The Alaska }Earthquake 

March 27, 1964 

Regional Effects 

rosion and Deposition on a 

Raised Beach, . Montague Island 

G E 0 L 0 G I CAL SURVEY P R 0 FE S S I 0 N A L PAPER 5 43 - H 

3 1819 00086457 5 

THE ALASKA EARTHQUAKE, MARCH 27, 1964: 

REGIONAL EFFECTS 

Erosion and Deposition on a 

Beach Raised by 

The 1964 Earthquake 

Montague Island, Alaska 

By M. J. KIRKBY and ANNE V. KIRKBY 

. A quantitative study of geomorphic 

modifications of uplifted coastal features 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 543-H 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1969 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402- Price 60 rents (paper cover) 
THE 

ALASKA EARTHQUAKE 

SERIES 

The U.S. Geological Survey is publishing theresults of investigations of the Alaska earthquake of 

March 27, 1964, in a series of six professional papers. 

Professional Paper 543 describes the regional effects 

of the earthquake. Other professional papers describe 

the history of the field investigations and reconstruction effort; the effects of the earthquake on communities; the effects on the hydrologic regimen; and 

the effects on transportation, communications, and 

utilities. 

CONTENTS 

Abstract- - ------------------ - 

Introduction-- - - - - - - - - - - - - - - - - Physical setting ------------- Methods------------------- 

Geology---------------------- 

Bedrock-------------------- 

Recent unconsolidated deposits 

in MacLeod Haxbor--- .- -

Sand-- - -- - - - - - - - - - - - - - - - - 

Silt---------------------- 

Gravel- - - - - - - - - - - - - __- - - - - - 

Beach deposits in Patton 

Erosion and deposition of bayhead deposits in MacLeod 

Harbor ---- - - - - - - - - - - - - - - - Formation of bay-head deposits- - - - - - - - - - - - - - - - - - - - 

Stream erosion -------- - - -- -- Data collected- - ---------- 

page 

H 1 

2 

2 

4 

4 

4 

Erosion and deposit,ion of bayhead deposits in MacLeod 

Harbor-Continued 

Stream erosion-Continued 

Influence of underlying ma 

terial on stream erosion- - 

Streams in silt- ----------- 

Streams in sand --------- -- 

Stream in gravel- - -------- 

Lateral and vertical erosionModel of erosion- --------- 

Recession of knickpoints- - - 

Other erosive agents in regrading of deposits -_-- ---- ---- - 

Erosion and deposition on uplifted beaches and rock plat+ 

forms ----------------- --- 

MacLeod Harbor beaches- - - - Beach profiles--- - - - - - - - - - - 

ILLUSTRATIONS 

FIGURES 

Page 

Erosion and deposition on uplifted beaches and rock platforms-Continued 

MacLeod Harbor beachesContinued 

Regrading of the beaches-- - 

Patton Bay beaches- - ------- 

Stratigraphic evidence for 

sea-level changes-- - ---- - 

Elevations of beach featuresTectonic deformation of 

beaches- - - - - - - - - - - - - - - - Obliteration of break in 

slope at top of beach- - - - Regrading of rock platformsRegrading of sand and 

gravel-- - - - - - - - - - - - - - - - - 

Conclusions ---- - - - - - - - - - - - - - - - 

1. Index map of Montague Island------_--------------------------------------------- 

2. Map of bay-head deposits in MacLeod Harbor- ................................... -- 

3. Panoramic photograph of eroded bay-head deposits in MacLeod Harbor--- - - - - - - - - - - - - - 

4. Diagram showing relationship of volumes removed by fluvial erosion to elapsed time----- 

5. Graph of variations in erosion rates of streams in MacLeod Harbor.. - - - - - - - - - - - - - - - - - - - 

6. Photograph of typical stream with silt banks in MacLeod Harbor-- ------------------- 

7. Map, long profile, and cross sections of typical stream in silt in bay-head deposits of 

MacLeod Harbor-- - -- - -- - - - ------- - -- - - -- - --- - -- ---- - - - - --- ----- --- - - -- --- - - - - 

8. Planetable map, long profile, and cross sections of stream in sand, MacLeod Harbor- - - - - 9. Cross profile of gravel sheet and lateral migration of streams, MacLeod Harbor- - - - - - - - - 

10. Graph showing inferred rates of lateral and of vertical erosion for streams, MacLeod 

Harbor--------------.--------------------------------------------------------- 

11. Graph showing postearthquake degradation of silt, by rainwash and marine erosion, along 

a line of pilings .--------_- -- - - --- ................................................ 

12. Sketch map of shoreline lithology and location of measured beach profiles---- - - - - - - - - - - - 

13. Graph of beach profiles around MacLeod Harbor .................................... 

14. Graph of beach profiles and stream profile through beach ............................. 

15. Photograph of rock platform showing differential erosion- ............................ 

Page 

H3 

6 

9 

9 

10 

11 

Page 
CONTENTS 

16. Photograph of rock platform, MacLeod Harbor- .................................... 

17. Map of Patton Bay showing coastal features and deformation ......................... 

18, 19. Stratigraphic sections and beach profiles, Patton Bay - --_--------------- ---------- 

20. Photograph showing cobbles of raised beach on truncated bedrock platform, Patton Bay-- 

21. Stratigraphic section showing old raised beach, Patton Bay .-_------------------------------ 

22. Stratigraphic sections and profiles of Nellie Martin River- - - ..---..--------------------- 

23. Graph showing distribution of elevations of beach features- -- - - - --_ - - - _ - --- - -- - _- - ---- 

24. Sketch of beach profiles in Patton Bay in relation to the preearthquake mean high water25. Graph showing variation in differences of elevation among coastal features in Patton Bay26. Sketch map of differences in elevation of raised beaches at the north end of Main Sandy 

Bay-------------------------------------------------------------------------- 

27. Sketch map and graph of differences in elevation of raised beaches at the south end of 

Main Sandy Bay-------------------------------------------------------------- 

28. Sketch map of cliff degradation and accumulation of talus, Patton Bay ---- _------------ 29. Sketch map of cliff degradation by landslides and gullying in North Sandy Bay - - - - - - - - - 30. Idealized profile through two raised beaches- - - ...................................... 31. Graph showing sequence of platform cutting in Patton Bay ............................. 

32. Graph showing relationship between change of eleration of sea level and cross-sectional area33. Graph showing position of preearthquake storm beaches-- - .. - _- -- -- -- - ------ -------- - - 

TABLES 

- 

Page 

1. Summary of data for streams in MacLeod Harbor --------------_ ..................... H10 

2. Relationship of width of platform regraded, cross-sectional area of material removed, and 

change of relative sea level- -- .................................................. 37 
THE ALASKA EARTHQUAKE, MARCH 27, 1964: REGIONAL EFFECTS 

EROSION AND DEPOSITION ON A BEACH RAISED BY THE 1964 

EARTHQUAKE,MONTAGUEISLAND,ALASKA 

During the 1964 Alaska earthquake, 

tectonic deformation uplifted the southern end of Montague Island as much 

as 33 feet or more. The uplifted shoreline is rapidly being modified by subaerial and marine processes. The new 

raised beach is formed in bedrock, sand, 

gravel, and deltaic bay-head deposits, 

and the effect of each erosional process 

was merumred in each material. Fieldwork was concentrated in two areasMacLeod Harbor on the northwest side 

and Patton Bay on the southeast side 

of Montague Island. In the unconsolidated deltaic deposits of MacLeod Harbor, 97 percent of the erosion up to June 

1965, 15 months after the earthquake, 

was fluvial, 2.2 percent was by rainwash, and only 0.8 percent was marine; 

52 percent of the total available raised· 

beach material had already been removed. The volume removed by stream 

erosion was proportional to low-flow discharge raised to the power of 0.75 to 

0.95, and this volume increased as the 

bed material became finer. Stream response to the relative fall in base level 

l Formerly Research Associate, Dept. of 

Geography, The Johns Hopkins University 

and now LPcturer in Geography, University of BJ"istol, England, 

• Graduate student, Dept. of Geography, 

The Johns Hopkins University, Baltimore, 

Md. 

By M. J. Kirkby 1 and Anne V. Kirkby 2 

ABSTRACT 

was very rapid, most of the downcutting in unconsolidated materials occurring within 48 hours of the uplift for 

streams with low flows greater than 10 

cubic feet per second. Since then, erosion by these streams has been predominantly lateral. Streams with lower discharges, in unconsolidated materials, 

still had knickpoints after 15 months. 

No response to uplift could be detected 

in stream courses above the former preearthquake sea level. 

'Vhere the raised beach is in bedrock, 

it is being destroyed principally by 

marine action but at such a low rate 

that no appreciable erosion of bedrock 

was found 15 months after the earthquake. A dated rock platform vaised 

earlier has eroded at a mean rate of 

0.49 foot per year. In this area the factor limiting the rate of erosion was rock 

resistance rather than the transporting 

eapacity of the waves. 

The break in slope between the top of 

the raised beach and the former seacliff 

is being obliterated by debris which is 

accumulating at the base of the cliffs 

and which is no longer being removed 

by the sea. Current cliff retreat by rockfall, mudflows, and landslides was estimated at 0.7 to 2.0 feet per year, and in 

parts of Patton Bay the accumulation of 

debris has obliterated 78 percent of the 

original break in slope in 15 months. 

Evidence of two relative sea-level 

changes before 1964 was found in Patton Bay. At a high stand of sea level 

lasting until about 2000 B.P. (before 

present), an older raised beach was 

formed which, over a distance of 5 miles, 

shows 40 feet of deformation relative 

to the present sea level. Peat deposits 

exposed by the 1964 uplift also record 

a low sea level that lasted until at least 

600 B.P. 

The 1964 raised beach was used to 

test the accuracy of identification of 

former sea-level elevations from raised 

beach features. The P·re-1964 sea level 

could be accurately determined from 

the height of the former barnacle line, 

so an independent check ·on high-water 

level was available. The most reliable 

topographic indicator was the elevation of the break in slope at the top of 

a beach between a bedrock platform -and 

a cliff. Even here, the former sea level 

could only be identified within 5 feet. 

The breaks in slope at the top of gravel 

beaches were found to be poor indicators 

of former sea level. 

On Montague Island, evidence of 

former high sea levels appeared to be 

best preserved ( 1) as raised bedrock 

platforms on rocks of moderate resistance in slightly sheltered locations and 

(2) as raised storm beaches where the 

relief immediately inland was very low. 

H1 
H2 

On March 27, 1964, arr earthquake shook parts of Alaska and 

caused great devastation in inhabited areas. However, maximum uplift took pla;ce, and large bedrock 

faults were active, on Montague 

Island, an uninhabited island off 

the southern coast of Alaska. 

The southern end of Montague 

Island was uplifted as much as 33 

feet or more during the earthqua,ke (fig. 1), and two active 

faults several miles long caused 

many landslides and broke or displaced many hundreds of trees 

(Plafker, 1967). Around the coast, 

marine deposits and a marineabraded bedrock platform were 

lifted above sea level and exposed 

to subaerial processes. 

The uplift associated with the 

earthquake has provided physiographic conditions on a scale much 

larger than could be simulated in a 

laboratory and with the added advantage that the features and 

changes were natural. The bayhead deposits of MacLeod Harbor 

(figs. 1, 2) on the northwest coast 

of the island provided an undissected surface, about 1 square mile 

in area and with a slope of only 20', 

which had been suddenly uplifted 

33 feet. On this surface, consequent drainage channels were init.iated and were rapidly eroded 

vertically and laterally; thus, a 

unique opportuni,ty was provided 

to study the effects of rapid uplift 

on the mode and rate of fluvial 

processes and the resultant valley 

forms and long profiles. 

The earthquake also brought to 

view a bedrock platform and associated deposits, which allowed 

observation and measurement of 

features that are usually under 

water and therefore difficult to 

study. 

ALASKA EARTHQUAKE, MARCH 27, 19'64 

INTRODUCTION 

On both sides of the island the 

amount of subaerial modification 

of the 1964 raised beach could be 

measured. Especially important 

was the opportunity to study the 

area only 15 months after the 

earthquake. In the MacLeod Harbor area, rivers in soft sediments 

have adjusted so swiftly that evidence of the postearthquake history of the area will soon be removed by erosion. Subaerial 

degradation of the raised beaches 

and abandoned seadiffs is so rapid 

that within a few years little of the 

original marine form will be left 

unchanged, and the growth of vegetation, which has already begun 

on the beach, will hasten its 

obliteration. 

Fieldwork was done in 1965 at 

two locations, MacLeod Harbor 

and Patton Bay (fig. 1). The project was a reconnaissance and, although the study was concerned 

with postuplift changes in raised 

beaches, differences in emphasis result from physiographic and tectonic differences between the two 

bays. The work was supported by 

a grant from the National Science 

Foundation, administered by The 

Johns Hopkins University. We 

would like to thank Professor M. 

G. Wolman of The Isaiah Bowman Department of Geography, 

The Johns Hopkins University, 

for his continuous scientific and 

administrative help; and A. T. 

Hohl, Department of Geology, 

Princeton University, and Thomas 

Dunne, Department of Geography, The Johns Hopkins University, for their assistance in 

carrying out the fieldwork. Mr. C. 

LaBounty, a longtime seasonal 

resident of Montague Island, provided valuable data on pre- and 

postearthquake conditions in the 

bay-head deposits. 

PHYSICAL SETTING 

Montague Island, one of the 

outermost islands of Prince William Sound, lies off the southern 

coast of Alaska at lat 147.5° W. 

and long 60° N. It is about 50 miles 

long, and has a maximum width of 

15 miles and a minimum width 

of less than 5 miles. The island 

is rugged and mountainous, and 

a chain of peaks forms its backbone at an average altitude of 

about 2,500 feet. Cliffs which 

occur almost all around the island 

make the coast dangerous. The 

1964 earthquake elevated and exposed a flat bedrock platform 

which encircles the island; the 

former marine cliffs, now beyond 

the reach of the sea, were broken 

by a few inlets and sandy beaches 

that allowed access to the island. 

Travel across Montague Island 

is difficult. It is uninhabited except 

for seasonal hunters and loggers; 

there are neither roads nor even 

paths, except those made by bears. 

The upper parts of the mountains 

are rocky and are snow-covered 

all year. Below the peaks the land 

is either steep and covered with 

alders or stunted conifers, or the 

slope becomes so gentle that waterlogging prevents tree growth, and 

patches of wet peat interspersed 

with pools of wa,ter form muskeg. 

At altitudes below about 1,000 

feet, the coniferous forest forms a 

dense cover down to sea level 

Only where the slope is gentle 

enough for muskeg to form or 

where it is so steep that it is totally 

unstable is there any break in the 

forest. Most of the island has not 

been logged and is covered with 

apparently virgin forest containing conifers more than 6 feet in 
EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H3 

I 

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14JD w 

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/ EXPLANATION 

UPLIFT. IN FEET 

.. 

>30 

20-30 

CJ 

10-20 

D 

< 10 

' ' ' 

--------~u~------- Fault active in 1964 earthquake 

Dashed where probable. U, 

upthrown side 

5 10 MILES 

1.-Index map of Montague Island showing amount of uplift (after Plafker and Mayo, 1965). A., area of figure 12; B, area of 

figure 17. 

280-547 0 - 69 - 2 
H4 

diameter and over 400 years old. 

The trees are as much as 200 feet 

high, and there is a thick, usually 

thorny, undergrowth. The forest 

floor is covered with shallow ponds 

and bogs ; large fallen tree trunks, 

many of them rotten, lie across one 

another in a chaotic pattern and 

provide unstable footing. 

Even on the postearthquake 

raised beaches, plant growth is 

rapidly establishing itself; only 15 

months after the earthquake, conifer seedlings were growing on 

surfaces that were formerly below 

sea level ( G D. Hanna, 1965, oral 

commun.). 

Most of the rivers on Montague 

Island are narrow, steep torrents 

that rush down the slopes in a series of rapids and waterfalls. Only 

the Nellie Martin River has 

achieved any substantial floodplain development, and it flows 

through a wide valley, whose floor 

near the coast is covered with thick 

peat deposits that form vertical 

riverbanks as much as 10 feet high. 

METHODS 

Mapping was done with a planetable and telescopic alidade. Beach 

and stream profiles were surveyed 

BEDROCK 

Little is known about the bedrock of Montague Island; most exposures are covered by forest and 

muskeg or, at higher elevations, by 

scree and snow. The recent uplift 

has improved accessibility to the 

coast and has brought nearly continuous exposures of bedrock into 

ALASKA EARTHQUAKE, MARCH 27, 1964 

with a hand level or Abney clinometer, and distances were measured by pacing. Preearthquake 

mean high water was assumed to 

be at the top of the abandoned barnacle line. (Plafker, 1965, p. 1-2). 

The level of postearthquake mean 

high water was obtained from the 

U.S. Coast and Geodetic Survey 

predicted tide tables. These 

agreed, within 1 foot, with the 

position of the new barnacle line 

where it was observed. 

Grain sizes of the MacLeod 

Harbor bay-head deposits were determined by counts (Wolman, 

1954) of 100 pebbles in stream 

beds and by sieve analyses of fine 

materials. On beach profiles we 

measured the size of the largest 

stones in a section and estimated 

visually the percentage of material 

finer than 1 inch in diameter. Permeabilities were determined approximately by measuring the 

time for 1 inch of water to infiltrate; the water was contained by 

a metal ring. 

Discharge of streams in McLeod 

Harbor was calculated as 

Surface velocity in center of 

stream X mean depth X width. 

This calculation gives values approximately twice the true value, 

because the centerline surface veGEOLOGY 

view along the shore. These strata 

are thought to be of Tertiary age 

(Plafker and MacNeil, 1966, p. 

66-67). 

A study of the coastal exposures 

indicates that the bedrock of the 

island consists of a sequence of interbedded thin layers of black argillite, thick beds of graywackelike 

sandstone, and occasional beds of 

locity is approximately twice the 

mean velocity of the whole cross 

section. Discharges were measured 

at ·a time of relatively low flow 

(see table 1, p. H10) but were corrected for daily variations by 

multiplying by a standardizing 

faotor: 

Discharge of stream 1 on June 

20, 1965 

Discharge of stream 1 on day 

of measurement 

To avoid confusion, the expression "1964 sea level"c has not been 

used in this paper. Instead, the 

expressions "preearthquake sea 

level" and "postearthquake sea 

level" are used. A sea level higher 

than the post-earthquake sea level 

is called a raised sea level, and a 

sea level lower than the preearthquake sea level is called a low sea 

level. However, inasmuch as the 

earthquake took place in 1964, it 

is correct, and gives rise to no ambiguity, to refer to the uplifted 

beach exposed by the earthquake 

as the 1964 raised beach. The 

changes in relative sea level are 

probably caused more by movement of the land than by absolute 

change of sea level, but in the absence of positive evidence it is customary (Sparks, 1960, p. 211) to 

refer to the land as fixed in relation to sea-level changes. 

conglomerate and volcanic rocks. 

The sedimentary rocks are uniformly bedded and have flat parallel bedding planes. The coarser 

clastic rocks are of two types : ( 1) 

conglomerate having coarse rounded quartz and rock-fragment clasts 

set in a gray sandstone matrix and 

(2) breccia of black angular argillite chips, also in a sandstone 
EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H5 

matrix. The volcanic rocks were 

not seen in an outcrop, but hillside 

boulders and beach cobbles indicate that they consist of buff and 

green medium- to fine-grained agglomerate. In the Patton Bay 

region, light-gray volcanic interbeds occur. 

Two major faults, both active in 

the 1964 earthquake, have been 

mapped on the island (Plafker, 

1967, fig. 2). The faults trend 

roughly parallel to the island axis 

and have dips of 50°-85° NW. The 

Hanning Bay fault is well exposed 

for about 4 miles between Hanning Bay and MacLeod Harbor; it 

has a maximum vertical displacement of 13 feet in bedrock and 17 

feet in unconsolidated beach material (Plafker, 1967). Inland, the 

fault can be traced by landslides, 

fallen trees, and earth fissures several feet deep and 1 foot wide. 

The Patton Bay fault can be 

traced for about 22 miles, but for 

most of that distance it is concealed beneath landslide debris, its 

maximum measured vertical displacement is 20 to 23 feet (Plafker, 

1967). This fault follows and accentuates a preexisting topographic break in slope. Near the 

Nellie Martin River, the Patton 

Bay faultline is characterized by 

a linear scarp covered with debris, 

including fallen trees, and modified by numerous landslides. The 

faultline scarp varies in height 

from 100 to 300 feet and has an 

average gradient of 40° made up 

of a series of irregular steps. At 

the top of the debris slope a clean 

exposure of rock 5 to 20 feet high 

slopes at an angle of 65° to 75°. 

There is no evidence of mineralization and no gouge. 

Surficial deposits comprise recently uplifted shallow-water marine silt and sand, coarse beach 

sand and gravel, and fluvial gravel. 

Extensive peat and some glacial 

deposits are also present. 

RECENT UNCONSOLIDATED 

DEPOSITS IN MAcLEOD 

HARBOR 

At the head of MacLeod Harbor, uplift has exposed unconsolidated sediments which formerly 

occupied ·beach, intertidal, and 

subtidal zones. Three main sediment types are recognized : sand, 

silt, and gravel. The areal distribution of these types is shown in figure 2. The deposits occupy a 

roughly rectangular basin that 

deepens seaward; they themselves 

also thicken sea ward, reaching a 

maximum exposed thickness of 18 

feet, but their total thickness is 

probably much greater. 

The sand occurs mainly on the 

south side of the bay and has a 

gradational contact with the silt. 

The silt occupies an area which 

was formerly occupied by quiet 

water in the deepest part of the bay 

head. The upper surfaces of the 

silt and sand were formerly partly 

intertidal and partly below spring 

low water level. 

SAND 

The sand of MacLeod Harbor is 

dark gray and uniform throughout the deposit. It stands in vertical cliffs as much as 20 feet high; 

for the most part it is well bedded, 

the thickness of individual beds 

ranging from 6 inches to 2 feet. 

The beds are continuous for a minimum of 200 feet and show the internal crossbedding that is characteristic of a mainly fluvial sediment. 

Scour channels and filled channels about 4 feet wide are common. 

Convolute bedding occurs at all 

levels in the sand, and some convolutions are trunca:ted by overlying 

uncontorted beds as much as 12 

feet below the present surface. The 

cause of the convolutions is not 

known, but the possibility that 

they were formed by thixotropic 

transformation p r o d u c e d by 

earthquakes prior to the 1964 

earthquake best fits the known 

facts. 

Occasional well-rounded pebbles 

as much as 3 inches in diameter are 

found within the sand. Throughout the deposit there are discontinuous stringers up to 2 inches 

thick of small wood pieces, shells, 

seaweed, and pine needles, similar 

to present high-tide swash accumulations. These stringers probably 

indicate forme.r positions of high 

tide. The permeability of the unsaturated sand is about 25 inches 

per hour. 

SILT 

The silt is black and uniform in 

composition, except for the percentage of sand present, which increases to·ward the contact with the 

sand. The silt shows an apparent 

lack of bedding and internal structure, although occasionally thin 

parallel bedding was suggested by 

differential erosion along streambeds. Clam shells (mostly Macona 

nasuta) that are found throughout the deposit are concentrated 

through erosion on the silt surface 

as a lag deposit. The permeability 

of the silt is about 0.01 inch per 

hour. 

GRAVEL 

Gravel deposits occur in three 

forms-as beaches around the inland edge of MacLeod Harbor, as 

fans around the former mouths of 

streams entering the bay, and as 

lenses and discontinuous sheets at 

or near present sea level. 

The beach gravel consists mainly 

of local rock types and occasional 

exotic pebbles of granodiorite, 

granite, or hornblende gneiss. The 

gravel is very well rounded and 

well sorted and has only minor 

amounts of sand matrix. The mean 

size of the pebbles varies widely on 

the beaches around the bay; on 

some beaches more than 80 percent 
H6 ALASKA EARTHQUAlKE, MARCH 2 7, 1 9'6 4 

N 

Cam psi 

---.po,ste,anthquake MHW 

(Approximate) 

EXPLANATION 

~~~~.::~ D 

Gravel Sand Silt 

Contact 

-------15------ Contours of land surface after postearthquake downcutting, in feet above postearthquake MHW 

Long-dashed where extrapolated 

--------20---- .... ····· 

Contours of preearthquake land surface, 

in feet above postearthquake MHW 

Dotted where extrapolated 

Eroded 

------------

margin of preearthquake land 

surface 

''''''''I I I I I I I I 

Preearthquake storm beach 

Main stream 

1!11 

Stream number 

X SECiiON 

Locality discussed in text 0 500 

2.-Bay-head deposits in MacLeod Harbor and June 1965 elevations af their upper (preearthquake) 

and lower (postearthquake) surfaces. 

1000 FEET 
E:ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H7 

are smaller than an inch in diameter, but on other beaches 80 percent are larger than 4 inches in 

diameter. On any individual beach 

the size range is much less, although generally pebble size decreases from the storm beach toward the sea, and sorting becomes 

better in the same direction. Permeabilities are about 450 inches 

per hour. 

Behind the gravel beach around 

most of the bay is a storm beach 

which is characterized by little or 

no sand matrix, poorer sorting, 

coarser grain sizes, steeper gradients on the upper surface both 

landward and seaward, and extrr,mr,lv high permeabilities. 

The river fan gravel deposits 

also consist of local rock types and 

vary in mechanical composition 

from their apexes in gaps through 

the storm beach to their sea ward 

edges. At their apexes they are 

similar in composition to the 

beach gravel. The percentage of 

sand matrix increases and the size 

of the pebbles decreases toward 

the seaward edges of the fans. 

Near the lower part of the fans 

(section 1, fig. 2) the deposit is of 

interbedded gravel and sand, the 

gravel beds ranging in thickness 

from one pebble (% inch) to 12 

inches. Individual pebbles vary 

from ¥s to 5 inches in diameter; 

many are disk shaped and most are 

well rounded but have low sphericity. The gray-sand matrix 

makes up 40 percent of an individual gravel bed; wood fragments 

and abraded logs are common. No 

graded bedding or cross bedding 

was observed. The upper and lower contacts with adjacent sand 

beds are uneven, and the gravel 

lenses out, the thinnest beds having 

the least lateral continuity. The 

sand beds are discontinuous and 

from 1 to 10 inches thick; planar 

crossbeds are present in a few 

beds. The total section at this location (fig. 2) consists of 60 to 70 

percent gravel beds and 30 to 40 

percent sand beds, not counting 

sand matrix in the gravel beds. 

The gravel lenses and discontinuous sheets occur only at or below 

1965 mean sea level. They are 

found beneath the sand and silt 

deposits and have been partly 

eroded and redistributed by postearthquake strPam action. They 

form a more or less continuous 

sheet where the streams have been 

eroded down to 1965 sea level. 

Near the present high-water level 

where the streams flow into the 

sea, gravel lenses occur beneath 

the streambeds. These lenses are 

as much as 3 feet thick, and in 

places a sand bed lies between 

them and the gravel sheet on the 

present surface. The lower gravel 

lenses have a sand matrix and a 

maximum pebble size smaller than 

that of the present stream gravel, 

BEACH DEPOSITS IN 

.PATTON BAY 

Bay-head deposits are not present in Patton Bay. Instead, uplift 

has merely widened the existing 

beach deposits or bedrock platforms. The distribution of the 

coastal forms is discussed on page 

H20 in relation to individual beach 

profiles. 

The coastline around Patton 

Bay consists mainly either ( 1) of 

wide gently sloping sand beaches 

with gravel storm beaches surmounted by large driftwood accumulations at their inland edges 

or (2) of cliff-backed bedrock 

platforms with or without narrow 

sand or gravel beaches and driftwood at their inward margins. 

Between zones of these two main 

coastal types, small areas of large 

boulders lie on bedrock. 

EROSION AND DEPOSITION OF BAY -HEAD DEPOSITS IN MACLEOD HARBOR 

FORMATION OF BAY-HEAD 

DEPOSITS 

Both the surface composition 

and the topography of the preearthquake deposits at the head of 

MacLeod Harbor (fig. 2) seem 

consistent with conditions of normal intertidal and shallow-water 

deposition by rivers at the head of 

an inlet. At their seaward limit 

the deposits seem to have formed a 

steeper slope. The evidence for 

this statement, although indirect, 

is fairly conclusive: (1) the U.S. 

Geological Survey 1 :63,360 topographic map and the U.S. Coast 

and Geodetic Survey charts show 

depths close inshore that indicate 

a steep offshore slope, and ( 2) before 'the earthquake local fishermen were able to anchor close inshore in areas that are still close to 

the seaward edge of the silt. A 

steeper slope, therefore, very probably existed rut the seaward edge 

of the deposits, although whether 

its form was erosional or depositional is unknown. It is also considered probable that this slope 

was partly exposed by the uplift 

associated with the earthquake, because major streams very rapidly 

trenched the newly uplifted de-
H8 

posits (C. LaBounty, oral commun., 1965) ; such trenching would 

be favored by the exposure o£ a 

steep slope by uplift above sea 

level. 

Figure 2 shows elevations o£ the 

bay-head deposits in June 1965. 

Surveyed contours o£ the remaining parts of the upper surface (in 

interfluve areas) have been interpolated in stream-eroded areas to 

reconstruct the preearthquake 

topography o£ the deposits. Surveyed contours o£ areas o£ postearthquake stream erosion have 

similarly been interpolated to estimate the final postearthquake topography o£ the bay-head deposits. The upper surface could still 

be fairly well defined in June 

1965, but the former seaward limit 

o£ the deposi,ts is less definite. 

Four independent levels £rom interpolated 1965 mean high water 

to the preearthquake barnacle line 

give a mean measurement o£ 33 

feet £or the uplift. The mean tidal range in MacLeod Harbor is 

9.2 feet, and spring low tides were 

formerly 19 feet above the 1965 

mean high water. The lowest elevations, which are still preserved 

on remnants o£ the preupli£t surface, are 13 feet above the postearthquake mean high water on 

sand and 9 feet 'U:bove on silt. 

Thus, the top o£ the seaward steep 

slope apparently was at least 10 

feet below spring low water 

( 1963) on the silt and at least 6 

feet below on the sand. The top 

o£ the steep slope may be close to 

the approximate position o£ the 

+10-foot contour o£ the upper 

surface shown in figure 2. 

The presence o£ a gravel sheet 

or sheets, both below the 1965 

stream courses and in the outliers 

o£ silt and sand, suggests that a 

former stream system similar to 

the one which is now developing 

produced these gravel sheets. Since 

ALASKA EARTHQUAKE, MARCH 27, 1964 

the gravel sheets could be traced 

down to the 1965 mean sea level, 

they very probably indicate a former relative sea level that was at 

least as low as the 1965 one and 

that predates the finer deposits. 

Cores taken in the present intertidal zone (Barrett, 1966, p. 997) 

show a series o£ pebbly layers extending down as low as 1 foot 

above mean lower low water. These 

sections seem to show deposition o£ 

25 to 30 feet o£ sand in and close 

to the intertidal zone, and imply 

a fairly steady rise o£ relative sea 

level before the earthquake. A similar steady rise has been reported 

for other parts o£ Prince William 

Sound (Plafker and Rubin, 1967). 

Some indication o£ the rate of 

rise o£ relative sea level has been 

obtained by the dating o£ two 

wood samples. C-14 sample W1768, from 4 feet above postearthquake mean high water (section 2, 

fig. 2), was interbedded with sand 

that probably was deposited above 

extreme low water level (14 feet 

below the 1965 mean high water) . 

The radiocarbon age o£ this sample 

is 820+200 B.P. C-14 sample W1764, from a tree in the position o£ 

growth approximately at the level 

o£ the preearthquake barnacle line, 

has a radiocarbon age of 380±200 

B.P. and indicates that mean high 

water was then at least 5 feet lower 

than the preearthquake mean high 

water. Rates o£ £all of relative sea 

level calculated £rom samples W1768 and W-1764 are 0.033±0.008 

foot per year and more than 

0.014+0.013, respectively. 

STREAM EROSION 

Bay-head silt, sand, and gravel 

were studied only at the head of 

MacLeod Harbor. Since the uplift 

o£ about 33 feet in March 1964, the 

area of intertidal and shallowwater deposits has been considerably dissected by rivers, few of 

which rise within the area that was 

formerly below sea level; the 

coastal perimeter of the uplifted 

deposits has been trimmed by wave 

action. By July 1965, about 50 percent of the available deposits had 

been regraded to a new level, accordant with the present sea level. 

Most of the regrading has been 

done by lateral erosion of the 

largest rivers (fig. 3). 

Long profiles were made of 20 

streams (numbered 1-17 and 20-

22, table 1), particular attention 

being paid to the position of knickpoints. At least three cross sections 

were made of each stream in order 

to estimate the total volume removed from each by erosion. A 

grain-size analysis was made of 

samples from near the mouth of 

each stream, and a relative value 

of low-flow discharge was calculated. Additional information 

about the earlier course of erosion 

was obtained from U.S. Coast and 

Geodetic Survey aerial photographs taken 8 weeks and 21 weeks 

after the earthquake. 

Interpretation of the information collected has been in terms of 

the overall course of erosion by the 

rivers, including the recession of 

knickpoints. This part of the interpretation is essentially a study 

of fluvial processes in an unusual 

environment. The data have also 

been studied in relation to the 

modifica:tion of a raised platform. 

The rapidity with which such a 

raised platform is altered, and the 

agents responsible for it, are discussed on pages H15-H16. 

DATA COLLECTED 

The banks of streams 1-9, 15, 

and 16 (table 1) are silt; the banks 

of streams 10-14 and 20-22 are 

sand; and the banks of stream 17 

are gravel. Bed material is not infrequently coarser than bank materia 1. Low-flow discharges 

ranged from zero for the smallest 
E-ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H9 

3.-Panorama of eroded bay-head deposits in MacLeod Harbor. 

streams in sand and 0.035 cfs ( cubic feet per second) for the smallest stream in silt (stream 6) to 301 

cfs (stream 10). The volume of 

material removed by all streams 

from the time of the earthquake 

to late June 1965 (fig. 4) was 

calculated from direct measurements. For the larger streams 

(streams 9, 10, 12, 21, and 22), estimates of volumes removed at 

earlier stages w e r e calculated 

from aerial photographs, which 

showed the width of erosion, and 

from planetable maps made in 

June 1965, which gave .the depth of 

erosion. For these large streams, 

errors arise mainly from incorrect 

interpolation of the initial elevation of the surface before stream 

erosion (fig. 2). This error in initial elevation is unlikely to be more 

than 1 foot near stream mouths 

where thicknesses removed are 10 

to 15 feet; it will be correspondingly smaller upstream where the 

rivers are narrower and thicknesses of material removed are 0 

to 5 feet. Measurements of widths 

of erosion, made from field measurements and from aerial photographs, are considered to be much 

more accurate than the depth 

measurements. For small streams, 

errors arise mainly from the intricacy of variation of width and 

bifurcation, but all major tributaries, were included in our calculations. An overall error of ± 10 

percent is considered a generous 

maximum allowance (95 percent 

200 ,-------,-------~-------,-------,,------- ------~-------, 

20 

EXPLANATION 

(187.000,000) 

Upper.confidence limit 

Estimated total available 

( 170,000,000 cu ft) 

(153,000,000) 

Lower confidence limit 

6 

Sand 

X 

Silt 

0 

Gravel 

Confidence limits 

TIME AFTER EARTHQUAKE, IN WEEKS 

4.-Relationship of volumes removed by fluvial erosion to time elapsed since the 

earthquake. 
HlO ALASKA EARTHQUAKE, MARCH 27, 1964 

TABLE !.-Summary of data for streams in MacLeod Harbor 

[*,no knickpoints apparent in streaml 

Stream number 

1 _____________ 

2-------------

3--------------

4 ____ ----------

5--------------

6--------------

1--------------

8 ____ ---------- g _____________ 

10 ____________ 

ll ____________ 

12 ____________ 13 ____________ 14 ____________ 

15 ____________ 

16 ____________ 

17------------ 20 ____________ 21 ____________ 22 ____________ 

TotaL _____ 

Distance of 

knickpoint 

from mouth 

(feet) 

76 

Many 

knick 

points. 

75 

66 

63 

255 

335 

1, 150 

0 

0 

* 0 

* * 50 

50 

110 

* 0 

0 

----------

Width at mouth 

(feet) 

25 

22 

15 

23 

18 

32 

20 

72 

190 

1, 625 

70 

665 

130 

30 

39 

490 

1, 450 

confidence). Streams appear to 

erode at a more or less uniform 

rate until they approach :the limits 

set by topography or by neighboring streams, at which time their 

rate of erosion becomes much less. 

Stream 10, for example, is continuing to erode at a uniform rate, 

whereas stream 22 appears to have 

neared its limit. The overall relationship of volume removed to 

time elapsed since the earthquake, 

for these streams and for all the 

streams combined, is shown in 

figure 4. 

Volumes removed by erosion up 

to June 1965 have been calculated 

by summing cross-sectional areas 

removed along the length of the 

stream. Figure 5 and table 1 show 

volumes removed, low-flow discharge, bed and bank material, and 

dther data. Each point in figure 5 

has been distinguished as to bank 

material, and streams flowing 

between silt banks have been further distinguished as to bed-material size on the basis of a pebble 

count. 

Relative 

low-flow 

discharge (cis) 

0. 37 

1. 73 

. 67 

. 051 

. 035 

. 35 

. 50 

5. 71 

73.0 

301. 0 

0 

80.8 

1. 10 

0 

. 0129 

. 0094 

1. 09 

2. 16 

9.30 

52.3 

100 

(/) 

z 

0 10 :::; 

..J 

~~--lJJ 

~ ~ 1.0 

ciu LJJ_ 

>m 

O::J 

~ u 0.1 

a:LJ.. 0 lJJ 

~ 

::J 0.01 ..J 

0 

> 

Crosssectional 

area at mouth 

(sq It) 

97 

180 

103 

128 

78 

198 

85 

480 

2, 600 

25, 200 

650 

9, 650 

1, 220 

120 

176 

6, 860 

21,700 

Total volume 

of material 

removed from 

the time of the 

earthquake to June 1965 (cu It) 

5, 025 

6, 160 

6, 250 

6, 300 

5,250 

41, 500 

22,400 

361,000 

3. 20X 106 

36. 7X 106 

164, 000 

18. ox 106 

710, 000 

19, 400 

----------

---------- 19, 600 

---------- 6. 04X 106 

29. ox 106 

94. 3X 106 

1 Zero discharge 

L 

Size of bed 

material 

D., (in.) 

2. l_ ____ 

1. 06 ____ 

2. 64 ____ Silt_ ____ ___ do ___ 

. 49 ____ 

. 74 ___ - 1. OL ___ . 96 ____ 1. 56 ____ 

Sand ____ 1. 27 ____ 

Sand ____ ___ do ___ 

--------

-------- 2. 9 _____ 

--------

--------

--------

--------

X 2.1 X 2.6 

Total volume of 

material removed 

(millions cu It) 

May 1964 Aug. 1964 (8 weeks) (21 weeks) 

0. 8L_ 2. 17--

3. 54.- 9. 50 __ 

3. 18.- 12. 8 __ 

. 92 2. 31 

5.20 24. 7 

13. 6 51. 5 

Type of bank 

material 

Silt _______ _ __ do _____ 

___ do _____ __ .do _____ ___ do _____ ___ do _____ 

----------

___ do _____ ___ do _____ 

Sand and 

silt. 

Sand_. _____ __ .do. ____ ___ do _____ ___ do _____ 

Silt _______ 

___ do _____ 

GraveL ___ Sand ______ ___ do _____ ___ do _____ 

----------

L 

Remarks 

Gravel bed. 

Do. 

Do. 

Data incomplete. 

Do. 

Gravel bed. 

Do. 

Sand-banked stream 

X 2.6 

Silt-banked stream 

0 2.9 

Gravel-banked stream 

(Stream 17) X 1.1 Figures for silt and gravel streams 

show median grain size of bed 

material, in inches 

0.001 c__ ____ _L _____ ...J..._ ______ .J__ ____ ........L _____ __j 

0.01 0.1 1.0 10 100 1000 

RELATIVE LOW-FLOW DISCHARGE, IN CUBIC FEET PER SECOND 

5.-Variations in erosion rate with low-flow discharge of streams in MacLeod 

Harbor. Arrow shows direction of increasingly coarse bed material. 

INFLUENCE OF UNDERLYING 

MATERIAL ON STREAM EROSION 

Three major characteristics of 

the underlying material apparently control stream erosion. 

The first is the permeability of 

the deposit, which determines the 

drainage area required to produce 

surface runoff. For example, surface flow commonly appears on the 

silt immediately below its upstream contact with a gravel bed, 

and results from the large difference o£ permeability. Differences 

in permeability probably also 

cause large differences in the ratios 

of high-flow to low-flow discharge. 

The second control is the variation in resistance to erosion. This 
EJROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND Hll 

control might be expected to result 

in marked differences in drainage 

densities and networks. A difference in networks is apparent 

(see fig. 7 for silt and fig. 8 for 

sand), but differences in drainage 

density are masked because many 

streams receive most of their water 

supply from streams outside the 

area of uplifted deposits. Their 

drainage density is therefore 

partly determined by the number 

and spacing of the feeder streams. 

The third control is the effect of 

bed armoring. Armoring is widespread in larger streams, mainly 

with gravel deposits that are 

eroded less readily than the bank 

materials. Extensive armoring is 

usually associated wi'th streams 

that have almost reached their 

limit of possible downcutting, so 

it is not immediately clear, from 

the data in this area, whether the 

slow rate of downcutting is a 

cause or an effect of the armoring. 

Differences in erosion rates of 

streams flowing in silt and in sand 

banks may be clearly distinguished 

in figure 5. The distinction seems 

to depend mainly on the differences in the ratio of high- to lowflow discharges, but the greater 

erodibility of sand may be a contributory cause. The silt channels 

can be distinguished by the size of 

the bed material except for stream 

2, in which the small volume of 

material removed is probably due 

to the fact that the stream's knickpoint had already reached gravel 

and it could not cut back farther. 

Note that the plot of the gravelbanked stream 17 falls with siltbanked streams with the same 

bed-material size. The data may 

indicate that armoring of the bed 

with coarser material tends to reduce the rate of erosion and that 

recession of the knickpoint is 

greatly slowed when it reaches the 

coarse gravel deposits which form 

the preearthquake storm beach. 

280-547 0 - 69 - 3 

6.-Typical stream with silt banks (stream 8, table 1), MacLeod Harbor. Note the 

conspicuous white clam shells on the silt surface. 

STREAMS IN SILT 

Initial stream courses m silt 

seem to have been consequent and 

to follow the directions of greatest 

slope, as exemplified by the 

streams that have not migrated 

laterally and that still follow the 

reconstructed consequent directions. The initial directions of 

larger streams, even though they 

have since moved laterally, also 

correspond to reconstructed consequent directions. Most of the 

streams in the silt are fed by runoff from the hillside along the 

north side of the harbor. 

A sharp division can be made 

between streams that can transport 

gravel effectively and those that 

cannot. Those that cannot carry 

gravel effectively are characterized 

by an abrupt ending of the channel at its upper end, at the lower 

margin of the gravel beach or at 

a gravel fan. The great difference 

in permeability between silt and 

gravel emphasizes this feature. In 

progressively larger streams, the 

bed material tends to change (provided suitable material is available) from silt to clam shells to 

gravel. The amount of coarse 

armoring material and the ability 

to move it thus increase together, 

although figure 5 shows that the 

armor has a net retarding effect 

on erosion. 

Small streams in silt (fig. 6) 

generally have steep banks, narrow 

valleys, and very marked knickpoints in their longitudinal profiles (fig. 7). Streambank gradients locally decline from the usual 

60°-70° to 10°-20°, where the silt 

is saturated. This decline is associated with extensive slumping 

and earthflows. The saturation is 

usually caused by unchanneled 

seepage from the edge of the 

gravel beach. Figures 6 and 7 illustrate typical silt valleys. 

STREAMS IN SAND 

The drainage areas of many 

smaller streams in sand (fig. 8) lie 

entirely within the bay-head sand 

area. These streams have no gravel 

or clam shells on their beds. 

Clearly defined c}1annels are small 

and are confined to the sea ward 

ends of the valleys. In the smallest 
H12 

A 

w 

:.: 

< & 30 

:X: 

Ill: 

;51-

tii t:j 20 -

0"-

0..z 

w-

> -

al ~ 10 

<:::!: 

z 

0 

i= 

< 0 > w 

_j 

w B 

350 300 

ALASKA EARTHQUAKE, MARCH 2 7, 1 9 6 4 

250 200 150 

DISTANCE. IN FEET 

EXPLANATION 

Upper silt surface 

Lower silt surface 

100 

• 

Knickpoint 

50 0 

~ 

D 

B 166 SQ ft 

A 198 SQ ft 

c 0 5 10FEET 

NO VERTICAL EXAGGERATION 

7.-Map (A.), long profile (B), and cross sections (C) of typical stream in silt (stream 6, table 1) in bay-head deposits of 

MacLeod Harbor. Letters on profile show position of cross sections. Numbers in cross sections refer to cross-sectional area 

eroded. 
EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND 

Upper surface section 

Valley bottom 

EXPLANATION 

Upper surface Damp sand 

Wet sand 

0 50 100FEET U 

Channel bank 

UPPER SURFACE 

Cross Cross Cross 

sectton sectlon sectlon 

B 

____---- 

---

posltlon f ZPZ surFC5 __/-- 

A_---- 

? -- 

- 

Do@" 

i 1 --y 0 

0 50 100FEEl 

--- Postearthquake MHW 

LONGITUDINAL PROFILE 

(Follow~ng t h e m o s t northerly tr~butary valley) 

------------------- Position of upper surface 

A\ Valley bottom 

20 

C gp~ppercrface 

Valley bottom 0 10 20 FEE1 

CROSS SECTIONS 

8.-Planetable map, long profile, and crws sections of a typical stream in sand (stream 13, table I), MacLeod Harbor. 
HI4 ALA,SKA EARTHQUAKE, MARCH 2 7, 1 9 6 4 

valleys, no channels at all are pressent. The downstream parts of the 

sand valleys are broad and have 

flat bottoms. These are joined, by 

sharp breaks in slope, to side 

slopes that are uniformly at the 

angle of repose. At the top of the 

side slopes there is a second break 

in slope, above which is an almost 

flat upper surface, which roughly 

corresponds to the original surface of the sand before and immediately after uplift. The whole 

flat section of the valley floor may 

be wet, even above the head and 

outside the banks of the channel; 

basal sapping of the side slopes apparently is chiefly responsible for 

valley widening. The upstream 

parts of the valleys are V-shaped 

in cross section and are dry on the 

valley bottom. The irregularity of 

the long profile suggests that there 

may be periods of infilling by wind 

between periods of fluvial erosion. 

Figure 8 illustrates a typical small 

stream valley in sand. 

Large streams, fed by water 

from the hillsides above the former sea level, are gravel floored 

and have steep vertical banks in 

sand, or in sand and gravel in their 

upper reaches (but still below the 

preearthquake water level). Clear 

knickpoints are not present in the 

longitudinal profiles of either 

large or small streams flowing over 

sandbeds. 

STREAM IN GRAVEL 

One small stream (stream 17) 

flows down the hillside and then 

across a steep gravel beach directly into the sea. The gravel had 

some finer matrix and was able to 

stand, at least for a short period, 

in vertical banks. The stream had 

a steep course and a knickpoint 

zone less well defined than those 

of the streams in sillt (fig. 14, p. 

H18). It is debatable whether 

placement of this single example 

with the streams in silt (fig. 5) is 

normal. 

'///A Valley of stream 22 1 

w- 

>S [&14 Valley of stream 10 

6 HORIZONTAL DISTANCE, IN FEET 

W 

0 8 weeks 21 weeks 65 weeks 0 8 weeks 21 weeks x--x-C x C Y Y * Y ---* X " L_J h 7 

Lateral migration of stream 10 65 weeks 

Lateral migration of stream 22 

9.-Cross profile of gravel sheet and lateral migration of streams 10 and !22 (table 

I), MacLeod Harbor. Arrows are drawn to scale, and show distances and direction 

of migration. 

LATERAL AND VERTICAL 

EROSION 

Figure 9 shows gravel sheets 

that crop out approximately at 

the surface of the present major 

streams. One or more gravel sheets 

at a similar level may extend beneath the remaining outliers of silt 

and sand. This preexisting gravel 

probably has some influence on the 

rate of downcutting of the streams. 

At one extreme, the control may be 

so complete that the present 

streams are simply being guided in 

their lateral erosion into hollows 

in the former gravel sheets and are 

thus prevented from further downcutting. At the opposite extreme, 

the gravel sheets may only slow 

downcutting temporarily, without 

tending to shift the streams laterally. The lateral migration of 

stream 10 shown in figure 9 might 

support either possibility, but the 

reversal of the direction of lateral 

era1 migration by the gravel. Figure 9 can thus be interpreted as a 

record of the rate of downcutting, 

which can be dated by reference to 

the lateral position of the streams 

in aerial photographs. 

The rates of lateral and vertical 

erosion deduced in this way (fig. 

10) tend to support the generally 

held view that lateral erosion 

mainly takes place after most 

downcutting is completed. Observations of Mr. C. LaBounty, 

(oral commun., 1965) indicate 

that the bulk of the vertical ero- 1 Stream 22 

500 

0 0 10 20 30 40 50 60 70 

migration of stream 22 shows that 

the stream does not simply move 

into a hollow and stay there; if 

streams did this, they mould never 

change their direction of migra- $ 15 

tion. Instead, it is concluded that ' O 0 20 

0 10 20 30 40 50 60 70 lower positions of the gravel sheet TIME AFTER EARTHQUAKE. IN WEEKS 

represent later times and that the 10.-hferred rates of lateral and of 

streams are downcutting vertical erosion for streams 10 and 22, 

without rigid control of their lat- MacLeod Harbor. 
EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H15 

sion in the major streams was very 

rapid, having been almost completed within 48 hours, and that 

the lateral erosion has been proceeding ever since. Erosion in unconsolidated sediments probably 

is limited by the rate rut which material can be transported by the 

stream. Thus vertical erosion and 

la;teral erosion compete for available capacity, as figure 10 seems 

to show for the first period of 8 

weeks. Vertical erosion will result 

in changes in stream gradientperhaps the most independent 

control of transport rate in a 

given cross seotion-and gradient 

is itself controlled by the rate of 

sediment transport from upstream. Material eroded from the 

bed will be redeposited if the slope 

is lowered too much, and the result will be very little net vertical 

erosion. No such regulatory 

mechanism applies to lateral erosion, whioh may therefore continue 

unchecked until aU available material is out of the possible meander belt of the river. Figure 10 

illustrates this general course of 

events, but gives no information 

about possible mechanisms. 

MODEL OF EROSION 

The results of these erosional 

studies are valid only within a 

limited framework. It would for 

example be quite improper to apply them to conditions in which 

the erosion was limited mainly by 

the resistance of the materials. 

Within the framework, however, 

the overall course of erosion following a sudden relative lowering 

of base level may be approximated 

by an equation of the form: 

V=A · Qr(1-e-7ct) 

where Q is the low-flow discharge, 

in cubic feet per second; V is the 

volume removed, in cubic feet; t 

is the time since the change of 

base level, in weeks; and A, r, and 

k are constants. 

For the MacLeod Harbor sediments, the constants A, r, and k 

have the following approximate 

values: 

Sand: A=1.9 X 106 ; r=0.75 

k=0.012. 

Silt: A=1.7 X 105 ; r=0.95 

k=0.012. 

The differences apparently due L~.> 

lithology may partly reflect differences in hydrology rather than 

in erodibility (seep. H10-Hll). 

Total volume removed seems to 

be the only consistent measure of 

the course of erosion. Recession 

of knickpoints (where present) , 

cross-sectional area at the mouth 

of the stream, and width at the 

mouth of the stream all showed 

much more scatter in their relation 

to other variables. 

RECESSION OF KNICKPOINTS 

The knickpoints in the smaller 

streams in silt apparently are controlled mainly by slight variations 

in resistance of the material; the 

streams seem to pick out certain 

bedding planes which are not otherwise distinctive. The knickpoints 

recede until they finally become 

fixed at the junction with the 

gravel beach above; this junction 

has alre,ady occurred for stream 2. 

On the larger streams, the gradient of the stream above the 

former high-water mark and the 

gradient on the gravel spread below it seem to be more or less independent. No identifiable knickpoints were observed in the long 

profiles, the only recognizable 

feature being a local steepening, 

probably associated with the 

former beach or storm beach. 

OTHER EROSIVE AGENTS 

IN REGRADING OF 

DEPOSITS 

The three principal types of erosional processes that modified the 

bay-head deposits were fluvial, 

marine, and slope processes. The 

most effective slope processes probably were the slopewash, caused by 

raindrop impact, and erosion by 

wind. 

In the broad expanse of deposits in MacLeod Harbor, fluvial erosion seems to be the most important 

process. The sea can only act along 

the perimeter, but rivers act over 

the whole surface. The volume of 

material removed by the rivers has 

been estimated from the planetable 

map of the deposits (fig. 2). 

Along the margin of the deposits, marine erosion produces cliffs 

as high as 18 feet. Below these 

cliffs a narrow beach is being 

formed, coarse material accumulating at its top. Some intertidal 

sorting of material may also take 

place, but its total effect in MacLeod Harbor is thought to be 

slight. 

On the silt deposits, clam shells 

protect small sloping pillars of 

material ; the silt around them is 

detached by raindrop impact and 

washed away by surface runoff. At 

the time of measurement, all these 

pillars sloped at about 30° to the 

horizontal in the same direction, 

which is presumed to be the angle 

and direction of the driving rain. 

Such an angle is probably not 

characteristic of all storms, so the 

heights measured probably represent only the last major storm. The 

median vertical height of 50 of 

these pillars was 1.9 inches (0.16 

ft). 

The total effect of this rainbeat 

since the time of uplift could be 

seen as follows: The shipworm 

( B ankia spp.) bores into wooden 

pilings underwater, but it cannot 

live within the silt; pilings become 

honeycombed with these boreholes 

and eventually break off within an 

inch of the surface of the silt; a 

line of old broken-off pilings projecting out of the present silt beach 

therefore records the former sur-
H16 

face level very accurately, and the 

total erosion of the surface may be 

measured. The rarity of surface 

runoff on the sand and gravel is 

thought to minimize erosion on 

these deposits by rainbe!llt and 

rainwash. 

The profile of the silt along the 

line of pilings is shown in figure 

11. Both the volume of material 

removed since the earthquake by 

surface wash above the top of the 

low seacliff (an average thickness 

of 0.7 ft) and the cross-sectional 

area removed by marine erosion 

( 17 5 square ft) can be calculated 

from figure 11. These computations can be compared with the 

volume removed by fluvial erosion. 

Volumes removed by the three 

main processes responsible for the 

regrading of the deposits are: 

By rivers (see fig. 4) =87 million cu ft 

By rainwash=0.7 ft X area of 

silt deposits= 2 million cu 

ft 

By the sea= 175 sq ft X coastal perimeter=700,000 cu ft. 

Wind U~Ction is locally evident 

on channel sides in the sand areas 

only. The total effect probably is 

a rearrangement rather than a net 

removal of material, because the 

ALASKA EARTHQUAKE, MARCH 27, 1964 

1--

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11.-Postearthquake degradation of silt, by rainwash and marine erosion, along a 

line of pilings. A, Profile of entire beach; vertical exaggeration X 10. B, Part of A, 

without vertical exaggeration, showing retreat of seacliff. 

form of the upper surface of the 

sand is remarkably even and shows 

no hollows, such as might be expected if deflation had occurred. 

It is concluded that, in MacLeod 

Harbor, the agents responsible for 

regrading are fluvial processes, 

rainwash, and marine aotion, in 

that order. However, a different 

order of importance might be obtained for, say, a narrow gravel 

beach. 

EROSION AND D-EPOSITION ON UPLIFTED BEACHES AND ROCK PLATFORMS 

Beach studies in MacLeod Harbor and Patton Bay had three objectives. The first was to provide 

data on the detailed form of 

beaches and offshore platforms 

that are difficult to study under 

normal conditions; the second was 

to observe the rates of erosion and 

deposition on the raised beach as 

an aid to the general study of 

raised beaches; and the third was 

to clarify the tectonic history of 

the island. These three objectives 

are closely related and data were 

collected from ( 1) profiles perpendicular to the shoreline, ( 2) surveys of selected geomorphic features and of amounts of erosion 

and deposition, and (3) stratigraphic sections and their associated radiocarbon dates. 

MACLEOD HARBOR 

BEACHES 

BEACH PROFILES 

Eight longitudinal profiles of 

preearthquake beaches and uplifted sea floors were made in the 

MacLeod Harbor area (locations 

shown in fig. 12). Most profiles 

were on rock platforms, but some 

were on beaches that were composed wholly or partly of sand or 

gravel. Interpretation of the profiles has been made in terms of the 

relief found on rock platforms, 

in terms of marine regrading of 

beaches, and in terms of knickpoint recession and fluvial regrading. 

The profiles seem to have gentler 
EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND 

12.-Shoreline lithology and location of surveyed beach profiles between MacLeod 

Harbor and Fault Cove. 
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ALASKA EARTHQUAKE, MARCH 27, 19•64 

Most exposed beac h 

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HORIZONTAL DISTANCE, IN FEET 

13.--Beach profiles around MacLeod Harbor showing the effect of increasing exposure to waves. Broken lines show effect of 

local modification by stream action. 

overall slopes below the preearthquake mean high water and broader rock platforms where the 

beaches are more exposed to waves 

of long fetch. This relationship is 

shown by the series of five representative profiles in figure 13. How 

much of the differences in profiles 

is due to varying wave exposure 

and how much, for example, to 

varying initial slopes of the coast 

is unknown. 

Sediments from the upper parts 

of both present and former beaches 

show a consistent change in grain 

size down slope; the finer material 

is found in deeper water and on 

lower gradients. In comparing 

profiles 3 and 8, a given grain size 

14 (right) .--Beach profiles 3 and 8 

(figs. 12, 13) , and stream profile 17 

(table 1), MacLeod Harbor. A, Beach 

profile 3 and stream profile 17, vertical 

exaggeration X 4; B, parts of beach 

profiles 3 and 8 referred to preearthquake spring low-tide level, no vertical 

exaggeration. 

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EROSION AND DE.POSITION ON A RAISED BEACH, MONTAGUE ISLAND H19 

on profile 3 is found on a steeper 

gradient and in shallower water 

than on profile 8. This difference 

may be partly explained by differences in sizes of material available, but it fits in well with the fact 

that profile 8 is the more exposed 

(fig. 13). 

Profiles 3 and 8 seem to have a 

step composed of different materials deposited over an earlier surface. The top of the step of both 

profiles is about 311z to 4 feet below the level of former spring low 

tides (fig. 14). The top of the 

steps seems to represent the shelf 

of deposits that was being built 

at the preearthquake sea level below the zone of effective wave 

transport. The relatively shallow 

depth compared with that for the 

bay-head sediments (about 10 ft 

below preearthquake spring low 

water) is consistent with the 

greater coarseness of the material 

in the beach profiles. On the more 

exposed beaches (profiles 1, 2, 5), 

the slope of the rock floor is probably too gentle for the extension 

of sediments from the preearthquake sea level to have produced a 

detectable step. The exposure of a 

former surface, on which this step 

rests, shows that at some previous 

time there rriust have been a sea 

level in the area at least as low 'as 

the postearthquake level. Such an 

earlier low sea level also explains 

the existence of gravel sheets beneath the bay-head deposi·ts, although no dateable material was 

found. 

A marked break in slope, similar 

in form to a wave-cut notch, was 

seen on the seaward side of an 

abandoned stack near profile 8, 

and in the rocky promontory at 

profile 7. This break is, at both 

places, about 16 feet below the preearthquake barnacle line. The 

break, which presumably was 

formed at or near a former hightide level since it does not seem to 

15.-Rock pJoatform showing differential erosion following the structure. 

16.- Rock platform near beach profile 8 (fig. 12), MacLeod Harbor. 

be structural, may mark a former 

sea level that is between the preand postearthquake levels, although it presumably predates 

them. 

Bedrock platforms are notably 

smooth in overall outline, but in 

detail vary from highly irregular 

surfaces with as much as 5 feet of 

local relief to much smoother surfaces with less than a foot of local 

relief. All the differences seem to 

emphasize lithological and structural differences in the near-vertical beds (figs. 15, 16). 

REGRADING OF THE BEACHES 

The uplifted MacLeod Harbor 

beaches have been modified by 

both marine and fluvial processes, 

but slope processes seem to have 

had little erosional effect. 
H20 

Marine processes act only in the 

zone below the storm beach. In the 

MacLeod Harbor area, storm 

beaches corresponding to the preearthquake sea level range from 9 

feet (at the head of MacLeod Harbor) to 12 feet (profile 4) above 

mean high water. There is no evidence of new storm beaches beginning to form, but, since uplift, the 

sea has regraded a zone between 

mean high water and 4 feet above 

it. In the short period since the 

earthquake, all unconsolidated material (but no bedrock) in this 

zone has been regraded. Marine regrading probably will eventually 

extend up to the full elevation of a 

new storm beach. Marine processes 

have had a much greater effect than 

fluvial processes in the regrading 

of all beaches studied. 

Fluvial processes are effective 

only where sand or gravel occurs 

on the beach. Beach profiles 4 and 

5 (fig. 13) are along streams, and 

stream 17 (table 1) is very close to 

profile 3. Comparison of profile 4 

with profiles 1-3 and 8 (fig. 13) 

shows the typical almost-straight 

beach profile that results from regrading by a competent stream 

flowing mainly through sand. The 

upper part of the course of the 

stream in profile 4 on the uplifted 

beach is erosional ; lower down the 

stream spreads out into a fan and 

effectively regrades a width of several hundred feet of the beach by 

tleposition. The low-flow discharge 

of the stream is 10.0 cfs. 

The stream in profile 5 has a lowflow discharge of only 0.03 cfs. 

This stream is competent to erode 

sand from an area underlain by 

cobbles but it cannot carry the cobbles, so the fluvial regarding is 

limited to erosion and deposition 

of sand. Not far from the line of 

profile 5, the stream is divided into 

several small channels, each 4 to 

10 feet wide and 1 to 2 feet deep, 

which do not appear to be cutting 

ALASKA EARTHQUAKE, MARCH 2 7, 1 9 6 4 

laterally ; hence the area regraded here is much smaller than 

that regraded by the stream in 

profile 4. 

Stream 17, cutting into beach 

profile 3, is a small stream with a 

low-flow discharge of 1.09 cfs, and 

a very steep course through unconsolidated gravel (fig. 14). This 

stream illustrates the tendency for 

a channel of uniform gradient to 

develop across a beach. Lateral 

erosion is at a minimum here, apparently because this stream is still 

actively downcutting. The area 

regraded is therefore very small. 

On the beach, which is both narrower and steeper than the uplifted bayhea.d, some resorting of 

beach deposits has already taken 

place, and streams flow at much 

steeper gradients than in the bayhead deposits. Thus stream 17 is 

steeper on the beach than it is 

farther upstream, whereas the 

stream in profile 4 has the same 

gradient above and below the preearthquake water level. The stream 

in profile 5, however, which falls 

down a steep gully onto the beach, 

is less steep in its beach section 

than immediately inland. The 

gradient of a stream across the 

beach, therefore, seems to be unrelated to its gradient above the 

preearthquake beach; in other 

words, the relative drop in sea level 

caused by the earthquake will not 

necessarily lead to the propagation of a knickpoint inland along 

every stream course. The formation of a knickpoint appears to be 

least likely in small streams flowing from steep hillsides. 

PATTON BAY BEACHES 

Seventeen beach profiles, on all 

types of material, were made in 

Patton Bay, across the island from 

MacLeod Harbor. These profiles 

show the relationships, both vertically and areally, between the 

following features, if they are 

present: postearthquake high-water mark, postearthquake storm 

beach, preearthquake high-water 

mark (the former barnacle line), 

preearthquake storm beach, and 

breaks in slope at the foot of cliffs 

corresponding to both the preearthquake sea level and to raised 

sea levels. Regrading of the uplifted beaches was also studied. 

Regrading of sand and gravel 

was similar to that on the other 

side of the island. An estimate was 

also made of the rate at which rock 

platforms on the Patton Bay side 

have been regraded since the 

abandonment of the older uplifted 

shoreline. The many seacliffs, some 

several hundred feet high, also 

provide an opportunity for examining the way in which the break 

in slope at the foot of a cliff is 

obliterated by subaerial processes. 

Most of the coastline of Patton 

Bay now exhibits a rock platform 

several hundred feet wide, the 

landward edge of which is generally covered by beach deposits (fig. 

17). This platform, which seems to 

accord with the preearthquake sea 

level, is backed, except in the center of the bay, by a steep seacliff 

about 100 feet high. Above this 

sea cliff the land is gently rolling 

and rises 200 to 300 feet above sea 

level. In the center of the bay 

coastline, however, the cliff is 

absent, and instead there is a low 

step and a raised beach platform 

80 to 500 feet wide, which is 

backed inland by slopes of 40° 

to 50° leading up to the main 

2,500-foot backbone of the island. 

Sandy beaches occur in three 

main areas above the coastline 

studied. Just south of the center of 

the bay is the most extensive 

stretch of sand, called in this 

report "Main Sandy Bay." The 

largest river on the island, the Nellie Martin River, debouches at the 

south end of the bay. At this point 

the present beach is backed by two 
E'ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND 

EXPLANATION 

PA TTON High ground 

Main Sandy Bay Sand 

Cobbles and boulders 

APPROXIMATE 

Illlll~lllll'I 

Cliff 

-:::::+ 

Storm beach 

- -- 15- - Contour of amount of 

1964 uplift, in feet 

-P21 

Location number 

Beach profile and 

number 

0 92 % 3/4 1 MILE storm beach I I I I I 

17.-Coastal features and deformation caused by the 1964 earthquake, Patton Bay. 
H22 

storm ridges, behind which is an 

extensive flat area drained by the 

Nellie Martin River and its tributaries. There is a second sandy 

area called in this report "Bay 

Mouth Bar" at the southern limit 

of the coast studied, where a lagoon 

is dammed up behind two stormbeach ridges. The third sandy area 

is to the north of the center of the 

bay. This area, called "North 

Sandy Bay," is backed by a cliff 

about 400 feet high, above which 

rise the main mountains of the 

island. There is also a small area of 

sand and gravel in the middle of 

the central rock platform, where 

two small rivers debouch across 

the platform. In several localities 

between a sand beach and an area 

of bare rock platform, a short section is covered by large boulders, 

2 to 10 feet in diameter, lying on 

bedrock; their undersurfaces are 

angular but their upper surfaces 

are smooth. This contrast bet>veen 

smoothed upper surfaces and 

angular undersurfaces suggests 

that marine action has not been 

powerful enough to overturn them 

during the period required to 

smooth their upper surfaces by 

rubrasion and solution. Smaller 

rounded cobbles, most of them as 

much as 6 inches in diameter, 

between the large boulders indicate that 6 inches is approximately 

the maximum size of stone that the 

waves were competent to transport. 

The main coastal features of the 

bay and the positions of the 17 

beach profiles and other key locations (indicated by series Nos. P1-

25) are shown in figure 17. 

STRATIGRAPHIC EVIDENCE FOR 

SEA-LEVEL CHANGES 

HIGH SEA LEVELS 

There is a continuous raised 

beach platform between Main 

Sandy Bay and North Sandy Bay 

that definitely represents a single 

former beach. Three profiles 

ALASKA EARTHQUAKE, MARCH 27, 1964 

slwwing the raised beach deposits 

are described below. 

The position of beach profile 

P23 is shown in figure 17. Figure 

18 shows the profile and a stratigraphic section located on it. The 

material in the bottom 24 inches 

of the section is similar to the 

modern cobbles in the upper part 

of the beach and is definitely identified as a gravel of the former 

beach. Its topographic situation 

might make it seem to be a part of 

the preearthquake beach; but the 

raised gravel is an integral part 

of the section above it. ·when sea 

level dropped from its raised level 

to the preearthquake level or lower, vegetation would have covered 

the former beach material, the 

cliff at the back of the beach 

would degrade, and the observed 

sequence of peat and angular gravel would be deposited. The date 

of the change of sea level must 

have been earlier than the deposition of the lowest peat in the section, which has been dated by radiocarbon as 2070±200 B.P. 

(W-1770). 

The position of profile P19 is 

shown in figure 17. Sections D1 and 

D2 (fig. 19) are close to the bank 

of a small river, which has exposed 

a 4-foot-thick bed of angular to 

subrounded gravel in a finer matrix overlying bedrock. Sections D1 

and D2 are close together and have 

been correlated on the basis of the 

position of the top of the angular 

gravel bed. The gravel is thought 

to be a former alluvial fan of the 

river, deposited while the sea \Vas 

at a higher level. In section D1 the 

level of the raised platform can be 

clearly seen in bedrock, and a small 

step in bedrock is exposed at the 

seaward edge of the vegetated 

platform. Section F (fig. 19) is 

close to this step, about 50 feet 

north of the river. In section F the 

material resting directly on bedrock is much more uniformly 

rounded, has less fine matrix, and 

contains a wider variety of rock 

types than the angular gravels of 

sections D1 and D2• This rounded 

material is interpreted as being a 

true beach deposit, beyond the 

edge of the alluvial fan. 

At profile P24 (fig. 17), at the 

south end of North Sandy Bay, 

the raised beach platform is much 

narrower; it is the northernmost 

exposure where beach gravel is 

clearly resting on a nonstructural 

rock platform. The position of the 

break of slope at the foot of the 

cliff can be located very accurately 

here (figs. 20, 21). 

LOW SEA LEVELS 

Possible evidence of a sea level 

slightly lower than the preearthquake level is shown by tree 

stumps, commonly found in the 

position of growth between preearthquake mean high water and 

the top of the preearthquake storm 

beach. These stumps all show some 

wave erosion. More substantial evidence of a former low sea level 

is found in peat layers in the banks 

of the Nellie Martin River. A 

continuous peat bed is exposed 

along the banks in the lower course 

of the river. The top of the bed 

slopes down toward the sea; it can 

be definitely identified as much as 

1.8 feet below the postearthquake 

high-water mark and seems to continue farther out to sea (fig. 22). 

Under present topographic conditions such peat is found behind 

storm beaches, that is, about 12 

feet above high-water mark. It 

therefore seems probable that this 

peat layer was formed at a time 

when sea level was at least 14 feet 

lower than the postearthquake 

level at this locality, or 25 feet below the preearthquake level. A 

sample taken from 2 feet below the 

top of the peat in section B (fig. 

22) has a radiocarbon date of 

600-+-200 B.P. (W-1766). 
E*ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND 

40 - preearthquake MHW 

30 - 

L 

W Y

Z SECTION 

2 20 - DEPTH, Vegetation: spruce P 

l- a forest and grass 

> Y 

W 2 

o-% in. angular 

10 - 

-Angular gravel in 

clay, as above 

Preearthquake MHW Peat, as above 0 Angular gravel plus 

10 percent rounded 

gravel in gray clay 

pebbles as much 

as % in. diameter 

Base of exposed 

I/ 

section 

0 100 200 300 

DISTANCE, IN FEET 

VERTICAL EXAGGERATION X5 

18.-Stratigraphic section and beach profile P23, Patton Bay. 
ALASKA EARTHQUARE, MARCH 2 7, 1964 

X 

50 

C 

Y 

L Vertical 

5 cliff 

f 40 

I 

Y r 

4 

3 

D 

x C 

2 30 

W W 

a (L 

W > 

u 

g 20 - 

II Height of preearthquake 

W -Istorm beach across river 

Y 

10 

0 

0 100 200 300 

HORIZONTAL DISTANCE, IN FEET 

SECTION F SECTION DZ DEPTH DEPTH BELOW SURFACE, 

SECTION D, BELOW SURFACE. 

S I 

0 Vegetation. dry mature spruce forest DEPTH 0 egetation: grass, fern. BEL Peaty humus wrth some mrneral 4 salmonberry, spruce, alder I materlal at base 9 

12 Brown-gray sllty sand with occasional 11 

13 lenses of well-rounded grit 

2 1 

22 Iron-stalned organic clay contalnlng thin organic bands 

Gray. mottled plastic clay with lenses of well-rounded 27 grit and occasional subangular to subrounded 

siltstone pebbles 

Mainly rounded material from sand sue up 

to 2 in.. and some large (6-50 in.) well. 

rounded boulders not of local rock, in a 

matrix of silt and some clay 

Base of section 

63 61 

Bedrock 

97 

Base of sectton 

121 Base of sectionzriver level 

19.-Stratigraphic sections Dl, Ds, and F, and beach profile P19, Patton Bay. 
EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND 

20.-Cobbles of raised beach resting on truncated bedrock platform, profile P24, Patton Bay. r 

L 

Obscured 

0 10 L_ ___ L__~ 

NO VERTICAL EXAGGERATION 

Fine-grained 

light-gray sandstone 

/ Preearthquake 

0 

~ 

_ beach gravel 

21.-Stratigraphic section showing old raised beach at P24, Patton Bay. 

H25 
ALASKA EARTHQUAKE, MARCH 2 7, 1 9,6 4 

SECTION B 

DEPTH 

BELOW SURFACE, 

IN FEET 

SECTION C 

DEPTH 

BELOW SURFACE. 

IN FEET 

Interbedded beach sand and 

gravel lndlvldual beds Storm beach gravel about 10 In th~ck 

SECTION A ~lX0ntlnuous stringers of small 

wood fragments (swash debr~s) 

DEPTH Grayslltysand wlth small woody 

BELOW SURFACE fragments Gradat~onal lower 

IN FEET contact Sharp upper contact 

0 ,, Surface Peat contalntng wood fragments 16 '"' of all slzes, as much as 20 ~n 

In d~ameter all abraded 

Present rlver level 

Z 

8 

2 15- 

5 1 Preearthquake MHW 

Y 

- ---- 10 - 

10 I I I I I I I 

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 

HORIZONTAL DISTANCE, IN FEET 

22.-Stratigraphic sections and long proflle of Nellie Martin River. 

It is concluded that there was an 

earlier raised sea level, higher than 

the sea level just before the earthquake, the exact elevation of 

which is discussed on pages H28 to 

H33. By 2070k200 B.P. the sea 

level was low enough relative to 

the land for a layer of peat to have 

grown upon the rock platform 

eroded at the higher sea level. A 

period of low sea level followed 

during which the sea was low 

enough for peat to form 1.8 feet 

below the 1965 high-water mark 

near the Nellie Martin River. The 

topographic position of the present peat layer behind storm 

beaches or higher indicates that 

the sea was locally at least 14 feet 

lower than at present relative to 

the land. The sea was at this low 

level at 600-c-200 B.P. Because the 

dated sample was near the top of 

the peat layer, it is assumed that 

relative sea level rose shortly after 

this time. 

ELEVATIONS OF BEACH 

FEATURES 

The sea level at which a raised 

beach was formed cannot be measured directly; fossil beach features are among the best available 

indicators of the former sea level. 

The range of elevation of several 

beach features, and the relationship of each to the high-water 

mark, has been studied in Patton 

Bay, mainly with reference to features formed at the preearthquake 

sea level. From these data the reliability of each feature as an indicator of a former high-water mark 

has been assessed, and some conclusions drawn about the causes 

of variation in elevation for each 

feature. The beach features chosen 

for this study were the break in 

slope between the foot of a cliff 

and a bedrock platform, the break 

in slope between a cliff and a 

gravel beach, and the top of the 

storm beach ridge. 

The elevation of a break in slope 

between a cliff and a bedrock platform could be measured in only 

three localities. The elevations of 

the break in slope above the relevant (preearthquake) mean high 

water, measured as the barnacle 

line, were f2.0 (P7), +0.6 (P4), 

and -0.8 (P5) feet. These figures 

suggest that, for the conditions 

found on Montague Island, the 

elevation of a break in slope in 

bedrock is within 2 feet of mean 

high-water level and that this feature is the best indicator of a former sea level. 

A gravel beach is a temporary 
E'ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H27 

deposit whose thickness varies 

with time. A break in slope ;between a cliff and a gravel beach is 

therefore likely to show a greater 

range in elevation between localities than one between a cliff and 

bedrock platform, and the lower 

limit of this range should be close 

to high-water mark. Seven measured elevations of this feature 

(fig. 2319) ranged from 0 to 16 feet 

above mean high water, with a 

mean value of +9.3 feet. At location P7 (fig. 17) the elevation of 

the break in slope increased by as 

much as 8 feet up small reentrants 

in the cliffs. It might be expected 

that the break in slope of a cliff 

with a gravel beach would give the 

most reliable evidence of high-water elevation ; however, the possible error of +8 feet shows that 

this feature is not a reliable indicator of this altitude. 

The difference in elevation between the top of the preearthquake 

storm beach and the preearthquake mean high-water level was 

measured at 11 points in Patton 

Bay. The values ranged from 6 to 

21 feet above mean high water, 

with a mean of 12.0 feet (fig. 23B). 

These values are consistent with 

the idea that a gravel beach 

banked against a cliff is in effect 

an incomplete storm beach ; therefore, elevations of breaks in slope 

in gravel beaches should be lower 

than the elevations of storm 

beaches. 

In several locations a new storm 

beach is probably beginning to 

form in response to the postearthquake sea level. Seven measurements show that the ridges formed 

are as much as 6 feet above mean 

high water (1965), with a mean 

value of 4.1 feet (fig. 23A). The 

present size and position of the 

ridges illustrates the initial stages 

in the formation of a mature 

storm ridge. 

The height of the top of a storm 

beach is dependent upon wave and 

shore environment, but it will approach a maximum elevation 

which is stable for its own environment. Mean high-water level 

can be determined from the devation of the top of a storm beach 

only within +8 feet, so this feature is also a poor indicator of sea 

level. However, the difference in 

elevation between two mature parallel storm beaches should be an 

accurate measure of -the elevation 

difference in sea levels when the 

two ridges were formed. This difference has been used in calculating the elevation of the raised sea 

level in Main Sandy Bay. 

In MacLeod Harbor the level of 

mean high water could be determined from the form of the beach 

profile, but no satisfactory correlation was found between beach 

form and water level in Patton 

Bay (fig. 24). The gradienk of the 

beach, the depth from high-water 

mark to the seaward edge of the 

beach gravel, and grain size of the 

beach material were all found to 

be of little use for the precise location of sea level. In MacLeod Harbor a consistent relationship was 

found between the preearthquake 

water level and the steep seaward 

edge of the gravel beach (fig. 14), 

but such a step is only to be expected where the gradient of the 

23.-Distribution of elevations of beach features; A, above postearthquake mean high water, of postearthquake storm beaches; 

B, above preearthquake mean high water, of preemthquake &om beaches; C, above preearthquake mean high water, of the 

bredlk in slope at heads of beachea 

es in MacLeodHeights of break 

In slope near 1 p7 1 

NUMBER OF MEASUREMENTS IN EACH HEIGHT RANGE 
H28 ALASKA EARTHQVAiKE, MARCH 2 7, 19 64 

1 FEET 

20 1 

%.-Beach profiles in Patton Bay in relation to the preearthquake mean high water, showing that the shape of the profile is 

not a reliable indicator of the po&ion of the mean high water. See figure 17 for location of profiles. 

seaward-building gravel is appreciably lower than the angle of 

the rock platform on which it is 

built. The evidence from MacLeod 

Harbor (p. H19) suggests that 

these crolnditions are found only 

in areas well sheltered from wave 

atback. Because the whole of Patton Bay is more exposed than any 

of the hches in MacLsod Harbor, 

no steps similar to those in Machod Harbor are to be expected in 

Pakton Bay. 

In summary, where a bedrock 

platform against the foot of a cliff 

is not covered by unconsolidated 

material, then the elevation of the 

wave-cut break in slope is considered to be the best 'available topographic indicator of mean high 

water, and the difference in elevation between two breaks in slope 

formed at different sea levels is an 

excellent measure of the difference 

between the two sea levels. 

Where unconsolidated material 

lies at the foot of a seacliff, the 

shape of the underlying bedrock, 

the amount of gravel available, 

and random factors probably control the height to which the gravel 

is accumulated. At one extreme 

there is no gravel, and at the other 

extreme a full storm beach is 

formed against the base of the 

cliff. The elevation of the break in 

slope between the gravel and the 

cliff therefore varies from near 

high-water mark to full stormbeach elevations. Sueh breaks in 

slope are not accurate indicators of 

former water levels. 

Storm-beach elevations increase 

with time to a maximum, which 

takes several years to reach. This 

maximum elevation is only constant for a given wave-exposure 

environment. Storm-beach elevations are therefore poor indicators 

of the absolute water level, but the 

difference in elevation between two 

mature parallel storm beaches 

should provide a good measure of 

the difference of elevation between 

the two sea levels at which they 

were formed. 

TECTONIC DEFORMATION OF 

BEACHES 

E1t;vations of beach features 

were measured at many localities 

over a 10-mile stretch of coastline 

in Patton Bay. The elevations of 

storm beaches, high-water marks, 

and breaks in slope were measured 

using the preearthquake high-mater mark as a standard. In figure 

25, each symbol represents a measured point, and the lines show the 

interpolated elevations of each feature between measured pints, so 

each line shows the change of elevation of a beach feature along the 

coast, in relation to the preearthquake high-water mark. The figure 

shows the elevations of pre- and 

postearthquake shoreline features 

and of raised shoreline features 

and the variation of height of these 

features along the coastline of Patton Bay. All elevations were mea- 
BROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H29 

25.-Variation in differences of elevation among coastal features in Patton Bay. 

sured relative to the preearthquake 

mean high water (which is taken 

as the top of the former barnacle 

zone). Postearthquake mean high 

water was estimated from U.S. 

Coast and Geodetic Survey tide 

tables. Where storm-beach crests 

and breaks in slope between 

beaches or rock platforms and the 

cliffs behind them are present, 

their elevations are shown. 

Elevations are shown for features which correspond to preand postearthquake sea levels and 

also to a raised sea level. The elevation of the raised mean high 

water has been calculated in one 

of the following mays: (1) in 

Main Sandy Bay, as the difference in elevations between the 

crest of the preearthquake storm 

beach and the crest of the raised 

storm beach, or (2) elsewhere in 

Patton Bay, as the difference in 

elevation between the top of the 

preearthquake barnacle zone (pre11 

60 

earthquake mean high water) and 

the break in slope between the 

raised beach platform and the 

steep slope behind it (raised 

mean high water). Figure 25 

shows the tectonic deformation of 

the shoreline from the time of the 

earlier raised beach until just before the 1964 earthquake, and also 

the deformation caused by the 

1964 earthquake. The deformation 

due to the 1964 earthquake is 

plotted in figure 17. These data 

are in general agreement with the 

smaller scale map by Plafker 

(1965, fig. 7), but they add detail 

for the Patton Bay area. 

The evidence for the raised sea 

level consists of: (1) a continu-ous 

platform from just north of location P24 (fig. 17) to the north end 

of Main Sandy Bay; (2) a continuous raised storm-beach ridge 

across Main Sandy Bay; and (3) 

a platform at two points immediately south of the Nellie Martin 

SOUTH DISTANCE ALONG SHORELINE. IN MILES NORTH 

Li 

W 

LL 

River, close to the southern end of 

the storm beach (locs. P9 and P10, 

fig. 17). The continuity of the 

landward edge of the platform and 

of the raised storm beach shows 

that they represent a single sea 

level throughout their extent. 

At Bay Mouth Bar an earlier 

storm beach within 2 feet of the 

elevation of the younger ridge has 

been preserved behind the younger 

ridge. This abandoned beach was 

presumably formed at a sea level 

almost at the preearthquake level, 

but no positive basis mas found for 

correlating it with the raised beach 

deposits described above. 

Figure 17 shows not only the 

distance over which the raised 

beach can be detected but also the 

15-idth of the platform which has 

has not yet been destroyed. At the 

north end of the coastline studied, 

the raised beach platform padually narrows and finally disappears in North Sandy Bay where 

EXPLANATION 

.G @ 

preearthquake break in slope Crest of postearthquake storm 

between foot of cliff and beach 

z gravel (G) or bedrock (6) 0 

Old raised beach MHW A X 5 Crest of preearthquake storm Postearthquake M HW w beach 

0 

B 

Postearthquake storm beach 

x-x @ -x- 0 

x-X-x 

Main Sandy Bay North Sandy Bay Nellie Martin W A 

m River *ma , A W N m 4.-+ hrn m 0-N me V) 

n Nm an 22: n c2 r; 222 22 N n 

40 1 I1 I I I I II I I lllll I II I I I II I I I II 

0 1 2 3 4 5 6 7 8 9 10 
H30 

the preearthquake seacliffs are several hundred feet high. The absence of the raised beach in this 

section can be attributed to the 

nature of the local rock, which 

offers very little resistance to marine erosion ; a preearthquake rock 

platform would have been eroded 

by marine action and its surface 

coV'ered with sand and gravel, in 

contrast to neighboring more resistant bare-rock pladorms. The 

lack of resistance of the rock is 

also indicated by ( 1) the relatively 

gentle gradient of the cliffs ( 40°-

500), (2) the tendency of the cliffs 

to degrade by landslides, mudflows, and gullies rruther than by 

rockfalls 'and talus accumulrution 

(this tendency indicat~ a high 

clay content in the rock or in its 

primary weathering derivatives), 

and ( 3) the considerable amount 

of erosion of the cliffs since the 

earthquake (fig. 29, p. H35). 

The coastline north of North 

Sandy Bay was not examined in 

any detail. The south side of Box 

Point (north of the area of fig. 17) 

has continuous cliffs 50 to 100 feet 

high; here low raised platforms 

apparently have not been preserved. The surface of the peninsula is relatively low and flat at 

altitudes of 100 to 200 feet, but 

there is too much relief for a raised 

platform. It is therefore tentatively assumed that, within the 

area studied, only in the central 

part of Patton Bay was wave attack sufficiently attenuated and 

the rock sufficiently resistant to allow the preservation of a raised 

beach platform. 

Since measurements of the relative heights of the raised beaches 

are based on the elevations of 

breaks in slope and of storm 

beaches, it must be recognized, for 

the reasons discussed above, that 

some measurements may be in 

error by 5 to 10 feet. Nevertheless, 

the amount of the warping is much 

ALASKA EARTHQUAJKE, MARCH 27, 1964 

greater than the amount of error 

inherent in the measurements, so 

the general pattern of the uplift 

can be accepted. For example, near 

the northern end of the raised 

beach (fig. 25), its elevation above 

the preearthquake mean high water rises from 10.2 feet (location 

P17) to 45 feet (profile P22) in a 

distance of 1 mile. This deformation is too great to be dismissed as 

an error of measurement, especially inasmuch as the pattern of 

warping in this area is confirmed 

by the elevations of breaks in slope 

both in gravel and in bedrock. 

The relationship between the 

two measures of the uplift of the 

raised beaches-(1) the preearthquake barnacle line to the break in 

slope of the raised beach and ( 2) 

the preearthquake storm beach to 

the raised storm beach-can be 

studied where the storm beaches 

abut against steep slopes at each 

end of Main Sandy Bay. Figure 

26 shows the relationship at the 

northern end of the bay and figure 

27 shows the relationship a,t the 

southern end. Both figures seem to 

show that, in the area of overlap, 

the difference in height between 

the two storm beaches tends to decrease and the difference in height 

between the break in slope and the 

barnacle line tends to increase. 

Inasmuch as the storm ridges are 

curved, the exposure to waves 

obviously will not be the same on 

each ridge at the points where a 

section line drawn at right angles 

to the ridge intersects them (figs. 

26, 27). The landward, older ridge 

will have been closer to the hillside, 

and therefore less exposed to 

waves. As a result, the landward 

ridge is lower for a given sea level, 

and the difference in height 

between the ridges is smaller than 

the difference between the sea levels at which the ridges were 

formed. Elevations of breaks of 

slope, on the other hand, increase 

up the inlets (p. H27), so the differences in elevation between the 

break in slope on the raised platform and the preearthquake highwater mark will tend to become 

progressively larger than the differences between the two sea levels 

in the zone where the steep hillside 

is farther from the shore. It is coneluded that the true change of sea 

level is greater than that shown by 

the storm beaches and less than 

that shown by the breaks of slope. 

An interpolated value of the sealevel change is shown by the 

broken lines in figures 25, 26B, and 

27B. 

South of theN ellie Martin River 

there are cliffs along the coast; no 

certain evidence of a raised beach 

was found, but with elevation differences as little as 5 feet above 

the preearthquake level, evidence 

would be very difficult to interpret 

in the absence of a continuous feature. Possible evidence exists in 

the second storm-beach ridge at 

Bay Mouth Bar and at isolated 

locations at the mouths of streams 

where the cliff is absent. However, 

the primary purpose of this investigation was not to locate possible 

raised beaches, but to use the evidence provided by a raised beach 

to support observations of changes 

to the beaches and platforms uplifted by the earthquake. 

Comparison of the radiocarbon 

dates for all samples from Patton Bay beaches indicates that 

observed data can be explained 

most simply by two relative sealevel changes before the 1964 

earthquake. Each change has left 

abandoned shoreline features at 

various elevations (fig. 25). Data 

from elsewhere in Prince William 

Sound suggest that the 1,000 years 

before the earthquake was characterized by slow submergence of 

the land ( Plafker and Rubin, 

1967), and the data from Montague Island are consistent with this 
EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND 

P14 Ra~sed storm beach/preearthquake storm beach=15 1 ft 

1 PV Raised storm beach/preearthquake storm beach=21.8 ft / 

Cab 

EXPLANATION J r f ,o 

MHW/break in slope=14.4 ft. 

Elevation difference between 

named feotures - 0' I "I I 

Profile location and number :\ f x:.. I 

# La U / t Raised storm beach/preearthquake storm beach=15.6 ft 

0 200 400 600 800 FEET 

26.-Relationship between measures of uplift of raised beaches at the north end of Main Sandy Bay. A, Diff-cea 

in elevation of raised break in slope, raised storm beach, preearthquake storm beach, and pmearthquake mean 

Mgh watar. B, Measurements; of sea-level change. 

30 

J u%i OW 

ZJ". 

- EXPLANATION 

X 

Sea-level change obtained from difference 

in elevation between preearthquake 

MHW and raised sea-level break In 

slope o 

Sea-level change obtained from difference 

in elevation between top of preearthquake storm beach and top of raised 

storm beach 

,. , u 

sea-level change 

B 

2 LO - -2 m 

b 3 - b N a> b 

0 ' 1 I1 I I I 1 1 0 1000 2000 3000 NORTH DISTANCE ALONG SHORELINE. IN FEET SOUTH 
ALASKA EARTHQUAKE, MARCH 2 7, 19,64 

EXPLANATION 

X 

Sea-level change obtained from difference 

In elevation between preearthquake 

MHW and raised sea-level break in 

113 W slope 0 

Sea-level change obtained from difference 

Assumed actual in elevation between top of preearth- I Ka10 saw quake storm beach and top of raised 

--__ storm beach K n Onw 

-3 

PI 1 P9 

I _I 

1000 2000 

DISTANCE MEASURED ALONG PREEARTHQUAKE STORM BEACH, IN FEET 

27.-Relationship between measures of uplift of raised beaches at the south end of Main Sandy Bay. A, Sketch showing 

difference in elevation of raised break in slope, raised storm beach, preearthquake storm beach, and preearthquake mean high 

water. B, Measurements of sea-level change. 
EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H33 

Sea level Location of 

Date relative to C-14 sample C-14 sample Basis for estimating mean high preearthquake 

level (feet) 

(fig. 17) water 

Sometime before 2070 

B.P. +8} W-1770 P23 Raised beach peat. By 2070 B.P ___________ ~-6 600 B.P _______________ ~-25 W-1776 P11 Nellie Martin peat. 

Preearthquake A.D. 0 -------- P11 Barnacle line. 

1964. 

Postearthquake A.D. -11 -------- P11 Estimate from U.S. 

1965. 

interpretation. If the central value 

for each radiocarbon date is used, 

then the sequence of events near 

the mouth of the Nellie Martin 

River seems to have been as shown 

in the table. 

These dates are consistent with 

relative sea-level changes at the 

same time ·and in the same direction on both sides of the island, 

although the amount of vertical 

movement varies considerably 

from place to place. No clear evidence of other sea-level changes 

was found in the area studied. 

OBLITERATION OF BREAK IN 

SLOPE AT TOP OF BEACH 

One immediate effect of the 

1964 earthquake was to initirute 

many landslides along the coastline, but much mass wasting of the 

former cliffs has taken place since 

the earthquake. The break in 

slope at the top of the former 

beach is being rapidly obliterated 

by several processes-talus •accumulation, landsliding, gullying, and 

vegetation growth. The uplift has 

prevented the sea from removing 

mass waste from the base of the 

cliffs, so all the material that has 

been eroded from the cliffs since 

the earthquake can be measured. 

From these measurements the 

rates at which the different slope 

processes were operating were 

determined. 

OBLITERATION BY TALUS 

ACCUMULATION 

At profile P3 (fig. 17), talus accumulations have already obCoast and Geodetic 

Survey tide tables. 

scured 85 percent of the break of 

slope at the foot of a vertical cliff. 

The distribution of fallen debris 

is shown in figure 28. Some of this 

material no doubt fell rut the time 

of the earthquake, but the major 

part probably has accumulated 

since the earthquake; the sound of 

falling stones was still frequently 

heard in 1965. 

The volume of talus material at 

profile P3 is estimated to be about 

210,000 cu ft. The cliff from which 

it is derived is about 110 feet high 

and 580 feet long. The talus accumulation therefore represents a 

mean cliff retreat of 2.4 feet in 

15 months (to July 1965), assuming a 30-percent porosity in the 

talus. At this rate, if the total 

accumulation postdates the earthquake and if the observed maximum angle of rest of 22° is 

correct (fig. 28), the cliff will be 

completely regraded in 71 years. 

If any of the present accumulation 

occurred at the time of the earthquake, then the time required for 

the cliff at profile P3 to become 

completely covered by its own 

talus will be as follows: 

Accumulation 

at time of 

earthquake 

(percent) 

0 

40 

60 

80 

Time required 

for total degradation of cliff 

(years) 

71 

117 

176 

350 

Unless almost all of the accumulation occurred at the time of the 

earthquake-and the continued 

falling of material makes this 

seem improbable-then the cliff 

obviously will be completely obscured by its own talus in no more 

than a century or two. 

OBLITERATION BY LANDSLIDES AND 

GULLYING 

In North Sandy Bay near profile P24 (fig. 1'7) , landslides and 

gullying seem to have been more 

important than simple talus accumulation in obliterating the 

break in slope at the foot of the 

cliff. A horizontal distance of 

2,940 feet of cliff was studied in 

·detail, and the distribution of fallen material is shown in figure 29. 

In this total distance, 22 percent 

of the break in slope is virtually 

unobscured by debris, 17 percent 

of the break in slope is obscured by 

alluvial fan material, and 61 percent of the break in slope is obscured by landslide material. It 

was evident that much of the 

movement postdates the earthquake; the stream in one gully was 

very rapidly transporting debris 

while studies were being made in 

July 1965, and material in some of 

the slides was still very wet and 

sticky. The extent of postearthquake erosion is also shown by the 

very marked increase between 

May and August 1964 in fresh 

landslide scars that appeared on 

air photographs taken by the U.S. 

Coast and Geodetic Survey at 

those times. The estimated volumes of material transported are 

6'70,000 cu ft in alluvial fans and 

370,000 cu ft in landslides. The average height of the cliff is 400 feet; 

so the mean distance of cliff retreat since the earthquake has been 

0.90 foot in 15 months. 

Landslides are far more effective than gullying in degrading 

the cliff, because they act along 

its whole length. If landslides 

alone are taken into account, and 

if all the volume of landslide debris has accumulated since the 
ALASKA EARTHQUm, MARCH 27, 196.4 

EXPLANATION 

21 " 50' 

Angle of scree slope 

0 50 100 FEET 

L I I 

28.-Degrading of vertical cliffs and accumulation of talus at prole P3, Patton Bay. 
BROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND 

EXPLANATION 

29.-Sketch showing degrading of cliff by landslides and gullying north of location P24 in North Sandy Bay. 

earthquake, then the time required 

for total degradation of the cliff 

would be 2,300 years. However, the 

gradient of the vegetated steep 

slope behind the raised beach is 

rarely less than 38", so it is unlikely that the process of degradation will proceed very far. At an 

early stage, vegetation will establish itself on the slope, and further 

degradation will be exceedingly 

slow, except perhaps locally along 

streams or gullies. 

The striking difference in behavior between the resistant vertical cliff and the nonresistant 50"- 

slope may help to explain some 

apparent anomalies of raisedbeach preservation in other areas. 

For example, rapid accumulation 

of talus may destroy topographic 

evidence of a raised beach within 

a short period, but it is this same 

talus cover which protects the 

raised beach from subsequent 

weathering and erosion and so 

over the long term preserves the 

beach in the stratigraphic section. 

OBLITERATION BY VEGETATION 

Where the upper limit of a 

raised beach is not a well-defined 

cliff, but a low step leading to another raised beach, the junction between the two beaches can be 

hidden by beach deposits and the 

growth of vegetation. It seems 

likely that this obliteration will 

occur extensively at the junction 

between the preearthquake beach 

and the raised beach, for example 

at the section shown in figure 19. 

Where the raised platform is well 

developed, the topographic step 

between the raised platform and 

the preearthquake beach is nearly 

always low, being nowhere more 

than a 10-foot step, even though 
H36 ALASKA EARTHQUAIRE, MARCH 27, 1986.4 

The development of the preearthquake platform at the expense of the raised platform has 

been in progress only during the 

period that relative sea level was 

rising to preearthquake level, that 

is, since 6002 200 B.P. or earlier. 

The probable sequence of sea levels 

and platform cutting is illustrated 

schematically in figure 31. 

30.-Idealized profile through two raised beaches, with soil and vegetation omitted. Figure 3123 shows two of the 

Step between two rock platforms indicated by A. possible extreme cases if the grathe difference between the two sea 

levels may have been 40 feet or 

more. Figure 30 shows such an 

idealized profile, with soil and vegetation layers omitted. Most of the 

topographic step between the two 

rock platforms, as visualized in 

figure 29, is obscured by beach deposits, so the surface step is very 

low; a vegetation cover will soon 

make it extremely difficult to distinguish between the two surfaces 

by topographic form alone. Two 

valid criteria may still be used: 

(1) there should be a marked subsurface step in the bedrock at A 

of figure 31; and (2) if datable 

soil layers (for example peat) 

continue to form on the abandoned 

beaches of Montague Island as 

they have in the past, then an age 

discontinuity should occur in the 

lowest layers at A. 

REGRADING OF ROCK 

PLATFORMS 

The rock platforms accordant 

with the preearthquake sea level 

appear to have been formed by the 

regrading of a preexisting platform accordant with the raisedbeach sea level. Regrading is well 

marked near location P21 (fig. 17) 

where many bedrock knobs on the 

upper part of the preearthquake 

platform stand as outliers, clearly 

accordant with the raised-beach 

platform, and in places surrounded by preearthquake beach 

deposits. 

Raised sea level 

Regraded cross section= h*x 

t 

h 

Preearthquake sea level 

31.-Probable sequence of platform cutting in Patton Bay. A, Cutting of raised 

platform. B, Cutting of main platform during a time af rising sea level: case 1, 

main platform at same gradient as raised platform; case 2, main platform at lower 

gnadient than raised platform. 
EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H37 

dients of the raised platform, main TABLE 2.-Relationship between change of sea level (h), width of platform 

regraded (x), and cross-sectional area of material removed (A = h.x) platform, and initial land slope are 

allowed to vary; it may be seen 

that the cross-sectional areas removed in forming the main platform range from a minimum of 

lhhx to a maximum of hx, where 

h is the difference between the 

elevations of the raised beach 

notch and the preearthquake notch 

and x is the width of the main 

platform, measured at right angles 

to the shoreline. 

In theory, the rate of regrading 

would depend on the resistance of 

the rock and on the thickness of 

rock to be removed and would decrease as either faotor increases. 

Rock resistance is an unknown 

variable, but the fall of relative 

sea level from the raised level to 

the preearthquake level is a measure of the thickness of the material to be removed ; it is compared 

with platform width in table 2. 

Figure 32 shows the relationship of the cross-sectional area removed by regrading (A), to the 

depth through which regrading 

has had to work. 1.t can be expressed by the best-fit 

relationship (fig. 32) 

Site (flg. 17) 

P24- _ _ - - - - - - - - - - - - - - - P23_----------------- 

P22-----_---_-------- 

P21--_--------------- 

P20------------------ 

PlQ-------_---------- 

- - - - - - - - - - - - - - - - - - - - - P17--_-_---------_--- 

- - - - - - - - - - - - - - - - - - - - - P16---------------_-- 

P9----__----_-----_-- 

Dividing this expression through 

by h, the approximate width of 

the rock platform is given by the 

relationship 

CHANGE OF SEA LEVEL (h). IN FEET 

h, in feet (raised level to preearth- quake level) 

43 

40 

45 

34 

21 

23 

17 

10 

13 

12 

6 

32.-Relationship between change of sea level and crass-sectional area of material 

eroded during regrading of raised bedrock platform, Patton Bay. Points are plotted 

from data in table 2. 

The mean rate of regrading can 

be found by dividing these two 

equations by the period during 

which relative sea level rose to the 

preearthquake level. Relative sea 

level was already rising at the time 

of formation of the low sea-level 

peat, dated at 60+200 B.P. (p. 

H33). The rise of relative sea level 

had probably begun before the 

deposition of sample W-1768 in 

MacLeod Harbor at 800+200 

B.P. The period of platform cutting has therefore been taken as 

1,000 years (T) , and the estimated 

mean rate of removal of material 

has been 

z in feet (base of 

iaised beach to 

postearthquake 

mean high water) 

400 

420 

400 

350 

320 

500 

550 

650 

450 

550 

750 

A/T=2.0+0.36h sq ft per ymr 

k30 percent. 

A-h.z) in square feet 

17,200 

16,800 

18, 000 

11,900 

6,500 

11,500 

9, 200 

6, 500 

5,800 

6,600 

4,500 

The mean rate of widening of the 

platform has similarly been 

x/T=0.36+2.0/h ft per year 

+-30 percent. 

These empirical equations can 

be compared with two limiting 

ideal situations, the first a constant 

rate of removal of material, and 

the second a constant rate of lateral 

cutting. The first situation typically would exist where the cliff is 

soft and is easily eroded, or the 

cliff is very high; the process is 
IASKA EARTHQU-, MARCH 27, 19'6 

limited by the capacity of the sea 

to carry away the eroded material. 

In the notation of the equations 

above, this situation gives: 

A/T = constant, 

x/T a l/h. 

The second situation is applicable 

to mistant cliffs where the limiting factor is the power of the 

waves to undercut the cliffs, and 

the transporting capacity of the 

water is more than adequate: 

A/T a h 

x/T = constant. 

Figure 32 shows that the data 

for Patton Bay more closely fits 

the second situation. The intercept 

of the area for zero net change of 

elevation is smaller than the hdependent term and may be interpreted as the amount of material 

removed from a coast where the 

sea returned to its original level 

and merely formed a steeper offshore platform. A sufficiently 

great relative lowering of sea level 

presumably would overload the 

transporting capacity of the system and thus would approximate 

the first ideal situation, but SO 

great a change was not observed in 

the field. 

In Patton Bay the resistance of 

the rocks seems to have been the 

main control of the erosion and regrading of the rock platform to 

its preearthquake level, and the 

transporting capacity of the waves 

has been more than sufficient to 

remove the debris produced. 

REGRADING OF SAND AND 

GRAVEL 

Where the beach is in unconsolidated materials, the whole area of 

the beach below the crest of the 

newly forming storm beach has 

apparently been regraded by marine processes. It seems probable 

that the vertical range of this regrading will increase as storms of 

greater magnitude build a higher 

storm-beach ridge. 

The upper limit of the future 

marine erosion and deposition of 

sand and gravel seems to be limited 

by the final storm-beach height, 

which should approximate the preearthquake storm-beach height, 

that is, 8 to 16 feet above highwater level. The horizontal extent 

of beach resorting up the bay is 

less clear. On a rock platform, the 

shape of the platform in cross section should determine the position 

of the storm beach. The beach profiles (fig. 24) indicate khat the base 

of the storm ridge forms from 4 to 

6 feet below high-water level, or at 

about mean sea level. 

Main Sandy Bay is the best example of a beach of unconsolidated 

material, where waves could 

change the position of mean sea 

level in a horizontal direction by 

erosion or deposition. Figure 33 

shows the presumed form of the 

raised beach before the preearthquake sea level and the position of 

the preearthquake storm beach for 

profiles P11, P12, and P13. Whether the effect of the sea-level change 

was erosional (PI2 and P13) or 

depositional (Pll) , the position of 

the new storm beach is immediately at the foot of the old one, the 

horizontal distance between the 

two storm beaches being almost 

constant at 300 feet despite variations in the amount of net relative 

sea-level change. The postearthquake storm beaches in profiles P1 

and P24 also are about 300 feet 

from the old storm ridges. 

In North Sandy Bay an estimate was made of the percentage 

of the area regraded by fluvial action (even if later covered with 

alluvial fan deposits). This estimate was made for the same section of beach as the estimates of 

obliteration of the break in slope 

by landslides and gullying (p. 

H33). At the top of the beach, close 

to the cliff, 24 percent of the area 

had been regraded by fluvial action; just above the postearthquake storm beach 33 percent had 

been regraded. The extent of regrading depends on the position 

of the streams flowing off the hillsides, and the lateral extent of regrading will increase as the 

streams change course on the 

beach. The streams flowing into 

North Sandy Bay are small and 

drain very steep slopes. Regrading consists mainly of erosion of 

the beach materials, followed by 

deposition of hillside material as 

alluvial fans, often above the 

original beach level. 

The overall course of regrading 

is illustrated by the beaches in 

MacLeod Harbor (profiles 3, 5, 8, 

fig. 13) and North Sandy Bay, and 

by the raised-beach section in Patton Bay (profiles P19, P23, fig. 

17). After a raised beach or rock 

platform that is backed by a steep 

hillside has been elevated, zones of 

marine and fluvial action are divided by a storm-beach ridge. On 

a beach, the storm ridge is formed 

about 300 feet away from the old 

storm ridge. On a rock platform, 

the storm ridge is formed with its 

seaward base at about mean sea 

level. This ridge remains until it is 

demolished by wave erosion of the 

rock, after which there will only 

be a gravel beach banked against 

a bedrock step as at A in figure 31. 

Below the storm-beach ridge a new 

equilibrium beach or bedrock platform will be formed by marine action ; the exact profile of this ridge 

cannot be predicted (fig. 24). 

Above the storm beach, infilling 

by alluvial fan or colluvial material alternates with the development of soil and peat until the 

streams are able to establish semipermanent courses and equilibrium gradients through the new 

storm beach. 
HROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND 

Raised 

k,n Preearthqua ke storm beaches /(13 

Raised MHW for P12 and P13 

20 0 I 1 I I I I I 

100 200 300 400 500 600 700 800 900 1000 

HORIZONTAL DISTANCE. IN FEET 

33.-Position of preearthquake storm beaches in relation to the presumed original surface of the raised beach. Profiles of 

original surface were obtained by transposing present profilee to raised high-water marks. 

Although most of the results of 

studies on Montague Island have 

only local relevance, some conclusions may be drawn which have a 

wider application. In MacLeod 

Harbor, the 33-foot uplift of the 

beach deposits and their subsequent erosion provided a rare opportunity to observe and measure 

the actual effect of a change in sea 

level on fluvial erosion. It was 

found that the streams flowing in 

sand and silt immediately cut a 

course of almost uniform gradient 

between the old and the new sea 

levels. This phase of the readjustment occurred within a matter of 

days for all streams flowing over 

unconsolidruted materials and having low-flow discharges greater 

than about 1 cfs. Whether the 

long-term adjustment of the long 

profile would be aggradation or 

headward recession of a knickpoint could not be determined, but 

it is here suggested that the longterm adjustment may depend on 

whether the new course is less 

steep or steeper than the course 

above the former sea level. Thus, 

in mountainous regions, where the 

streams have relatively steep gradients, a fall in sea level might 

lead to either aggradation or 

erosion; there would not necessarily be a relation between 

changes of sea level and the presence of knickpoints. In lowland 

regions, however, where the lower 

courses of rivers have very low 

gradients, it is anticipated that 

knickpoints will develop and miCONCLUSIONS 

grate upstream following a relative fall in sea level. 

In addition to being an example 

of a dissected uplifted surface, the 

head of MacLeod Harbor is also 

an example of an area where 

fluvial processes are occurring so 

fast that changes may be readily 

studied. A mean rate of net sediment removal of 1 cfs in a stream 

of low-flow discharge of 300 cfs 

may be unusually rapid, but the 

fact that the erosion rate varies in 

direct proportion to low-flow discharge may have wide application, 

inasmuch as it shows the effect of 

discharge on the rate of erosion 

and the transport of uniform materials. 

The relationships between the 

preearthquake sea level and the 
H40 ALASKA EARTHQUAKE, MARCH 2 7, 19164 

beach features associated with it 

have been very clearly exposed by 

the uplift. Measurements over 

about 10 miles of coastline show 

that the height of storm beaches 

varies from 6 to 22 feet above highwater level, the mean being 12 feet. 

Where gravel was banked against 

bedrock cliffs at the top of a beach, 

the height of the break in slope 

between the gravel beach and the 

cliff varied from 2 to 15 feet above 

high-water level, the mean being 

9 feet. Therefore, neither of these 

features can be depended upon to 

indicate the relative sea level within less than 5 to 10 feet unless the 

exposure and configuration of the 

shoreline is known in unusually 

great detail. 

The area studied was not large 

enough to serve as the basis for 

general deductions about where 

raised beaches are best preserved, 

but some observations were made. 

Gently sloping shores in unconsolidated materials seem well suited to 

preservation of raised storm beaches. On rocky shores, such factors as 

rock resistance and wave exposure 

tend to influence the rates of formation and destruction of rock 

platforms in the same way, so there 

is no reason to assume that these 

factors influence preservation. In 

Patton Bay, however, the raised 

be~ach platform has in fact been 

best preserved where the rock 

seems to be most resistant (although not highly resistant), and 

the platform narrows dramatically 

and disappears where the rock becomes less resistant. Other factors 

which should theoretically aid 

beach preservation are (1) a low 

gradient on the earlier shoreline, 

which favors more rapid formation of a platform at the old sea 

level, and (2) a large uplift where 

the lowered relative sea level exposed a larger area of platform 

and thus delayed its total destruction. The influence of the slope of 

the former coast could not be 

clearly seen in the field, but 

broader platform remnants do 

seem to be associated with some of 

the areas of greater uplift. In Patton Bay, therefore, rock platforms 

seem to be best preserved in ztn area 

(1) slightly protected from wave 

attack, (2) where the rock is moderately resisbant and (3) where 

relatively great uplift has followed 

the formation of the platform. 

The 1964 deformation has provided information about the way 

in which a raised beach is modified 

after uplift. Rates of widening of 

a rock platform by erosion have 

been calculated and have been 

shown to be roughly constant at 

0.49 feet per year for cliffs of 

moderate height. The rate of recession of 0.7-2.0 feet per year for 

cliffs on Montague Island should 

indicate the order of magnitude of 

recession in other areas. Storm 

beaches form on old platforms 

and their bases are roughly at 

mean sea level ; they are converted 

to gravel banks against a step as 

the platform is eroded by the 

waves. 

Beaches of sand and gravel 

(other than bay-head deposits) 

seem to be regarded by marine action within 1 year, except for the 

upper part of the beach, where a 

new storm beach forms about 300 

feet seaward of the previous one, 

provided the sea level drops at 

least 5 feet. The new storm beach 

may take many years to attain its 

full height. Behind a storm beach, 

there is a rapid fluvial regrading 

of beach material near the mouths 

of streams and a general slow infilling by alternate layers of vegetation and alluvial and colluvial 

d e p o s i t s. Larger streams cut 

through the storm beach and drain 

part of the area behind it. Within 

a period estimated to be about 5 

years, areas of extensive deposits, 

such as the bay-head sediments of 

MacLeod Harbor, will be regraded 

mainly by fluvial action before a 

new storm beach develops. 

The break in slope between the 

top of the beach and a cliff is almost completely obscured after a 

few years unless the cliffs are particularly resistant. Talus and landslides may degrade some or all of 

the cliff, unless the rocks weather 

to fine-grained material and are 

stabilized at an early stage by vegetation. Where the top of the beach 

is backed by another older beach, 

quite large (40 ft) differences in 

sea level were found to produce 

only low (5 ft) topographic steps. 

These steps could be so obscured 

by unconsolidated beach deposits 

and vegetation that a series of 

raised beach levels might appear 

on the surface to be one continuous 

slope. 

Raised beaches formed at a 

known date and having their initial profiles almost perfectly preserved are very rare, but they did 

exist on Montague Island iininediately after the 1964 earthquake. 

Thus an ideal opportunity JYRS provided to study the rates and processes of destruction of raised 

beaches. Such studies of presently 

forming geomorphic features are 

an excellent basis for understanding and identifying similar features formed in the past. 
E'ROSION AND DGPOSITION ON A RAISED BEACH, MONTAGUE ISLAND 

Barrett, P. J., 1966, Effects of the 1964 

Alaska earthquake on some shallow-water sediments in Prince William Sound, southeast Alaska : 

Jour. Sed. Petrology, v. 36, no. 4, p. 

992-1006. 

Plafker, George, 1965, Tectonic deformation associated with the 1964 

Alaska earthquake : Science, v. 148, 

no. 3678, p. 1675-1687. 

I'lafker, George, 1967, Surface faults 

on Montague Island associated with 

the 1964 Alaska earthquake: U.S. 

Geol. Survey Prof. Paper 543-G, 

42 P. 

REFERENCES CITED 

Plafker, George, and MacNeil, F. S., 

1966, Stratigraphic significance of 

Tertiary fossils from the Orca 

Group in the Prince William Sound 

region, Alaska, in Geological Survey research 1966: U.S. Geol. Survey Prof. Paper 55@B, p. B62-B68. 

Plafker, George, and hIayo, L. R., 1965, 

Tectonic deformation, subaqueous 

slides, and destructive waves associated with the Alaska March 27, 

1964, earthquakean interim geologic evaluation : U.S. Geol. Survey 

open file report, 21 p. 

Plafker, George, and Rubin, Meyer, 

1967, Vertical tectonic displacements in south-central Alaska during and prior to the great 1964 

earthquake : Jour. Geosciences 

[Osaka City Univ.], v. 10, art. 1-7, 

p. 1-14. 

Sparks, B. W. 1960, Geomorphology: 

London, England, Longmans, Green 

and Co., Ltd., 371 p. 

Wolman, hI. G., 1954, A method of samling coarse river-bed material : Am. 

Geophys. Union Trans., v. 35, no. 

6, P. 951-956. 

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