall descriptions. Descriptions of 

some rock falls include incomplete estimates of volume, such as 

the length of trail blocked by rock-fall debris. In order to estimate 

volume, such descriptions require additional assumptions (for 

example, debris width, thickness, and porosity) that are often 

difficult to evaluate. An historical estimate of rock-fall volume 

blocking a road may represent only a small fraction of the total 

volume because other portions of the rock fall may have stopped 

uphill or been transported downhill beyond the road. In such cases, 

the reported volume is a minimum estimate. In general, volumes 

reported prior to 1980 are depositional volumes; since 1980 an 

effort has been made to estimate volumes from source areas. In 

certain instances, older historical rock-fall volumes have been 

updated based on field or remote sensing measurements of their 

source areas or deposits. Other descriptions lack any information 

relative to size. Historical volume records reported here generally 

refer to final volumes of deposition rather than detached volumes. 

If tonnage was reported, volume was calculated assuming an 

average unit weight of 2,700 kg/m3

 (168 lb/ft3). Inaccuracy in 

estimating volume can also result from differences in average unit 

weight between intact bedrock and rock-fall deposits. 

Momentum transfer can also complicate estimates of rock 

fall volumes. The removed volume at a rock-fall scar is at most 

a measure of the initial minimum volume from the source area; 

during descent down the cliff face, a transfer of momentum 

can mobilize additional intact rock from bedrock ledges or 

talus which can substantially increase the overall volume of 

mobilized material. For example, the reported volume of the 

Ahwiyah Point rock fall on March 28, 2009 (ID 2009032801, 

located within the Half Dome general location; see fig. 3), 

consists of approximately 25,400 m3 from the source area, 

2,000 m3 from intermediate blocks dislodged after sliding, and 

19,300 m3 dislodged when the initial rock mass struck bedrock 

ledges as it moved down the cliff face (Zimmer and others, 

2012). Thus, in some events the reported volume may differ 

considerably from the initial source area volume. 

When a series of rock falls occurred in the same location, 

but their individual volumes could not be discerned from one 

another, only a total volume estimate is provided. For example, 

on March 10, 1987, a large rock fall from the face of Middle 

Brother (Three Brothers general location) spread rapidly across 

the talus cone at its base, covered the adjacent road (Northside 

Drive), and sent a few boulders across the Merced River (ID 

1987031001). Another large rock fall occurred later that day 

from the same source area (ID 1987031002). The combined 

volume of these two rock falls is an estimated 600,000 m3 and 

represents the largest event in the database. This value is listed 

as a single cumulative volume in the entry for the first event 

(ID 1987031001), whereas the volume for ID 1987031002 

is listed as “See ID 1987031001” indicating that the reported 

value for the preceding event is a summation of the two rock 

falls. In only a few instances were rock-fall volumes precisely 

determined (for example, IDs 2008100701–2008100801, 

2009032801, 2009082601–2009091401), usually by using 

repeat laser scanning or other quantitative remote sensing 

techniques (see, for example, Stock and others, 2011, 2012; 

Zimmer and others, 2012). Considering the qualifications 

regarding volume estimates, the volumes reported in this 

inventory, especially those prior to 1980, are best viewed as 

order-of-magnitude estimates unless specific volume or weight 

data are reported, or if the dimensions of the source area or 

entire deposit were more accurately measured, as indicated in 

the narratives or relevant publications.

In addition to quantitative estimates of volume, we also 

characterize the range of volumes in relative terms, ranging 

from “extremely small” to “extremely large” (table 2). A 

correlation between these relative volume terms and the range 

of volume and assigned median volume was developed by 

Wieczorek and others (1992) based on detailed examination 

of specific rock falls and the overall distribution of rock-fall 

volumes, and was continued in the database of Wieczorek 

and Snyder (2004). The assignment of volumes based on 

qualitative terms provides a consistent method for estimating 

volumes across the entire inventory of rock falls.

For many rock falls, especially those that occurred early 

in the reporting period, event volumes are not well known, and 

a median volume (logarithmic scale) is reported for specific 

order-of-magnitude estimates (table 2). Rather than reporting 

volume ranges in the “Estimated volume” column, we report 

the median volume there and show the range in the minimum 

and maximum estimate columns. In some cases, the volume of a 

rock fall is not known exactly but can be constrained by a range 

that is more precise than an order of magnitude; in those cases, 

the minimum and maximum estimates are also shown. Finally, 

in some cases the volume is well-constrained and the estimate 
10 Historical Rock Falls in Yosemite National Park, California (1857–2011)

is considered to be reasonably accurate; in those cases the 

minimum and maximum estimates are intentionally left blank.

Rock Units

Rock units associated with rock-fall source areas are 

determined in most recent events by on-site inspection of 

source areas and the resulting new talus beneath these areas. 

For older events and for events in which the general rock 

fall location is known, but no definite source area or fresh 

talus identified, we cross-referenced the event locations 

with published geologic maps (Kistler, 1973; Calkins 1985, 

Dodge and Calk, 1987; Huber and others, 1989; Peck, 2002) 

to determine source area rock units. In some cases, the exact 

rock unit is indeterminate either because the exact location 

of the rock fall is unclear, or because there is a mixture of 

different rock units in a particular area (for example, Glacier 

Point contains large areas of both Half Dome Granodiorite and 

granodiorite of Glacier Point); in these cases, more than one 

geologic unit is listed.

The oldest rock unit listed in the database is undivided 

metamorphic or metasedimentary rock (Pzme), and is found 

at a few small enclaves inside the park (for example, along 

Wawona Road) and just outside the park (for example, the 

Merced River gorge near El Portal). The remainder of the rock 

is granitic and dioritic rock of Cretaceous age and is part of the 

larger Sierra Nevada batholith (Bateman, 1992). The database 

includes rock falls that have occurred from rocks representing 

19 mapped intrusive units (table 3). Some slope movement 

events, such as debris flows, debris slides, debris slumps, 

earth slides, and earth slumps, consist primarily of previously 

deposited material that has remobilized. In these cases, the rock 

unit is designated as Quaternary colluvium and (or) talus (Qat).

Probable Triggering Condition(s)

We assign a probable triggering condition for each rock 

fall or other slope movement based on available evidence at the 

time of the event. Referring to landslides in general, Wieczorek 

(1996) defines a trigger as “an external stimulus such as 

intense rainfall, earthquake shaking, volcanic eruption, storm 

waves, or rapid stream erosion that causes a near-immediate 

response in the form of a landslide by rapidly increasing the 

stresses or by reducing the strength of the slope materials.” 

This definition encompasses two concepts: the environmental 

conditions under which the event occurs, and the physical 

processes and mechanistic forces that result in movement of 

the rock or soil. Whereas the majority of triggering conditions 

can be inferred through observations or temporal coincidence, 

triggering mechanisms can, in most cases, only be determined 

by detailed monitoring and instrumentation of rock fall 

detachment surfaces (see discussion in Wieczorek and others, 

2008). Triggering mechanisms include such processes as 

increased water pressure from precipitation-induced seepage, 

inertial forces from seismic shaking, and fracture propagation 

Table 2. Relative size categories of rock falls in Yosemite 

National Park.

 Relative size Volume range (m3) Median volume 

(m3

)

Extremely small 0–0.5 0.2

Very small 0.5–5 2

Small 5–50 20

Medium 50–500 200

Large 500–5,000 2,000

Very large 5,000–50,000 20,000

Extremely large 50,000–500,000 200,000

Table 3. Rock units comprising rock falls in Yosemite National 

Park.

Geologic symbol Rock unit

Qat colluvium and/or talus

Kcp Cathedral Peak Granodiorite1

Khd Half Dome Granodiorite2

Kgp granodiorite of Glacier Point2

Ks Sentinel Granodiorite2

Kyc granodiorite of Yosemite Creek2

Kgi quartz diorite of Mount Gibson3

Krd granodiorite of Red Devil Lake1

Kbv Bridalveil Granodiorite2

Klt Leaning Tower Granite2

Kic granodiorite of Illilouette Creek2

Kid quartz diorite1

Kt Taft Granite2

Krm granodiorite of Rancheria Mountain3

Kec El Capitan Granite2

Ktf granodiorite of Tueeulala Falls4

Ka granodiorite of Arch Rock2

Kga tonalite of the Gateway1

Kdg diorite of the Rockslides5

Kg granitic rocks, undivided1

Pzme metasedimentary rock, undivided1

1

Nomenclature from Huber and others (1989).

2

From Peck (2002).

3

From Kistler (1973).

4

From Dodge and Calk (1987).

5

From Calkins (1985).
Data Files 11

from thermal heating or freeze-thaw cyclic forces. In general, 

none of these processes have been directly measured at 

particular rock fall sites in Yosemite and thus our assignment of 

triggering mechanisms must be considered tentative.

We use the term “probable” with respect to triggering 

conditions to reflect the inherent uncertainty in assigning 

exact triggers. For example, a very light rainfall event may 

be temporally coincident with a rock fall, but the amount 

of rainfall might not be sufficient to cause a change in the 

stability of the rock mass. Similarly, a period of very hot 

temperatures might be associated with a rock fall but cannot 

be strictly determined to have triggered failure.

Wieczorek (1996) noted the importance of timing in 

assigning triggers, stating that “the requisite short time frame 

of cause and effect is the critical element in the identification 

of a landslide trigger.” Some rock falls recorded in the 

accompanying database were observed by eyewitnesses and 

directly associated in time with triggers such as rainstorms 

or earthquakes. In other accounts, although the rock fall was 

not observed as it occurred, the trigger was reported to be a 

temporally associated event, such as an earthquake, rain storm, 

period of rapid snowmelt, wind, or period of extreme freezethaw conditions. In this manner, the triggering condition is 

inferred primarily by temporal coincidence. As such, assigned 

triggers should be considered provisional assessments subject to 

change pending more detailed analysis. However, in general, it 

is unrealistic to assume that more information will be obtained 

for the vast majority of the events reported in the database. 

We assign sixteen types of triggering conditions (with 

associated abbreviations) to the event database, described in 

the following sections.

Earthquakes (EQ)

Earthquakes generate seismic ground shaking that can 

result in inertial forces to rock and soil masses, causing failure 

(see, for example, Keefer, 1984). Earthquakes can also cause 

cyclic deterioration of the strength that initially keeps rock or 

soil masses stable. We assigned earthquake (EQ) triggers to 

events by temporal correlation (within seconds to minutes) 

with observed or measured ground shaking. We evaluated 

earthquake activity based on information provided by the 

USGS National Earthquake Information Center (USGS, 2011).

Precipitation (P), Rain-on-Snow (R/S), Snow Melt 

(SM)

Precipitation consisting of rain and snow, rain falling 

on snow with resulting snow melt, and seasonal snow 

melt can lead to erosion of interstitial material between 

originally stable rock masses, reduction of tensile stresses 

in soils, and development of water pressures within both 

soil and rock masses. We assigned these triggers to events 

by temporal correlation with observed or measured weather 

and temperature conditions. We assign a precipitation (P) 

trigger collectively to both rain and snow events because 

both may occur during a single storm and often vary greatly 

depending on elevation. We further distinguish rain-on-snow 

(R/S), and snow melt (SM) conditions occurring at the time 

of a rock fall, when known, to highlight particular types of 

seepage conditions (for example, rain-on-snow conditions can 

sometimes generate greater runoff than precipitation alone). 

Problems assigning precipitation triggers to a particular event 

are primarily with timing; infiltration of rainfall may take days 

or weeks following storm events, and snowmelt may occur 

even more gradually over several months during seasonal 

warming.

Freeze-Thaw (F/T)

Freeze-thaw conditions consist of temperature 

fluctuations around 0 oC that result in the freezing of water 

and thawing of ice within rock fractures. The approximate 9 

percent volumetric expansion of water into ice during freezing 

can lead to increased stresses in rock fractures that force 

partially detached rock masses to move outwards from the cliff 

face. Thawing conditions can release destabilized blocks that 

were previously frozen in place (see, for example, Matsuoka 

and Sakai, 1999). We assign freeze-thaw (F/T) triggers to 

events by temporal correlation with observed or measured 

weather and temperature conditions.

Snow Avalanche (SA)

Snow avalanches can trigger rock falls by directly 

impacting unstable rock masses along their downward 

trajectory or by mobilizing unstable rock masses through the 

basal shear stress they impart to the surfaces that they move 

over. We assign a snow avalanche (SA) trigger to events by 

temporal correlation with observations or documented reports 

of snow avalanches.

Wind (W)

Wind can loosen rock masses by causing pressure 

differences between interior fractures and exterior surfaces, 

or by forcing trees to leverage root networks embedded in 

rock masses. We assign a wind (W) trigger based on temporal 

correlation with observed windy conditions (typically winds in 

excess of 30 km/hour).

Lightning (L)

Lightning can weaken and detach intact rock masses by 

the discharge of electrostatic energy during a strike, sometimes 

through superheating of water into steam that causes increased 

pressures in rock fractures. We assign a lightning (L) trigger 

based on temporal correlation with observed or reported 

lightning strikes.
12 Historical Rock Falls in Yosemite National Park, California (1857–2011)

Root Wedging (RW)

Root wedging consists of the expansion of rock fractures 

from the growth of vegetation roots. This can cause either 

an increase in stress to the interior of rock masses or cause 

a decrease in rock fracture strength by the breaking apart of 

rock mass bonds. We assign a root wedging (RW) trigger to 

rock falls based on inferred correlation with observations that 

showed evidence of root growth in bounding rock fractures 

but with no other identified trigger.

Thermal Stress (TS)

Thermal stress as a rock-fall trigger consists of the 

expansion and contraction, and resulting deformation, of 

rock masses from cyclical changes in temperature (see, for 

example, Vargas and others, 2012). Although not reported 

as a rock fall trigger in previous versions of this database 

(Wieczorek and Snyder, 2004; Wieczorek and others, 1992), 

recent research (for example, Collins and Stock, 2010) 

has shown that thermal stresses may be responsible for 

the deformation and subsequent failure of thin, exfoliation 

flakes formed along sheeting joints, a feature common in 

Yosemite. Further, this research has shown that there may be 

a temporal correlation of rock falls with summer afternoons 

and early evenings in which temperatures are typically at 

their maximum during the year. We therefore assign a thermal 

stress (TS) trigger to rock falls based on inferred correlation 

with periods of measured high temperatures and strong daily 

temperature gradients, and for which no other obvious trigger 

could be identified.

Crack Propagation (CP)

Crack propagation is caused by changing stress conditions 

within a rock mass. Although these changes can be induced 

by a number of factors, including water or ice as previously 

identified, we reserve the assignment of a crack propagation 

trigger to conditions where a rock fall can be plausibly linked 

to stress redistribution and cracking following an earlier failure 

of an adjacent rock mass. This can occur, for example, when a 

rock fall occurs in an overhanging region and the loss of support 

results in a new stress condition that leads to an additional rock 

fall (for example, progressive rock falls from the Rhombus 

Wall in 2009–2010; Stock and others, 2012). The cracking 

observed by Wieczorek and Snyder (1999) and modeled by 

Chau and Lin (2011) that followed the June 13, 1999, Curry 

Village rock fall (Glacier Point general location) is another 

example of this type of process. Assigning temporal coincidence 

for crack propagation triggers can be difficult because cracks 

may take days, weeks, months, or longer to propagate through 

rock masses. However, in some instances, cracking noises 

were heard just prior to, or just after, a rock fall occurred, 

indicating crack propagation. Therefore, we assign a crack 

propagation (CP) trigger when this condition was suggested 

by the adjacent proximity of new rock falls to existing source 

areas, the presence of cracking sounds, and where no other 

obvious trigger could be identified. Because of the difficulty of 

reliably identifying crack propagation conditions, this trigger is 

considered particularly tentative at this time.

Blasting (BL), Construction (C)

Human-induced blasting of rock for construction or 

hazard mitigation purposes can induce vibratory forces and 

cyclical strength degradation similar to earthquakes such that 

rock masses may become destabilized. Similarly, human slope 

disturbance in the form of construction (but not including 

blasting) can cause undercutting and destabilization of rock 

masses that may lead to rock falls and other slope movements. 

We assign a blasting (BL) or construction (C) trigger to events 

by temporal correlations (usually within seconds to minutes) 

with observed or reported blasting and construction activities.

Ground Vibration (GV)

Ground vibration induced from other rock falls can 

trigger subsequent rock falls by causing vibratory forces and 

cyclical strength degradation similar to earthquakes such that 

rock masses may become destabilized. We assign a ground 

vibration (GV) trigger to events by temporal correlation of 

rock falls from nonadjacent rock masses, and in the absence of 

any documented seismicity.

Unrecognized (UNR) and Unknown (UKN)

Referring to landslides in general, Wieczorek (1996) 

noted that some landslides “may occur without an apparent 

attributable trigger because of a variety of combination of 

causes, such as chemical or physical weathering of materials, 

that gradually bring the slope to failure.” In cases where rock 

falls in Yosemite were closely observed, but the triggers are 

still unrecognized, they are listed in this database with an 

unrecognized (UNR) trigger. The concept of unrecognized 

triggers is not unique to Yosemite and this category has been 

included in other rock fall inventories (for example, Sanderson 

and others, 1996). Unrecognized triggers are distinguished 

from unknown triggers where, in the latter case, little to no 

information is known about the event. In cases where a trigger 

is unreported and no additional information regarding possible 

triggering conditions is given, they are listed in this database 

with an unknown (UKN) trigger. Rock falls whose timing is 

not well known are also assigned unknown triggers.

Impacts to Humans

The impacts of a particular rock fall to humans are 

categorized in three different ways: number of human fatalities, 

number of human injuries, and damage to trails, roads, 
Data Files 13

structures, utilities, or other infrastructure or assets. The degree 

of seriousness of injuries was not characterized separately in 

most historical accounts; the number of injuries recorded in 

the database therefore includes both minor and major injuries. 

For a more comprehensive evaluation of the hazards or 

risks posed by these rock falls, additional details of reported 

damage are provided in the narrative descriptions. In some 

narrative descriptions, the estimated costs of damage or repairs 

were included (reported for the time that the event occurred, 

that is, uncorrected for inflation), but we did not attempt to 

characterize these with respect to the magnitude of damage. 

Likewise, we did not distinguish severe from minimal damage 

caused by rock falls. For example, severe damage from a rock 

fall may have torn apart or covered a large portion of a road, 

requiring days or weeks for repairs, whereas in other cases, 

only a few small rocks falling onto a road may have resulted in 

the road being closed for shorter periods of time until the site 

was judged safe to reopen. The narratives provide additional 

qualitative information about the severity of damage.

Within the database, fatalities and injuries are designated 

with the number of persons killed or injured. When no one was 

affected, a “0” was assigned. Review of historical information 

between publication of this version of the database and the 

previous version identified 3 additional rock-fall-related human 

fatalities and numerous additional rock-fall-related injuries, 

bringing the total number of documented fatalities to 15 and 

the total number of injuries to 85 from 1857 through 2011.

Published References

Within the accompanying database, we provide a listing of 

published references relevant to the rock falls and other slope 

movement events reported in this database. These include papers 

from scientific journals, conference proceedings and abstracts, 

published reports from the USGS and other sources, Master’s 

theses and Ph.D. dissertations, and newspaper articles. In cases 

of large or otherwise notable rock falls, many newspaper articles 

exist; in those cases, we reference one or two examples. 

The published references column in the database has been 

updated from the previous version (Wieczorek and Snyder, 

2004) to include additional references for rock falls occurring 

between 1857 and 2004, and to include new material for rock 

falls occurring after 2004. We have included all published 

references in this column, even if information in them is not 

specifically cited in the “Narratives” column. Full citations 

for references listed in the “Published References” column are 

provided in the third sheet of the database, titled “References.”

Narratives

The “Narratives” column in the accompanying database 

and the accompanying file, “Yosemite_rock_fall_database_

narratives_1857–2011.pdf” (see example in fig. 7), contains 

written descriptions of the events, where available. These 

descriptions vary greatly in length and detail. Descriptions of 

rock falls were compiled using both published and unpublished 

information. The many sources of unpublished information 

include field notes, internal NPS, USGS, and other reports, 

memoranda, emails, website forums, and oral communications 

from park visitors, employees, and residents. 

The first collection of rock-fall information was an 

unpublished rock-fall inventory prepared by Yosemite National 

Park Historian James Snyder (NPS, unpub. data, 1990). 

Subsequent investigations by Wieczorek and others (1992) 

and Wieczorek and Snyder (2004) derived additional historical 

rock-fall information from a variety of sources. These sources 

include, among others, (1) historical publications, such as those 

by geologists Josiah Whitney (Whitney, 1868) and Joseph 

LeConte (LeConte, 1875); (2) the unpublished notes, journals, 

and letters of John Muir from the Holt-Atherton Collections, 

University of Pacific Libraries, Stockton, California; (3) the 

Guardians’ Reports, Cavalry Administration Reports, and 

the Superintendent’s Monthly Reports of Yosemite National 

Park from the Yosemite Research Library and at the National 

Archives in Washington, D.C.; and (4) the field notes and draft 

maps of Francois Matthes at the USGS Photo Library and Field 

Records in Denver, Colorado, and Matthes’ personal diaries 

at the Bancroft Library, University of California, Berkeley. 

Beginning in the 1980s, rock falls were generally documented 

by USGS and/or NPS personnel in field notes, letters, and 

memoranda. Information pertaining to most rock falls occurring 

since 2005 are recorded in the field notebooks and digital files of 

the Yosemite National Park geologist. Other unpublished sources 

are available for examination at the Yosemite Research Library 

in Yosemite Valley. 

As described previously, rock falls from the same source 

area that occurred at observed separate times, even only several 

seconds apart, are listed as separate events (for example, ID 

1996071001, 1996071002); however, the description of the several 

events is sometimes combined for continuity in the narratives, (for 

example, IDs 2003060601–2003062901). Finally, in some cases, 

photographs or maps are referenced in the descriptions of the rock 

falls from the cited sources; these photographs or maps are not 

included in this inventory, but are available for examination in the 

Yosemite Research Library in Yosemite Valley, or in the files of 

the Yosemite National Park geologist. 

Environmental Conditions

To further evaluate the assigned probable triggering 

conditions, we compiled temperature and precipitation data 

for each rock fall starting in October 1985 (the time of earliest 

available meteorological data from nearby stations) for a 

total of 537 events and tallied this data in the second sheet 

of the Excel spreadsheet, titled “Environmental Conditions.” 

We acquired data from the California Department of Water 

Resources (CDWR, 2010) for five meteorological stations 

in Yosemite (fig. 1): four stations are located on the rim 

surrounding Yosemite Valley—Gin Flat (GIN), Crane Flat 

(CFL), Ostrander (STR), and Mariposa Grove (MPG)—and 
14 Historical Rock Falls in Yosemite National Park, California (1857–2011)

Figure 7. Example format of 

the rock fall database narrative 

document. The narratives are 

accompanied by only the key 

information from the organizational 

categories included in the database.

one station is located in Yosemite Valley proper (YYV). Data 

presented for the floor of Yosemite Valley (1,215 m elevation) 

are only available since December 1998 when the YYV station 

started operating.

We calculated minimum, maximum, and average 

temperatures for the 24 hours prior to the rock fall and 

assigned the temperature at the time of the rock fall for two 

locations, the floor and rim of Yosemite Valley. We include 

both locations to highlight the winter temperature inversion 

and summer “oven” effect that produces lower and higher 

temperatures, respectively, on the floor of Yosemite Valley 

than might otherwise be expected. Temperature statistics for 

the rim of Yosemite Valley (mean elevation 2,377 m) are 

elevation-corrected averages of available data from the four 

stations surrounding Yosemite Valley (GIN, CFL, STR, and 

MPG). Temperature is corrected for elevation by assuming 

an ambient temperature gradient of -0.65 °C per 100 m 

of elevation (Lundquist and Cayan, 2007). We assigned 

cumulative seasonal precipitation (defined as July 1 to June 

30) and preceding 30-day, preceding 72-hour, and preceding 

24-hour precipitation to each rock fall using data from the 

Yosemite Valley station (after 1998) and the Gin Flat station 

(for 1985–1998). It should be noted that not all of the rock 

falls were assigned an environmental condition as there are 

rock falls with unknown dates, and also time periods when the 

meteorological stations were not operational.

Summary

The inventory database presented herein provides detailed 

information on 925 rock falls and other slope movement 

events in Yosemite National Park spanning more than 150 

years. Since deglaciation occurred some 15,000 to 19,000 

years ago, rock falls have been the dominant geologic process 

contributing to landscape erosion; this is particularly true for 

the glacially steepened cliffs of Yosemite Valley. Rock falls 

and related events can sometimes pose a threat to the visitors, 

employees, and residents of Yosemite National Park. In 

addition to providing pertinent information for use in scientific 

rock fall studies, the inventory database serves as a reminder 

of the common occurrence of rock falls in steep landscapes. 

The database therefore not only provides valuable information 

for understanding rock falls in Yosemite National Park, and for 

mitigating the resulting rock-fall hazard and risk, but can also 

improve understanding of rock-fall processes worldwide.

Acknowledgments

We are indebted to the numerous park employees, 

residents, and visitors who provided eyewitness accounts, 

photographs, and descriptions of rock fall and other slope 
References Cited 15

movement events included in this inventory. We thank Butch 

Farabee, Jr., for providing new information on fatal incidents 

involving rock falls. Robert Sas, Jr., and Heather Roger helped 

to compile data for 2006 to 2009, and were supported by the 

Geological Society of America’s GeoCorps America Program. 

Kenneth Bodhi (USGS) developed macros for transferring the 

narrative data to the supplemental report format. Chuck Powell 

(USGS) reviewed the geologic names referenced in the text and 

database. Finally, Mark Reid (USGS) and Paul Santi (Colorado 

School of Mines) provided helpful reviews of this report. 

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Menlo Park Publishing Service Center, California

Manuscript approved for publication May 1, 2013

Edited by Claire M. Landowski

Design and layout by Jeanne S. DiLeo
Stock and others—Historical Rock Falls in Yosemite National Park, California (1857–2011)—Data Series 746