Investigating the Landscape of Arroyo Seco—

Decoding the Past

A Teaching Guide to Climate-Controlled Landscape 

Evolution in a Tectonically Active Region

Circular 1425

Version 1.1, September 2017

U.S. Department of the Interior

U.S. Geological Survey

Investigating the Landscape of Arroyo Seco—

Decoding the Past

A Teaching Guide to Climate-Controlled Landscape 

Evolution in a Tectonically Active Region

By Emily M. Taylor, Donald S. Sweetkind,and Jeremy C. Havens

Circular 1425

Version 1.1, September 2017

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2017

Revised: September 2017 (ver. 1.1)

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visit https://store.usgs.gov.

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U.S. Government.

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

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

Suggested citation:

Taylor, E.M., Sweetkind, D.S., and Havens, J.C., 2017, Investigating the landscape of Arroyo Seco—Decoding the past—

A teaching guide to climate-controlled landscape evolution in a tectonically active region (ver. 1.1, September 2017): 

U.S. Geological Survey Circular 1425, 45 p., https://doi.org/10.3133/c1425.

Library of Congress Cataloging-in-Publication Data

Names: Taylor, Emily M., author. | Sweetkind, Donald S. (Donald Steven),

 author. | Havens, Jeremy C., author. | Geological Survey (U.S.), issuing

 body.

Title: Investigating the landscape of Arroyo Seco- decoding the past : a

 teaching guide to climate-controlled landscape evolution in a tectonically

 active region / by Emily M. Taylor, Donald S. Sweetkind, and Jeremy C.

 Havens.

Other titles: U.S. Geological Survey circular ; 1425.

Description: Reston, Virginia : U.S. Geological Survey, 2017. | Series:

 Circular ; 1425 | Includes bibliographical references.

Identifiers: LCCN 2017009447 | ISBN 9781411341272 (pbk.)

Subjects: LCSH: Seco, Arroyo (Monterey County, Calif. : River) | Fluvial

 geomorphology--California--Monterey County. | Landscape

 changes--California--Santa Lucia Range. | Climatic

 changes--California--Santa Lucia Range.

Classification: LCC GB565.C2 T39 2017 | DDC 557.94/76--dc23 | SUDOC I

 19.4/2:1425

LC record available at https://lccn.loc.gov/2017009447

ISSN 1067-084X (print) 

ISSN 2330-5703 (online)
iii

Acknowledgments

Thank you to the many families in Arroyo Seco who provided permission for land access. With the exception of Arroyo Seco 

recreation area, land is private, and access is difficult. Thank you also to the Big Sur Land Trust for allowing us into Arroyo 

Seco Ranch. Lew Rosenberg and John Tinsley provided valuable time and ideas talking about work they had completed in 

Arroyo Seco. Thank you to Dan Sudan, director of the San Francisco-based Mission Science Workshop, for suggesting this 

project to make geology and climate change more accessible to middle and high school students. 
iv

Contents

Acknowledgments........................................................................................................................................iii

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

Background.....................................................................................................................................................3

Physical and Geologic Setting of Arroyo Seco.........................................................................................4

Strath Terraces in Arroyo Seco.................................................................................................................11

Alluvial Fans and the Salinas River Valley...............................................................................................16

Landscape Evolution Resulting from Changes in Global Climate 

and the Effects of Active Tectonics.............................................................................................18

Climate Cycles and Their Effect on Global Sea Level...................................................................18

Marine Sediments—A Geologic Record of Past Climate Cycles ...............................................21

Geologic and Climatic History Recorded by Marine Terraces....................................................24

Tying Arroyo Seco Terraces to the Coastal Record......................................................................26

Road Log Map...............................................................................................................................................28

Road Log...............................................................................................................................................30

Stop 1 ....................................................................................................................................................31

Drive from Arroyo Seco Bridge to Arroyo Seco Recreation Area..............................................32

Stop 2 ....................................................................................................................................................35

Stop 3 ....................................................................................................................................................36

Stop 4 ....................................................................................................................................................37

Stop 5 ....................................................................................................................................................38

Stop 6 ....................................................................................................................................................39

Stop 7 ....................................................................................................................................................39

Selected References...................................................................................................................................40

Faulting History....................................................................................................................................40

Geologic Map and Strath Terrace Formation.................................................................................40

Geologic Reconstruction of Arroyo Seco Valley and the Salinas River Valley ........................40

Glacial Interglacial Cycles ................................................................................................................41

Sea-Level Fluctuations and Terrace Formation.............................................................................41

Glossary of Geologic Terms .......................................................................................................................42

Figures

1. Map of the location of Arroyo Seco ..........................................................................................2

2. Map showing a perspective view of Arroyo Seco, looking upstream, with 

terrace deposits............................................................................................................................3

3. Schematic diagram of strath terrace formation......................................................................5

4. Quaternary geologic map of Arroyo Seco, Monterey County, California............................6

5. Photographs of a narrow canyon formed in pre-Cenozoic basement rocks 

and the broad valley formed in the Monterey Formation.......................................................9

6. Contoured map showing the elevation of the top of pre-Cenozoic rocks prior 

to offset on the Reliz and Rinconada faults and present faulted configuration...............10

7. Photographs of exposures of rounded gravel deposits .......................................................11

8. Schematic diagrams of physical characteristics of Arroyo Seco strath terrace 

evolution as stream gradient decreased over time ...................................................................13
v

9. Schematic diagram showing development of soil horizons for time-related 

development of soil horizons and schematic cross section showing 

development of soil profiles on terraces in Arroyo Seco ....................................................15

10. Schematic diagram of hydrologic cycle and map of the distribution of ice during 

cold wet cycles (glacial events) and warm dry cycles (interglacial events) ...................19

11. Map of the distribution of ice and vegetation density on earth in the last glacial

maximum and present day ........................................................................................................20

12. Marine cores are collected and sampled for analysis to distinguish past climates...........21

13. Graphs and table showing geologic time versus 18O curve and sea level........................23

14. Photographs of the modern marine beach and sea cliff at Capitola and 

ancient marine terraces at Watsonville..................................................................................24

15. Schematic diagram of marine terrace evolution in tectonically stable and 

actively faulting environments .................................................................................................25

16. Field guide map ...........................................................................................................................28

17. Photograph of Reliz fault zone exposure................................................................................31

18. Photograph of the active drainage in Arroyo Seco ..............................................................31

19. Photographs of Monterey Formation exposed in road cuts and natural exposures...........33

20. Arroyo Seco recreation area map ...........................................................................................34

21. Photograph of terrace gravel deposit on basement rocks..................................................35

22. Photograph of the Civilian Conservation Corps Camp Arroyo Seco, 1936 ........................36

Conversion Factors

Inch/pound to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Investigating the Landscape of Arroyo Seco—

Decoding the Past

A Teaching Guide to Climate-Controlled Landscape 

Evolution in a Tectonically Active Region

By Emily M. Taylor,1 Donald S. Sweetkind,1 and Jeremy C. Havens2

Introduction

Arroyo Seco is a river that flows eastward out of the 

Santa Lucia Range in Monterey County, California. The Santa 

Lucia Range is considered part of the central California Coast 

Range. Arroyo Seco flows out of the Santa Lucia Range into 

the Salinas River valley, near the town of Greenfield, where 

it joins the Salinas River. The Salinas River flows north into 

Monterey Bay about 40 miles from where it merges with 

Arroyo Seco. In the mountain range, Arroyo Seco has cut 

or eroded a broad and deep valley. This valley preserves a 

geologic story in the landscape that is influenced by both 

fault-controlled mountain building (tectonics) and sea level 

fluctuations (regional climate).

Broad flat surfaces called river terraces, once eroded by 

Arroyo Seco, can be observed along the modern drainage. 

In the valley, terraces are also preserved like climbing stairs 

up to 1,800 feet above Arroyo Seco today. These terraces 

mark where Arroyo Seco once flowed. The terraces were 

formed by the river because no matter how high they are, the 

terraces are covered by gravel deposits exactly like those that 

can be observed in the river today. The Santa Lucia Range, 

Arroyo Seco, and the Salinas River valley must have looked 

very different when the highest and oldest terraces were 

forming. The Santa Lucia Range may have been lower, the 

Arroyo Seco may have been steeper and wider, and the Salinas 

River valley may have been much smaller.

Arroyo Seco, like all rivers, is always changing. Sometimes rivers flow very straight, and sometimes they are curvy. 

Sometimes rivers are cutting down or eroding the landscape, 

and sometimes they are not eroding but depositing material. 

Sometimes rivers are neither eroding nor transporting material. 

The influences that change the behavior of Arroyo Seco are 

mountain uplift caused by fault moment and sea level changes 

driven by regional climate change. When a stream is affected 

by one or both of these influences, the stream accommodates 

the change by eroding, depositing, and (or) changing its shape. 

In the vicinity of Arroyo Seco, the geologically young 

faulting history is relatively well understood. Geologists have 

some sense of the most recent faulting event and of the faulting in the recent geologic past. The timing of regional climate 

changes is also well accepted. In this area, warm climate 

cycles tend to cause the sea level to rise, and cool climate 

cycles tend to cause the sea level to fall. If we understand 

the way the terraces form and their ages in Arroyo Seco, we 

can draw conclusions about whether faulting and (or) climate 

contributed to their formation.

This publication serves as a descriptive companion to the 

formal geologic map of Arroyo Seco (Taylor and Sweetkind, 

2014) and is intended for use by nonscientists and students. 

Included is a discussion of the processes that controlled the 

evolution of the drainage and the formation of the terraces in 

Arroyo Seco. The reader is guided to well-exposed landscape 

features in an easily accessible environment that will help 

nonscientists gain an understanding of how features on a geologic map are interpreted in terms of earth processes.

1

U.S. Geological Survey

2

ADC Management Services, Inc.
2 Investigating the Landscape of Arroyo Seco—Decoding the Past

Figure 1. Map of the location of Arroyo Seco.

Monterey

Bay

PACIFIC

OCEAN

San Antonio 

Reservoir

Nacimiento 

Reservoir

King City

Salinas

Paso Robles

Monterey

Santa Cruz Capitola

Watsonville

SAN LUIS 

OBISPO COUNTY

MONTEREY 

COUNTY

SAN BENITO 

COUNTY

SANTA CRUZ

COUNTY

SANTA CLARA

COUNTY

MERCED

COUNTY

FRESNO

COUNTY

Salinas

Gabilan Range

FAULT

ZONE

River

Carmel

River

Rive

T

r

ularcitos

Salinas

River

Salinas River

Santa Lucia Range

TULARCITOS

Sierra de Salinas

RINCONADA

SAN GREGORIO

SAN

ANDREAS

FAULT

ZONE

ZONE

FAULT

Carmel

Valley

Greenfield

Valley

122°00' 121°30' 121°00' 120°30'

37°00'

36°30'

36°00'

35°30'

RELIZ FAULT

Castroville

CALIFORNIA

Map area

NEVADA

PACIFIC

OCEAN

San Francisco

Los Angeles

EXPLANATION

FAULT

ZONE

Area of published geologic

 map of Arroyo Seco

 (Taylor and Sweetkind, 2014)

Fault

River, drainage, or reservoir

0 10 20 MILES

0 10 20 KILOMETERS

101

101

Area of

geologic

map Arroyo Seco

Monroe

Canyon

Gonzales

Base map data from Google, 

DigitalGlobe 2015,

Image date April 9, 2013

Fault lines from Jennings and Bryant (2010)
Background 3

Background

Arroyo Seco is a perennial drainage (text in bold is 

defined in the glossary) in the Santa Lucia Range of the central 

Coast Ranges of California and is one of the largest tributary 

drainages of the Salinas River (fig. 1). In Arroyo Seco, the 

geomorphic features record the complex evolution of a 

stream drainage in the Coast Ranges. Here, one can observe a 

sequence of strath or abandoned erosional terrace surfaces 

carved into an upward-migrating mountain range (Taylor and 

Sweetkind, 2014). Strath terraces exposed in Arroyo Seco are 

spectacular examples of an environment altered over a long 

period of geologic time by processes controlled by global 

climate change and regional tectonics. Not every environment responds to climate change and tectonics in the same 

way, although parts of the processes observed in Arroyo Seco 

are shared in many environments. This guide provides a general introduction to these geologic processes as well as a road 

log that can be used to locate examples of features formed by 

the described processes in the field.

A recently published geologic map of Arroyo Seco 

includes a more regional map centered on Arroyo Seco, 

mapped at a scale of 1:50,000, and an inset map that focuses 

on the Arroyo Seco drainage, mapped at a scale of 1:24,000 

(Taylor and Sweetkind, 2014). On these maps, terrace deposits 

have been identified and correlated using a variety of geologic 

mapping techniques and digital datasets. Figure 2 shows an 

example, in perspective view, of the geologic mapping of 

the terrace deposits, with terraces of similar age shown in 

the same color and label, as viewed looking upstream up the 

Arroyo Seco drainage.

During the course of the geologic mapping, several local 

stakeholders expressed interest in a layperson’s guide to the 

geology and evolution of the Arroyo Seco drainage for use as 

a teaching and explanatory tool. This publication serves as a 

descriptive companion to the formal geologic map of Arroyo 

Seco (Taylor and Sweetkind, 2014) and is intended for use by 

nonscientists and students. In it, the reader will be guided to 

well-exposed geomorphic features in an easily accessible environment that will help the nonscientist gain an understanding 

of how features on a geologic map are interpreted in terms of 

earth processes.

This guide includes

•	 A discussion of the processes that controlled the evolution of the drainage and the formation of the terraces 

in Arroyo Seco. The majority of the discussion is 

specific to the processes that occur here and not in 

every geologic environment. Readers will learn how 

geologists interpret observations in terms of earth 

processes and link physical features to other lines of 

evidence related to past climate and tectonic activity.

•	 A description of the types of geologic features 

observed, their shape and extent, and the criteria for 

mapping and identifying specific geologic features.

•	 A road log that describes a driving route and short 

hikes to view examples of geomorphic features and 

representative exposures.

Figure 2. Perspective view of Arroyo 

Seco, looking upstream, with terrace 

deposits mapped by Taylor and 

Sweetkind (2014).

G16

Perspective view looking to the west at an elevation of about 6,000 feet. Geology from Taylor and Sweetkind (2014)

Scale is variable because of the effect of perspective; field of view at the

lower edge of the image is about 1,100 feet

Base map data from Google, 

DigitalGlobe 2015,

Image date December 13, 2015,

VERTICAL EXAGGERATION ×1

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ARROYO SECO ROAD
4 Investigating the Landscape of Arroyo Seco—Decoding the Past

Physical and Geologic Setting of Arroyo Seco

Geologists create geologic maps by studying the location, 

distribution, and physical and chemical properties of rocks and 

sediments. These rocks and sediments are typically exposed at the 

surface, but they can also be studied in the subsurface by drilling 

boreholes. Rocks and sediments are altered over time by weathering, erosion, and faulting. Weathering alters rocks and sediments, 

erosion transports weathered rocks by water and gravity, and 

faults rearrange rocks and sediments. Geologists can distinguish 

the effects of geologic time on different kinds of rocks and sediments over time. They interpret and distinguish the effects of 

tectonic activity, faulting, and physical and chemical weathering. 

A geologic map attempts to separate different geologic units in 

a meaningful way so that geologists can better understand the 

evolution of the regional rocks and sediments. Our goal was to 

better understand the formation of the strath terraces that are 

easily observed in the broad valley and surrounding ranges at 

Arroyo Seco.

Strath terraces are surfaces cut into bedrock by eroding 

streams. Streams commonly alternate between periods of eroding 

(or downcutting), and periods of depositing (or accumulation) 

of sediments (fig. 3A). Stream terraces form during pauses in the 

downcutting processes so each terrace level records the profile of 

the stream. An eroding episode typically ends with a prolonged 

period where the stream stays at the same position in the landscape (stream equilibrium), and a flood plain is formed. Stream 

gravel is both eroding the bedrock on the flood plain and being 

transported downstream (fig. 3B). This relatively brief pause 

when the flood plain is forming is followed by renewed streamchannel downcutting that preserves the beveled terrace surface 

(Bull, 2007). A change takes place for climatic and (or) tectonic 

reasons, and the stream or river cuts down to a new level, leaving the former flood plain as a strath terrace, underlain by a thin 

veneer of gravel (fig. 3C). This continues to happen episodically, 

and the result is strath terraces at different elevations in the valley 

with higher terraces being older (fig. 3D). The challenge to the 

geologist is to figure out the terrace ages, the relation between 

terraces across the drainage, and establish whether climate or 

tectonics drives the formation of the terrace (fig. 3E).

On the geologic map of the Arroyo Seco drainage, different colored polygons represent different aged geologic units and 

are labeled according to the identity of the unit (figs. 2 and 4). 

Note that deposits associated with each strath terrace consist of 

many separated polygons of the same unit (same color and label). 

These now discontinuous features record the presence in the past 

of a once-continuous surface that was an older flood plain of 

Arroyo Seco, now uplifted and dissected by erosion. Older terrace 

surfaces are higher in elevation than the youngest terrace adjacent 

to Arroyo Seco. In addition to the mapped terrace deposits, the 

surrounding bedrock is mapped as undifferentiated basement 

rock shown in purple in the southwestern part of the map (labeled 

as MzPzub on fig. 4) and the Monterey Formation in brown 

elsewhere on the map (labeled Tms on fig. 4).
Physical and Geologic Setting of Arroyo Seco 5

Although Arroyo Seco extends well up into the Santa 

Lucia Range, the region of interest (and covered by this field 

guide) extends from the contact of the resistant basement rocks 

(fig. 4, MzPzub), which underlie the higher parts of the Santa 

Lucia Range, and the downstream end where Arroyo Seco 

leaves the mountains to join the Salinas River. For the most 

part, the stream section runs through the comparatively softer 

and more easily eroded sedimentary rocks of the Monterey 

Formation (fig. 4, Tms). The different rock types play an important role in the evolution and resulting shape of the eroded valley where Arroyo Seco flows. Arroyo Seco is constrained to a 

narrow canyon where it flows through resistant basement rocks 

(figs. 5A and 5B), but opens eastward to a broad, 11-mile long 

valley with broad terraces where the drainage flows over of the 

Monterey Formation (figs. 5C and 5D). Arroyo Seco declines in 

elevation from about 850 feet to 150 feet in the area of interest.

Figure 3. Strath terrace formation.

A

E

B C D

Strath

terrace Qf2

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same age?

Flood plain is abandoned

and strath terrace forms Over time multiple strath terraces form

River cuts down Flood plain forms

Braided

channel

Tectonic and

climatic straths

cut in bedrock

Cut terrace

Alluvial fan

flanking valley

Flood plain Salinas

River

Minor buried

strath terrace

Internal adjustment of the Arroyo Seco

Alluvial fan eroded to

form a fill terrace

Modified from Bull, 2007, fig. 2.15B, p. 43.

Salinas River at the

base level of erosion

Climate

aggradation

surfaces

RELIZ FAULT

Terrace gravel

Bedrock

Valley fill

Alluvial fan

EXPLANATION
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Carmel

2

Base from U.S. Geological Survey 1:100,000-scale 

digital data, 1982, Universal Transverse Mercator, 

zone 10, North American Datum of 1983 (NAD 83).

121°

30’

36°14’

28’

24’

26’

121°

30’

28’

28’

16’

26’

28’

24’

WILLOW

CREEK FAULT FAULT

ARROYO SECO

Piney Creek

Paloma

Cree k

Arroyo Seco

recreation

area entrance

STOP 2

STOP 4

STOP 5

STOP 3

STOP 6

STOP 5

Road trip stop with mileage 

 (mileage begins at Arroyo 

 Seco recreation area entrance)

Faults (dashed where concealed)

Major rivers and drainages

Contact

Road trip point of interest

Approximate location and 

 orientation of figure 2

EXPLANATION

6 Investigating the Landscape of Arroyo Seco—Decoding the Past

Figure 4. Quaternary geologic map of Arroyo Seco, Monterey County, California. Modified from Taylor and Sweetkind, 2014.
APPROXIMATE MEAN

DECLINATION, 2014

Map location

CALIFORNIA

13 1⁄2°

TRUE NORTH

MAGNETIC

NORTH

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2

Greenfield

16’

24’

24’

22’

20’

121°18’

36°18’ 36°18’

121°

18’

22’

20’ 36°14’

RELIZ

FAULT

CONTOUR INTERVAL 33 FEET (10 METERS)

NATIONAL GEODETIC VERTICAL DATUM OF 1929

0 0.5 1 KILOMETER

0 0.5 1 MILE

STOP 6

STOP 7

STOP 1

Physical and Geologic Setting of Arroyo Seco 7

Quaternary geologic map 

of Arroyo Seco, 

Monterey County, California
8 Investigating the Landscape of Arroyo Seco—Decoding the Past

Figure 4. Quaternary geologic map of Arroyo Seco, Monterey County, California.—Continued.

QUATERNARY DEPOSITS BEDROCK

Holocene

Pleistocene

Late

Pleistocene

Middle 

Pleistocene

Early 

Pleistocene

Pliocene

Paleocene

QUATERNARY

(Q)

TERTIARY

(T)

MESOZOIC–

PALEOZOIC

(MzPz)

Qfr2

Fluvial

deposits

 Fluvial and

alluvial-fan 

deposits

Marine

sediments

[Angled boxes represent estimated range of age uncertainty; Holocene and Pleistocene subdivisions and boundary ages

are from Gibbard and others, 2010, p. 99]

CORRELATION OF MAP UNITS

Basement

Eolian

deposits

Qfr1

Qfy2

Qei

}|ub

Qfai3

Qao

Qfy1

Qfo8–

Qfo1

QTpr

Oligocene

Miocene

Eocene

Terrestrial

sediments

Qfai2y

Qfay

Qfai4

Qcb

Tpr

Tsa

Tms

Qfai2o

Qfai1

2,588,000 years

781,000 years

126,000 years

11,700±99 years

Qfo8

Qfo7

Qfo5

Qfo4

Qfo3

Qfo2

Qfo1

Qfo6

Cenozoic

pre-Cenozoic
Physical and Geologic Setting of Arroyo Seco 9

The Monterey Formation exposed in Arroyo Seco is 

bounded on the east by the Reliz fault, a north-south trending 

fault on the west side of the Salinas River valley (figs. 1 and 4; 

Rosenberg and Clark, 2009). Vertical offset on this fault is one 

of the causes of changes in base level contributing to the formation of strath terraces in Arroyo Seco (Finnegan and Balco, 

2013). The Reliz fault begins 5 miles south of Arroyo Seco 

at Monroe Canyon and continues northward along the west 

side of the Salinas River valley to Monterey Bay (fig. 1). The 

Reliz fault extends north and slightly east from the terminus of 

the Rinconada fault (fig. 1), a much longer fault that extends 

from Monroe Canyon southeast for 140 miles. These faults are 

mapped as separate features because they have different structural characteristics, do not respond to offset in the same way, 

and ruptured at different times in the past. The Rinconada fault 

moves predominantly in a horizontal direction (strike-slip), 

and the Reliz fault moves predominantly in a vertical direction 

(dip-slip). The Rinconada fault has evidence for recent offset; 

in contrast, the last offset on the Reliz fault occurred more 

than 100,000 years ago (Rosenberg and Clark, 2009).

Figure 5. A narrow canyon (A) formed in pre-Cenozoic basement rocks (B) and the broad valley (C) formed in the Tertiary Monterey 

Formation (D).

Photographs by E.M. Taylor, U.S. Geological Survey, March 2015

A B

C D
10 Investigating the Landscape of Arroyo Seco—Decoding the Past

The landscape around Arroyo Seco has looked different in the geologic past (Durham, 1974; Dibblee, 1976; 

Graham, 1978). The area that is now called Arroyo Seco 

was once a deep trough (fig. 6A) in the pre-Cenozoic basement rocks and later filled in with a thick accumulation of 

marine sediments that form the Monterey Formation. This 

deep trough was mapped in the subsurface using evidence 

from the geology exposed at the surface and from the depths 

at which pre-Cenozoic basement rocks are intercepted in 

deep drill holes (fig. 6B). Horizontal (strike-slip) motion on 

the Rinconada fault has split the once-continuous trough 

and offset it nearly 24 miles (fig. 6B). The marine sediments 

deposited in the deep trough form the Monterey Formation 

and are up to 12,000 feet thick in Arroyo Seco and to the 

east are buried by younger sediments that fill the Salinas 

River valley.

Figure 6. Contoured map showing the elevation of the top of pre-Cenozoic rocks: A, prior to offset on the Reliz and Rinconada faults and 

B, present faulted configuration.

FUTURE RINCONADA FAULT

FUTURE NACIMIENTO FAULT

Junipero Serra

Uplift

Arroyo Seco Trough

South Salinas Basin

Coast Ridge Uplift

Paso Robles High

?

?

?

?

?

?

Lockwood High

SAN ANDREAS FAULT

FUTURE SAN ANDREAS FAULT

South Salinas Basin

−4000

0 −12,000

−4000

−6000

Paso Robles

San Ardo

King City

Jolon

RELIZ

RINCONADA FAULT

NACIMIENTO FAULT

Arroyo Seco Trough

Coast Ridge Uplift −8000

?

?

?

?

?

? −12000

−4000

−2000

0

0

−4000

−8000

−6000

−10000

Junipero Serra

Uplift

FAULT

0

Paso Robles High

Lockwood

High

FUTURE RELIZ FAULT

?

?

?

?

? ? ? ?

36°

35°45’

35°30’

36°15’

121°30’ 121°15’ 121° 120°45’

Exposed pre-Cenozoic rocks

Well that intersects pre-Cenozioc rocks

Paleotopography Low

EXPLANATION

High

Fault

Inferred fault

0

−2000

2000

−4000

−6000

−8000

−10,000

−12,000 Elevation, in feet

EXPLANATION

Modified from Durham (1974), Dibblee (1976), and Graham (1978)

Contour on top of buried pre-Cenozoic 

 rocks, dashed where inferred. 

 Queried where uncertain. Contour 

 interval 2,000 feet, data mean sea level

−2000

Pre-Cenozoic rocks

Contour on top of pre-Cenozoic

 rocks. Contours are not

 labeled; they schematically

 show paleotopographic

 highs and lows

Future trace of fault

0 5 KILOMETERS

0 5 MILES

0 5 KILOMETERS

0 5 MILES

Top of pre-Cenozoic basement rocks—

before fault offset

A Buried top of pre-Cenozoic basement rocks—

 present faulted configuration

B
Strath Terraces in Arroyo Seco 11

Strath Terraces in Arroyo Seco 

The erosional strath terrace is the fundamental geomorphic feature in Arroyo Seco and was the focus of the 

Arroyo Seco geologic map (fig. 4). Strath terraces provide 

an important record for interpreting tectonic and climate 

history. In many actively incising river canyons, gravelcapped bedrock strath terraces are tens to hundreds of 

feet above the active channel and extend discontinuously 

for miles. Strath terraces represent ancient and nowabandoned river flood plains that formed during periods 

of valley-bottom widening and planation (DeVecchio and 

others, 2012; Finnegan and Balco, 2013).

Strath terraces preserve the incision history of 

Arroyo Seco. Older strath-terrace deposits (shown in 

shades of green in figs. 2 and 4, map units Qfo1–Qfo8)

are preserved to elevations as much as 1,800 feet above 

the modern drainage whereas younger terraces (shown 

in shades of orange and yellow in figs. 2 and 4) are from 

500 to less than 3 feet above the modern drainage. The 

terraces are capped by coarse, rounded gravel consisting of 

granitic rocks (figs. 3E and 7A) derived from the upstream 

basement rocks in the Santa Lucia Range. These gravelly 

alluvial deposits are typically less than 6 feet thick and lie 

directly on the bedrock; the contact between the alluvial 

deposits and the bedrock is clear and abrupt (figs. 3E and 

7B). Road cuts and natural exposures in Arroyo Seco also 

expose younger alluvium deposited by tributary drainages 

in some drainages and colluvium that bury terrace gravel 

(figs. 3E and 7C).

When looking at a stream or river, geologists consider the shape of the river to help understand the history 

of the drainage. Stream channel shapes or patterns are 

straight, meandering, braided, or a combination of these 

shapes. Pattern is controlled by gradient (valley slope), 

sediment load (amount of gravel, sand, silt, and clay) and 

stream power (amount of water and velocity). Channel patterns have been classified by experimentally varying these 

parameters (Schumm and others, 2000). By understanding 

what we observe experimentally, we can say something 

about what we observe in nature. For example, braided 

streams are defined by a broad flood plain with multiple 

shallow channels, high stream gradient, and coarse sediment load, whereas meandering streams may also have a 

broad flood plain but are typically confined to a narrow, 

deep channel and transport much finer sediment.

Figure 7. Exposures of rounded gravel deposits as 

follows: A, rounded gravel deposits can be observed on 

terrace surfaces; B, road cut exposure of gravel directly 

overlying the Monterey Formation; and C, road cut 

exposure of gravel buried by younger fine-grained alluvium.

A

B

C

Terrace gravel 

Monterey Formation

Terrace gravel

Contact

Contact

Photographs by E.M. Taylor, U.S. Geological Survey, April 2010

Young alluvium from

tributary drainage
12 Investigating the Landscape of Arroyo Seco—Decoding the Past

Strath terraces preserved well above the modern drainage 

and occurring at the same elevations on both sides of Arroyo 

Seco suggest the previous existence of a wide, erosive channel cutting a broad flood plain. In the past, the Arroyo Seco 

drainage appears to have consisted of a large braided network 

of channels that migrated across a broad flood plain, reflecting 

a high stream gradient and a coarse bedload (fig. 8A). Today, 

Arroyo Seco is a form of braided stream called a single channel, gravel-bed river—a braided system with the reaches of a 

single channel, indicating that the factors controlling stream 

channel shape have changed. The gradient has decreased, the 

bedload has decreased, and the stream power has potentially 

increased. In places over time, as the stream gradient decreased, 

the Arroyo Seco flood plain evolved into a single channel and 

confined to a narrow river system with steep banks (fig. 8B).

The observed rounded gravel at Arroyo Seco is consistent 

with the model that alluvium smooths the stream bed and remains 

as a thin mantle when the surface is abandoned. Braided stream 

systems typically deposit thin mantles of gravel. Three independent actions occur over time to form and abandon a strath terrace: 

(1) the depositional cycle—increased bed load and decreased 

stream power or the amount of water, (2) the erosion cycle—the 

formation of the flood plain caused by increased lateral erosion 

and decreased vertical incision, followed by (3) vertical incision 

and abandonment of the flood plain (Pazzaglia, 2013).

Factors that control the shape of a stream are directly 

controlled by tectonics and (or) climate. Lateral erosion and 

formation of strath terraces begin when sediment is available to 

scour and erode the stream bed (fig. 8). Abrasion of bedrock in 

a channel only occurs if sediment is available. Sediment tends 

to be available when climate changes from a relatively cooler, 

wetter climate to a warmer, drier climate because decreases in 

vegetation during drier climates result in bare slopes that are 

easily eroded.

Strath terraces preserved 

well above the modern drainage 

and occurring at the same elevations 

on both sides of Arroyo Seco 

suggest the previous existence 

of a wide, erosive channel 

cutting a broad flood plain.
Strath Terraces in Arroyo Seco 13

Figure 8. Physical characteristics of Arroyo Seco strath terrace evolution as stream gradient decreased over time.

Terrace/flood plain remnants

Bedrock

Fluvial gravel and sediment

EXPLANATION

A

B

Braided river

• High stream gradient

• Coarse bedload • Broad and active flood plain

• Stream gradient decreases

Alluvial fan
14 Investigating the Landscape of Arroyo Seco—Decoding the Past

Once a surface is abandoned by a stream, soil formation can begin on the terrace surface, but soils develop 

slowly (Birkeland, 1999). Unconsolidated gravel begins 

to weather by physical and chemical processes that break 

down the gravel to smaller mineral fragments. Windblown 

dust begins to infiltrate into the weathering gravel. In 

the most general terms, soils develop layers in a vertical 

sequence over time called A, B, and C soil horizons, and 

their sum constitutes a soil profile. Soil A horizons consist 

of the accumulation of decomposing organic matter mixed 

with the mineral fraction. Soil B horizons, below the 

A horizons, tend to be weathered from the original sediment or rock and include materials moved downward from 

the overlying horizons and deposited in the B horizon. 

Soil C horizons lack properties of the A and B horizons 

and are most like the original sediment or rock where the 

soil is developed.

Soils exhibit a development sequence with time 

(fig. 9A; Birkeland, 1999). Young soils are poorly consolidated, have little or no soil horizons, and retain many 

of the characteristics of the original deposit. Older young 

soils have an A horizon over a C horizon. A clay-enriched 

B horizon forms over time, and the total amount of clay 

increases. Gravel clasts in the soils also show progressive 

weathering with time.

In Arroyo Seco, the soils formed on the strath 

terraces demonstrate a clear progression of soil characteristics over time (fig. 9B). Soils less than 9,000 years 

old (Qfay) (IRSL from Taylor and Sweetkind, 2014) 

consist mainly of an A horizon. Older soils become more 

cemented and exhibit soil horizons that are progressively 

thicker and B horizon colors change from yellow-brown 

to reddish-brown to red. B horizon clay content also 

increases and reaches a maximum of 60 percent in the 

oldest soils. Soils older than 120,000 years old (IRSL 

from Taylor and Sweetkind, 2014) have decomposed 

granitic clasts. The oldest terrace remnants (Qao) tend to 

be eroded, and thus the soils are eroded and thin (fig. 9B). 

The Arroyo Seco geologic map (Taylor and Sweetkind, 

2014) contains a detailed discussion of the ages of the 

soils and terrace deposits.

Once a terrace surface

is abandoned by Arroyo Seco, 

soil formation can begin. 

Older soils look very different 

than younger soils.
Strath Terraces in Arroyo Seco 15

Figure 9. Development of soil horizons for A, time-related development of soil horizons and B, schematic cross section showing 

development of soil profiles on terraces in Arroyo Seco.

Qfr

Flood plain

Qfy

3,000–

11,000

years

old

Qfai4

9,000 

years old

Qfai3

35,000–40,000

years old

115,000–126,000 years old

Qfai2

45,000–65,000 

years old

Qfai1

126,000–300,000? years old

Qao

Holocene Late Pleistocene Middle Pleistocene

Bedrock Bedrock Bedrock Bedrock

Organic matter

A horizon A horizon

B horizon

C horizon C horizon

Increasing age of soils

Increasing distance above Arroyo Seco

Bedrock

Eolian silt in profile

Argillic B horizon

Secondary calcium 

 carbonate or silica

Gravel

EXPLANATION

A

B

Arroyo

Seco

SCHEMATIC NOT TO SCALE

River gravel scours bedrock on 

the flood plain, which is abandoned 

by the river and leaves gravel in 

abrupt contact with bedrock.

Rainfall washes dust into gravel. 

Dust and gravel begins to weather 

into mineral fragments of sand, silt, 

and clay. Vegetation is established 

and decomposes over time.

Soil horizons begin to form as

mineral fragments physically and 

chemically weather.

Developed soil has discrete 

horizons. Clay accumulates in the 

B horizon, and in time that horizon 

gets more enriched in clay, getting 

redder and thicker.
16 Investigating the Landscape of Arroyo Seco—Decoding the Past

Alluvial Fans and the Salinas River Valley

Where Arroyo Seco enters the Salinas River valley, east 

of the Reliz fault, the river deposits the sediments eroded from 

upstream as a broad alluvial fan. Some of the younger alluvial 

fans can be correlated to the ages of terraces because the material that eroded to form the strath terraces has not been buried. 

The predominant terrace levels (Qfai3 and Qfai4; fig. 4), 80 to 

100 feet (ft) above the active drainage, correlate with a large 

alluvial fan adjacent to the range front and east of the Reliz 

fault. Both erosion of the strath terrace and deposition of the 

fan occurred at the same time. Some younger terrace and alluvial fan deposits were formed at the same time.

We know from drill hole data that the Salinas River 

valley is a sedimentary basin filled with marine sediments

and terrestrial sediments. Below the sediments, at depth in 

the Salinas Valley, are ancient basement rocks similar to those 

exposed today at high elevations in the Santa Lucia Range. 

Arroyo Seco has eroded an extensive valley that is the source 

of some of the material that fills the Salinas River valley. 

Preserved stream-terrace deposits within and near sedimentary basins are a record of the erosional history that provide 

a context for downstream basin aggradation; basin-margin, 

fault-related uplift; and paleoclimate events. Understanding 

the erosion of sediments from Arroyo Seco is a significant 

contribution to understanding the evolution of sedimentation 

in the northern part of the Salinas River valley. Arroyo Seco 

strath terraces mark stable periods between base-level changes 

driven by tectonics or sea-level fluctuations, an indirect result 

of climate change.

Understanding

the erosion of sediments from 

Arroyo Seco is a significant 

contribution to understanding 

the evolution of sedimentation 

in the northern part 

of the Salinas River valley.
Sali nas Ri ver

Alluvial

fans

Arroyo Seco

A

r

royo

Se

co

Alluvial Fans and the Salinas River Valley 17
18 Investigating the Landscape of Arroyo Seco—Decoding the Past

Landscape Evolution Resulting from Changes in Global Climate 

and the Effects of Active Tectonics

Terrace treads are capped by gravel that records the past existence of 

a broad stream flood plain. Subsequent stream erosion into bedrock and 

abandonment of the old flood plain indicates a change in stream equilibrium, probably in response to changes in the base level. In Arroyo Seco, 

both mountain uplift by fault movement and base-level changes driven by 

climate change contributed to the timing of terrace formation. We know 

that the Reliz fault (fig. 4) has had some movement during the time the 

terraces were forming (Rosenberg and Clark, 2009; Finnegan and Balco, 

2013), and we know that during that same time period, the sea level has 

fluctuated as a result of global climate (Alexander, 1953; Bradley and 

Addicott, 1968; Imbrie and others, 1984), resulting in periods of rapid 

incision downstream of the Reliz fault (Bull, 1990). Both tectonics and 

climate change have directly influenced the formation of strath terraces in 

Arroyo Seco.

Climate Cycles and Their Effect on Global Sea Level

The amount of water on Earth is always the same and is constantly 

cycling between the sea, the land, and the sky (fig. 10A). This cycle of 

evaporation, transpiration, and precipitation is referred to as the hydrologic 

cycle. Water changes phase or form as it moves through the hydrologic 

cycle. Water is most often a liquid, sometimes it is solid in the form of ice 

and snow, and sometimes it is a gas in the form of water vapor (fig. 10A).

The fluctuations in global climate change where water is stored on 

Earth. During relatively warm climate cycles, water is liquid and stored 

primarily in the oceans. During relatively cold climate cycles, precipitation is frozen in the form of snow and is stored as ice. During the last 

glacial maximum, about 18,000 years ago, about 73 percent of the fresh 

water was stored in the form of ice; today, the amount of fresh water 

stored in the form of ice has decreased to about 69 percent (fig. 10B). 

Recent estimates of the last glacial maximum are as old as 19,000 to 

20,000 years ago (Clarke and others, 2009).

During cooler climate cycles, water is stored in glaciers and ice 

caps at high latitudes (closer to the north and south poles) and elevations 

(height above sea level) (fig. 11A). During the last major glaciation, about 

18,000 years ago, the Arctic Ocean was frozen and glaciers covered the 

northern parts of North America, Europe, and Asia (fig. 11A). Today, massive ice in the Northern Hemisphere is isolated primarily to Greenland 

and the Arctic Ocean (fig 11A). Because the amount of water on Earth 

is constant, as the climate cooled and glaciers formed, the transfer of 

water to land caused sea level drop and new coastlines to from at a lower 

elevation. The last low stand at about ˗425 ft (Chappell and Shackleton, 

1986; Clark and others, 2009) was 18,000 years ago (fig. 11A). When the 

global landscape is dominated by fluctuating snow and ice, it is referred 

to as a glacial cycle. As the global climate warmed, melting snow and ice 

returned water to the oceans and sea level rose. Sea covered the exposed 

coast that once surrounded the ice-covered continents.
Landscape Evolution Resulting from Changes in Global Climate and the Effects of Active Tectonics 19

Figure 10. A, Hydrologic cycle and B, the distribution of ice during cold wet cycles (glacial events) and warm dry cycles (interglacial events). 

%, percent.

Glaciers

Surface water

Rain

Snow

Transpiration

Evaporation

Infiltration

B Distribution of fresh water during the last glacial maximum compared to today

A The hydrologic cycle

Salt water

in oceans

(97.5%)

Fresh water

(2.5%)

Permafrost (0.8%)

Surface and atmospheric water (0.4%)

Total Water

Glaciers

(68.8%)

Ground

water

(30.0%)

Fresh Water

Fresh water

(6%)

Salt water

in oceans

(94%)

Permafrost (0.8%)

Surface and atmospheric water (0.4%)

Total Water

Glaciers

(72.6%)

Ground

water

(26.2%)

Fresh Water

Last Glacial Maximum Present Day

Estimates of Last Glacial Maximum water distribution from Lambeck and others, 2000; Joan Fitzpatrick, written communication 2014, U.S. Geological Survey, Denver. Estimates of present day water 

distribution from United Nations Educational, Scientific, and Cultural Organization (UNESCO) and Cambridge Energy Works.
20 Investigating the Landscape of Arroyo Seco—Decoding the Past

Parts of the glacial cycle can be quantified. During the 

last glacial maximum, about 32 percent of the total land area 

was covered by ice, and sea level was at least 425 ft lower 

than it is today (Chappell and Shackleton, 1986). A series of 

shelves under the water mark former shorelines. Today—an 

interglacial—10 percent of land area on Earth is covered in 

glacial ice, and sea level is at the present position (fig. 11B). 

During warm interglacials, global sea level generally rises to 

a level close to the present level. But during the last major 

interglacial, about 120,000 years ago, the seas were about 15 

to 20 ft higher than today.

Ice sheets exerted considerable pressure on the Earth’s crust 

around and beneath them, and these areas adjusted to the ice load 

by elastic bending and plastic flow. Conversely, the ocean floors 

lighten by the removal of water. These cycles are referred to as 

the global eustatic record and contribute to the evolution of 

marine terraces near Arroyo Seco (Andrews, 1975).

The climate change record is a complex but well-understood 

history although the ultimate causative factors that drive swings 

between warm and dry and cool and wet cycles is less understood 

and probably more controversial. The climate cycles in the last 

2 million years are generally well understood. Sea level rises 

during warm and dry interglacials and falls during cold and wet 

glacials. The Earth today is in an interglacial cycle. Sea levels are 

relatively high, and the climate is relatively warm and dry. These 

changes in base level, driven by the changes in sea level, directly 

influence the terrace formation in Arroyo Seco.

During the last glacial maximum, 

about 18,000 years ago, 

the sea level was at least 425 feet 

lower than it is today.

Figure 11. Map of the distribution of ice and vegetation density on earth in the A, last glacial maximum and B, present day. Snow is 

white, desert and grassland are tan, and forest is green.

Arroyo Seco

North Pole

U.S.A.

U.S.A.

NORTH

AMERICA

ASIA

EUROPE

GREENLAND

ATLANTIC

OCEAN

PACIFIC

OCEAN

ARCTIC OCEAN

CANADA

Present day

shoreline

Glaciers or sea ice Dense vegetation Sparse vegetation

EXPLANATION

Ice Age sea level modified from National Oceanic and Atmospheric Administration, 2008. Vegetation density map simplified from Adams and Faure, 1997.

Last Glacial Maximum

(~18,000 years ago)

A B Present Day
Landscape Evolution Resulting from Changes in Global Climate and the Effects of Active Tectonics 21

Marine Sediments—A Geologic Record of Past Climate Cycles

Marine-sediments have physical, chemical, and biological properties that can be used directly and indirectly to distinguish 

cool climates from warm climates. These sediments can be sampled by drilling and sampling the sea floor. Large ships mounted 

with drill rigs are used to core and retrieve ocean sediments (fig. 12A). Marine sediment cores display a variety of sediment 

types that are sampled and analyzed (fig. 12B). Cores are typically split in half, where half the core is sampled and half is 

archived for future analysis (fig. 12C).

Source: https://en.wikipedia.org/widi/File:JOIDES_Resolution_2009.jpg, 

Photo is released into the public domain by its author 

Integrated Ocean Drilling Program.

Source: https://commons.wikimedia.org/wiki/File:

PS1920-1_0-750_sediment-core_hg.jpg,

Photo is released into the public domain by its author 

Hannes Grobe.

Source: http://www-odp.tamu.edu/public/slideset_photos/slide23.jpg, 

Photo is released into the public domain by its author 

Integrated Ocean Drilling Program Slide Set.

A

B

C

Figure 12. Marine cores are collected and sampled for analysis to distinguish past climates.
22 Investigating the Landscape of Arroyo Seco—Decoding the Past

Data from sampled intervals from cores of marine 

sediments are used to interpret global ice volume, and this 

is used to infer climate. Shells of tiny plants, animals, and 

corals in marine sediment layers are typically made of calcium carbonate (CaCO3

). As the shells form, they incorporate oxygen from the seawater. The concentration of 

different forms or isotopes of oxygen in the CaCO3 reflect 

both the ice volume and the temperature of the seawater. 

During cool climate cycles, heavy oxygen (18O) is found 

in higher concentrations relative to light oxygen (16O), and 

the reverse is true during warm climate cycles. The higher 

concentrations of 18O in the ocean are the result of 16O 

being preferentially transferred from oceans to glaciers. 

When glaciers melt during interglacials, the 16O is returned 

to the oceans. This fluctuation in the ratio of the isotopes 

(

18O/16O) gives a regular pattern of ice volume with time 

(fig. 13A). The highs and lows are numbered and known 

as marine isotope stages (MIS), with MIS 1 being the 

last 18,000 years, and the numbers increase with age. The 

marine-sediment record suggests that relatively long glacial periods are spaced about 100,000 years apart, develop 

slowly, and terminate abruptly with warm interglacial 

periods lasting between 10,000 and 15,000 years (fig. 13) 

(Imbrie and others, 1984; Chappell and Shackleton, 1986). 

In the last 400,000 years, sea level has only reached 

current levels during the warmest part of the interglacial 

cycles (fig. 13). The last major sea level rise occurred 

about 120,000 years ago, during MIS 5 (fig. 13).

Marine sediments have 

physical, chemical, and 

biological properties that 

can be used directly 

and indirectly to 

distinguish cool climates 

from warm climates.
Landscape Evolution Resulting from Changes in Global Climate and the Effects of Active Tectonics 23

Figure 13. Graphs and tables showing geologic time versus A, 18O curve and B, sea level. Altitude from modern sea level, in feet

Modern sea level

0 100,000 200,000 300,000 400,000 500,000 600,000 700,000 800,000

Years ago 

Warmer Colder

18O/16O variations from average

From Chappell and Shackleton, 1986

From Morrison, 1991

2

3

4

5

6

7

8

9

10

11

12

12

13

14

15

16

17

18

19

20

21

Numbers on curve represent 

 marine isotope stage

Ages of strath terraces in 

 Arroyo Seco (Taylor and 

 Sweetkind, 2014)

EXPLANATION

EXPLANATION

150

−150

−300

−350

−400

0

A

B

C

Sample number

(map unit) Age (years)a

AS10-9-7 (Qfy1) 

AS10-9-8 (Qfy1)

AS12-6-1 (Qfay) 

AS12-6-2 (Qfai4)

AS12-6-3 (Qfai3) 

AS10-9-9 (Qai2y) 

AS10-9-6 (Qfai2o) 

AS10-9-5 (Qfai1)

1,030±150

1,590±190

9,570±680

8,770±350

35,350±3,680

50,640±2,980

45,960±1,840

<65,000

>65,000

>120,000

a

Modified from Taylor and Sweekind, 2014, table 3.
24 Investigating the Landscape of Arroyo Seco—Decoding the Past

Geologic and Climatic History Recorded 

by Marine Terraces

Along the California coast, marine terraces provide a 

geologic record of sea level fluctuation and tectonics. Erosion 

at the present shore creates a seaward-sloping platform overlain with beach sediment (fig. 14A). At the inner edge of the 

beach is a sea cliff. Higher platforms are called marine terraces 

and are markers of sea level in the geologic past (fig. 14B). 

Marine terraces are capped usually by marine sand and include 

coral and shells. North of Salinas, on Monterey Bay, a terrace 

occurs about 100 ft above the beach and the ancient sea cliff 

(fig. 14A). This terrace has been interpreted as having formed 

during the last major interglaciation or warm climate cycle, 

124,000 years ago (Lajoie and others, 1991).

The coast of California has been affected by both the rise 

and fall of the sea level caused by global climate change and 

by regional tectonics (Muhs and others, 2003). In a tectonically stable, near-shore environment, the highest marine 

terrace was eroded about 124,000 years ago during the last 

major interglacial warm climatic cycle (fig. 15A). In an 

environment undergoing active faulting, terraces record uplift 

events and climate cycles (fig. 15B). During this interglacial 

warm climate-cycle, the sea level was higher than it had ever 

been in the last 400,000 years (fig. 13), and this terrace will be 

preserved as long as present-day beach erosion has not been 

extensive enough to remove it.

In contrast, a coast undergoing uplift has a flight of marine 

terraces that record past interglacials (fig. 15B). Once formed, 

uplift takes them above present-day beach erosion, thus preserving them. Ideally, all major interglacials are recorded. Along the 

California coast, north of Monterey, the coast is being uplifted, 

and there are terraces above the dated 124,000-year-old terrace 

(MIS 5). Regional faults cause tectonic movement, consistently 

upward, that result in a constantly and slowly emerging coastline along much of the California coast (fig. 15B). Thus, the 

regional effects of climate change resulting in sea-level fluctuations and regional tectonics both explain the marine terraces.

Marine terraces can be used to estimate the rate of coastal 

uplift. Figure 13B indicates the elevation of global sea level. If 

one can identify the marine terrace elevation corresponding to 

each interglacial, then the difference in elevation is the amount 

of uplift, and combining that with the estimated age gives the 

estimated rate of uplift. Tectonic uplift must be considered 

in conjunction with the eustatic record before uplift rates are 

calculated because not all uplift is tectonic. Some uplift is the 

result of the earth’s surface rebounding as the ice sheets melt 

(Muhs and others, 2012). If the age of a single terrace and the 

eustatic record are known, an uplift rate can be calculated. If 

the rate is assumed to be constant over time, ages of terraces at 

higher and lower elevations can be estimated. For the California coast, Muhs and others (2003) calculate a rate that varies 

from 0.005 to 0.5 inches (in.) per year. Using the higher rate, a 

MIS 7 terrace is expected to be 888 ft above sea level.

Figure 14. Photographs of the A, modern marine beach and sea 

cliff at Capitola and B, ancient marine terraces at Watsonville.

Sea cliff

Marine terrace

Photographs taken by E.M. Taylor, U.S. Geological Survey, March 2015

B Watsonville marine terraces

A Marine terrace at Capitola

Shoreline
Landscape Evolution Resulting from Changes in Global Climate and the Effects of Active Tectonics 25

Figure 15. Marine terrace evolution in A, tectonically stable and B, actively faulting environments.

No uplift Uplift

Climate driven marine terraces Climate and tectonics driven marine terraces

A Tectonically stable B Active faulting environment

Stage 9

Stage 7

Stage 5

Modern shoreline

Stage 2

Warmer period shoreline

~120,000 years ago (Stage 5)

Cooler period shoreline

~18,000 years ago

Modern shoreline

Bedrock

Active beach deposit

Marine sediments mainly sand, 

 with shells and coral

Ocean

EXPLANATION
26 Investigating the Landscape of Arroyo Seco—Decoding the Past

Tying Arroyo Seco Terraces to the Coastal Record

Because some of the terraces at Arroyo Seco have been dated (Taylor 

and Sweetkind, 2014; fig. 13B), they can be tied to the coastal terrace record 

because the formation of the stream terraces is correlative in time to the formation of the marine terraces. Marine and stream terraces in central California 

formed as a result of sea-level high stands superimposed on a tectonically 

uplifting coast (Alexander, 1953). Driven by tectonics, mountains move up, 

and driven by climate change, sea level fluctuates. When the mountains go 

up, or the sea level drops, streams actively respond by eroding a new flood 

plain. By looking at the shape of the stream and the shape of the terraces in 

the mountains, geologists know if and when the stream is moving fast or slow, 

and if the stream is eroding or depositing. When a stream is in equilibrium, it 

is essentially neither eroding nor depositing, but rather maintaining the stream 

channel and flood plain. However, when there is tectonic activity or climate 

change, the base level of the stream changes and new processes begin. If 

the mountain goes up or the sea level drops, streams erode until they reach 

equilibrium. When the stream reaches equilibrium, erosion stops and again 

the stream is constrained to its channel and flood plain. With each cycle of 

mountain building or sea level drop, the stream responds by abandoning its 

former flood plain and eroding downward. All these processes are at play in 

Arroyo Seco.

When sea level fluctuates, the Salinas River responds by changing 

its gradient in an attempt to reach equilibrium. Arroyo Seco follows the 

response of the Salinas River. When the sea level goes down during glacial 

cycles, the rivers cut down, and when sea level comes back up, the Salinas 

River should aggrade to the interglacial sea level. During interglacials, 

seawater extended from the Monterey Bay to the town of Gonzales (about 

24 miles). At depth, two clay-rich marine estuarial deposits are recognized in 

drill holes. They are distinctive, contain marine fossils, and represent periods 

of marine encroachment from high sea level (Tinsley, 1975). These clay 

deposits occur at approximately –180 and –400 ft and define the tops of two 

aquifers. The base of the –180-ft aquifer (–145 ft) and the base of the –400-ft 

aquifer (–270 ft) are in the same range for elevation estimates for sea level 

lows during the last glaciation (Muhs and others, 2003; MIS 2–4, fig. 13).

The ages of terrace formation in Arroyo Seco (fig. 13; Taylor and 

Sweetkind, 2014) suggest that strath terraces formed at high stands of sea 

level. At high sea stands, Arroyo Seco cut back and forth on the Monterey 

shale, formed a flood plain, and then with constant coastal uplift, the river 

downcut and formed a new flood plain leaving a strath terrace above stream 

level. Flights of paired strath terraces observed in older higher terraces in 

Arroyo Seco are common in tectonically active terrains (Bull, 2007).

Terraces formed in Arroyo Seco that postdate the last known movement 

on the Reliz fault must be eroded and abandoned as a result of base-level 

changes beyond the range front fault driven by climate change. In this area 

of California, the upward migration is one driving force causing erosion and 

abandonment of terrace surfaces. The raising and lowering of sea level also 

influences Arroyo Seco’s erosional history. Both processes contribute to the 

strath terrace formation.
Landscape Evolution Resulting from Changes in Global Climate and the Effects of Active Tectonics 27
121°

30’

36°14’

28’

24’

26’

121°

30’

28’

28’

16’

26’

28’

24’

Monterrey

PACIFIC

OCEAN

0 0.25 

CONTOUR 

NATIONAL G

Piney Creek

Paloma

Creek

Area of

Figure 19

Arroyo Seco

recreation

area entrance

G16

EAST CARMEL VALLEY ROAD

Carmel

ARROYO SECO ROAD

STOP 2: Gorge

parking

STOP 4: Day

use parking

STOP 3:

Campground

0.0

EXPLANATION

Major rivers and drainages

Road trip stop with mileage (mileage

 begins at Arroyo Seco recreation

 area entrance)

Road trip point of interest

Abandoned

airfield

Miller’s Ranch

Alluvial fan

Country store

Alluvial terrace

capped by

stream gravel

STOP 6: S

Flat Road

6.7

STOP 5: Arroyo

Seco Ranch 3.0

3.0 STOP 5

Q

s

a

Base from U.S. Geological Survey 1:100,000-scale digital data, 1982, 

Universal Transverse Mercator, zone 10, North American Datum of 1983 (NAD 83).

Fault (dashed where concealed)

28 Investigating the Landscape of Arroyo Seco—Decoding the Past

Road Log Map

This road log 

describes suggested stops 

on the 11 mile-long road along 

Arroyo Seco.

Figure 16. Field guide map. Modified from Taylor and Sweetkind, 2014.
Greenfield

Salinas

Map location

101

1

CALIFORNIA

0.5

R INTERVAL 33 FEET (10 METERS)

GEODETIC VERTICAL DATUM OF 1929

Sycamore

Faulted Monterey Formation in

contact with unfaulted alluvium

Qfai3 and Qfai4 on the 

south, older terraces 

above, to the north

RELIZ

FAULT

16’

24’

24’

22’

20’

121°18’

36°18’ 36°18’

121°

18’

22’

20’ 36°14’

APPROXIMATE MEAN

DECLINATION, 2014

13 1⁄2°

TRUE NORTH

MAGNETIC

NORTH

Monterey

Paso Robles

San Ardo

Greenfield

Salinas

PACIFIC

OCEAN

Map location

101

46

1

198

5

CONTOUR INTERVAL 33 FEET (10 METERS) CALIFORNIA

NATIONAL GEODETIC VERTICAL DATUM OF 1929

0 0.5 1 KILOMETER

0 0.5 1 MILE

G16

G16

G17

ARROYO SECO ROAD

Greenfield

ELM AVENUE

Alluvial terrace

capped by

stream gravel

STOP 6: Sycamore

Flat Road

Arroyo Seco

Bridge

Rounded gravel on

terrace surface

Barn on Qfai4

Gravel quarry on Qfy1

Alluvial fan buries

terrace gravel

Faulted Monterey Formation in exposed in road cut

contact with unfaulted alluvium

6.7

STOP 7: Road

to quarry 8.4

STOP 1: Exposure of

Reliz fault in road cut

Qfai3 and Qfai4 on 

the south, older terraces 

above, to the north

Road Log Map 29
30 Investigating the Landscape of Arroyo Seco—Decoding the Past

Road Log

This road log describes suggested stops on the 11 milelong road along Arroyo Seco (fig. 16). The log begins at a 

general meeting place at the Arroyo Seco bridge at the intersection of Arroyo Seco Road and Elm Ave, about 6 miles west 

of the town of Greenfield (fig. 16). After that, for logistical and 

safety reasons, the reader is directed to drive to the west end of 

the canyon. Stops are described from upstream to downstream 

starting at the Arroyo Seco U.S. Department of Agriculture 

Forest Service recreation area entrance, 11 miles west of the 

Arroyo Seco bridge, eastward to the Salinas River valley. 

Mileage is given from the west end of the road and is listed as 

total miles. Pullout locations are described as of spring 2016; 

road conditions may have changed since then. Arroyo Seco 

Road is narrow and winding and can be busy. Pullouts in this 

road log are typically for one vehicle. Do not expect to be able 

to pull out if you are in a larger group. Drive cautiously and 

avoid summer weekends. All exposures and overviews are 

accessed on public land along the right of way. If there is a 

road or a fence that says private property, keep out, or no trespassing, do not drive or walk beyond those signs. As of this 

writing (fall 2016), no cell phone coverage exists in Arroyo 

Seco beyond the bridge.
Road Log Map 31

Arroyo Seco recreation area is popular in the warm summer months when families come to camp, swim, picnic, and hike 

(http://www.fs.usda.gov/recarea/lpnf/recarea/?recid=10906). 

There is limited parking April through September. Parking in 

the park is on a daily basis and day use closes at 6 p.m. It is a 

good idea to begin early in the day when the sun angle is low and 

you are sure to have access and parking in the recreation area. 

Descriptions of pullouts, road conditions, and fees may have 

changed since this report was published. 

Stop 1

At the intersection of Arroyo Seco Road and Elm Avenue, 

on the north side of the Arroyo Seco bridge, there is a small 

pullout on the north side of the road, directly in front of an 

exposure of the Reliz fault (fig. 17). In the fault exposure, 

younger terrestrial sediments of the Pancho Rico Formation 

(23–2.6 million years old) are to the east of the fault, and 

older marine sedimentary rocks of the Monterey Formation 

(65–5.3 million years old) are to the west of the fault. Relative 

to the Monterey Formation, the Pancho Rico Formation has 

been offset in both a downward direction and southward. The 

Reliz fault is the range-front fault along which the Santa Lucia 

Range is uplifted. Offset causes change in the base level and 

results in Arroyo Seco downcutting in response to the uplift. 

Outcrops of the Monterey Formation continue westward from 

this point until nearly the west end of Arroyo Seco Road.

Notice to the left and right of the fault, rounded gravel are 

preserved at the top of the bedrock exposures. Immediately above 

the fault, however, the gravel has been eroded, but the surface 

does not appear to be offset. This gravel is a former alluvial flood 

plain, and when Arroyo Seco cut down, it was left as a strath terrace. This happened, at this location, more than 120,000 years ago 

(fig. 13C) (IRSL from Taylor and Sweetkind, 2014).

Look down into Arroyo Seco, and you will see rounded 

gravel in and adjacent to the river (fig. 18). These rounded gravel 

are derived from the resistant basement rocks far upstream. As the 

bedrock eroded, rocks were transported to the river by gravity and 

running water. They were rounded by tumbling together as they 

moved downstream. Most of the flat terrace surfaces you see in 

Arroyo Seco are capped by similar rounded gravel (fig. 7A) and 

mark where Arroyo Seco once eroded a flood plain. The gravel is 

usually exposed overlying the Monterey Formation and sometimes buried by younger deposits in road cuts. 

Figure 18. Photograph of the active drainage in Arroyo Seco.

Figure 17. Reliz fault zone exposure.

Photograph by E.M. Taylor, U.S. Geological Survey, April 2010

Unfaulted late Pleistocene fluvial-terrace deposits (Qfai1)

Monterey Formation

(marine sediments)

RELIZ

FAULT

Pancho Rico

Formation

(terrestrial

sediments)

Monterey

Formation

Arroyo Seco

Rounded gravel

on flood plain

Photograph by E.M. Taylor, U.S. Geological Survey, March 2015
Monterey Formation

reworked in slope wash

Terrace gravel

Monterey Formation

buried by terrace gravel

32 Investigating the Landscape of Arroyo Seco—Decoding the Past

Drive from Arroyo Seco Bridge to Arroyo Seco 

Recreation Area

The drive up to Arroyo Seco recreation area provides an 

excellent opportunity to observe the geology and erosional 

processes. Figure 16 is annotated with things to observe on 

your drive to the Arroyo Seco recreation area. Above Arroyo 

Seco bridge, Arroyo Seco is confined to a meandering and narrow flood plain (fig. 5C). A few reaches are braided. Almost all 

of the flat surfaces you observe were eroded by Arroyo Seco 

in the geologic past. Higher terraces are preserved both north 

and south of the stream. When Arroyo Seco was flowing over 

those high surfaces, the river flood plain was much wider.

In Arroyo Seco where the stream is currently eroding bedrock, the Monterey Formation is exposed at or near the surface. 

Road cuts commonly expose Monterey Formation, overlying 

terrace gravel and associated soils, and at the top sometimes, 

a younger deposit derived from the slope above or from a 

tributary stream. The Monterey Formation can be weathered 

(fig. 19A), tilted (fig. 19B), cemented and vertical (fig. 19C), flat 

lying (fig. 19D), and folded (fig. 19E). Slope deposits moved 

downslope under the influence of gravity and may bury alluvial 

terrace deposits (fig. 19F). Weathered Monterey Formation can 

also be moved down the slope as slope wash (fig. 19G), debris 

flows, colluvium, and landslides.

Drive up Arroyo Seco Road and stop at the Arroyo Seco 

recreation area (fig. 20) kiosk to pay the entrance fee ($10 as 

of summer, 2016), and ask for the parking pass to the Gorge 

parking area. If it is not too crowded, ask if you can briefly 

visit the day use area on your way out of the park. There are 

two different colored parking passes for the two parking areas. 

Drive over the bridge, past the campground, and continue to 

the end of the road to the Gorge parking area. On the way, 

look at the exposed rocks and consider how different this part 

of the Arroyo Seco looks compared to the broad agricultural 

valley downvalley. When you get to the Gorge parking area, 

you will be in a narrow canyon because the surrounding basement rocks are much more resistant to erosion than the softer 

Monterey Formation. These resistant rocks are the source of 

the rounded gravel in the river and the strath terraces. The 

relatively soft Monterey Formation rocks and the resistant 

basement rocks are separated by a fault at the mouth of the 

canyon (fig. 20). You will see the abrupt change in the rocks 

shown by color, to your right (north), across Arroyo Seco, as 

you drive to the parking area.

Arroyo Seco recreation area 

provides an excellent opportunity 

to observe the geology and 

erosional processes.
Road Log Map 33

Figure 19. Monterey Formation exposed in road cuts and natural exposures.

Monterey Formation

buried by terrace gravel

Terrace gravel

Monterey Formation

reworked in slope wash

A Weathered

E Folded F

G Reworked in slope wash

B Tilted

C Cemented and vertical D Flat lying

Photographs by E.M. Taylor, U.S. Geological Survey, April 2010
34 Investigating the Landscape of Arroyo Seco—Decoding the Past

Figure 20. Arroyo Seco recreation area map.

0 500

0 250 500 METERS

scale = 1:8,000

1,000 FEET

Campground

Fault (dashed where concealed)

Qfai3 Geologic map unit, see fig. 4

EXPLANATION

Polygon colors represent different aged terrace deposits.

See figure 3 for geologic explanation of colors and symbols.

Base from U.S. Geological Survey 1:100,000-scale digital data, 1982,

Universal Transverse Mercator, zone 10, North American Datum of 1983 (NAD 83)

STOP 4

STOP 2

(short hike)

STOP 3

(short hike)

Arroyo Seco

recreation

area entrance Day Use

Area

CCC Camp

(demolished) Dam

Bridge

Ranger Station

Gorge

Parking

Gate

Lake Parking

Airfield

(demolished)

ARROYO SECO ROAD

Basement rocks

Monterey

Formation

Qfai3

Qfai3

Qfai3

Qfai3

Qfai2y

Qfai2y

Qfai2y

Qfai2y Qfai2y

Qfo7

Qfy1

Qfy1

Upper Lake

Lower

Lake

Upper Primitive Campground

Group Campground

N

Qfai2y
Road Log Map 35

Stop 2

Park in the Gorge parking area 

and prepare for an easy walk up the old 

Arroyo Seco Road, west of gate. The 

parking area is on a terrace surface. 

Until 1995, when a landslide buried 

part of the road, cars could drive this 

road through the east side of the Santa 

Lucia Mountains to Fort Hunter Liggett 

and Mission San Antonio de Padua 

near the town of Jolon. The old road 

provides beautiful views up and down 

Arroyo Seco Canyon.

The goal of the canyon walk is 

to look for rounded gravel capping 

terraces that are eroded into the 

resistant basement rocks. The best 

exposures are across the stream 

(fig. 21); a few small terrace remnants are preserved above the road. 

Rather than the broad flat terraces 

that dominant the lower part of the 

drainage, terraces in the canyon are 

poorly preserved and narrow.

Figure 21. Terrace gravel deposit on basement rocks.

Terrace gravel

Basement

rocks

Contact

Photographs by E.M. Taylor, U.S. Geological Survey, April 2010

Terrace gravel

Basement

rocks

Contact
36 Investigating the Landscape of Arroyo Seco—Decoding the Past

Stop 3

Return to the car and follow the signs to the Upper Primitive Campground, but 

before you reach the Upper Campground (fig. 20), park on the left at the Group Camp and 

“Information Board” (fig. 20, Lake Parking). Take the short trail to the upper lake, east of 

where your car is parked. On your left, as you walk to the Upper Lake (fig. 20), is a strath 

terrace. Rounded gravel clasts are present at the surface and are exposed in the bank. The 

terrace gravel here is larger than those farther downvalley, a common characteristic of most 

streams. The Upper Primitive Campground is on this terrace. You can walk to the terrace 

surface by walking down the road past the Lower Lake and following the sign to the Upper 

Campground. Look north across the valley to see more terraces surfaces. All three campgrounds are on terrace surfaces. 

Streams deposit larger stones closer to the mountain front and gravel size decreases farther from the mountain front. Enormous rounded stones, deposited where the recreation area 

is today and collected from the terrace surface, were used to make the road and parking area at 

the Upper Campground. This is a common tactic where gravel is available on strath terraces. 

Geologists often look at landscaping material to locate terrace surfaces where the gravel may 

be buried or removed.

Return to the car and continue down the one-way road. The Lower Lake can be viewed 

to the left and was once connected to the upper lake. In wetter years, the lakes are both filled 

with water and connect. The two lakes were part of a meander sweep of Arroyo Seco that 

eroded into the surrounding bedrock (fig. 20). Now the stream is located to the north, and the 

lakes are isolated from the drainage. These isolated lakes are called oxbow lakes and are common in meandering stream systems.

At the end of the lower lake is a dam probably built by the Civilian Conservation Corp 

(CCC) to enhance the natural lake. As you drive over the dam, you are near the former CCC 

camp that has been demolished (fig. 22). A few stone chimneys still exist. In this area, the 

CCC worked on building and maintaining bridges, roads, and trails for about 10 years beginning in 1933.

Source: Salinas Public Library, https://www.flickr.com/photos/67808015@N05/9307677747/

in/phptpstream/.

Figure 22. Photograph of the Civilian Conservation Corps Camp Arroyo Seco, 1936.
Road Log Map 37

Stop 4

As you approach the park entrance, the day use area is on your left. It is a great place 

to picnic and swim; however, on a hot summer day, it can be crowded. The parking area 

is on a young terrace just feet above the active drainage (fig. 20). This young surface 

is often covered with large sycamore trees that do not grow on older terraces, probably 

because sycamore trees require groundwater at a shallow depth. The soil developed on 

this surface is weak with little or no soil horizons. Look north across Arroyo Seco from 

the day use parking area at the Monterey Formation; it extends from here to the Reliz 

fault where the river intersects the Salinas River valley. The fault-contact between the 

Monterey Formation and the basement rocks can also be seen from the bridge just before 

you leave the park and the guard shack.

0.0 Leave the day use area and proceed to the guard shack at the Arroyo Seco 

recreation area entrance where your odometer should be reset to 0.0 to begin 

the drive down Arroyo Seco Road. Just outside the park entrance is Arroyo 

Seco Resort, a cluster of privately owned houses nestled along a young terrace 

of the Arroyo Seco.

0.4 The terrace across Arroyo Seco to your right was once an airfield or landing 

strip probably used by the CCC (fig. 20) and later for firefighting planes. There 

is a small pullout at 0.6.

0.8 The road cut on the left is colluvium. The colluvium is pale brown to brown, 

poorly sorted with little or no bedding. The clasts are typically angular, less 

than 3–4 in. long, in a fine-grained matrix of silt and sand.

1.1 The road cut on the left is Monterey Formation. The top has been eroded flat 

by Arroyo Seco. The overlying rounded gravel is a strath terrace and was 

deposited by Arroyo Seco.

1.5 Look straight ahead at the multiple terrace surfaces as you descend the valley. 

Each surface is capped by rounded gravel and records where the Arroyo Seco 

once flowed. Mountain building by tectonics has raised the terraces above the 

elevation at which they formed.

1.7 Big Sur Land Trust manages Arroyo Seco Ranch, the property on your right. 

The property includes a 2-mile stretch of the Arroyo Seco, which has been 

named a priority steelhead restoration site.

2.1 On your left is a good example of a narrow canyon; the wall provides the 

alluvial and colluvial material that buries the terrace gravel and can be observed 

in the road cuts.

2.4 The narrow terrace tread on your right is used as a house site.
38 Investigating the Landscape of Arroyo Seco—Decoding the Past

Stop 5

3.0 Entrance to Arroyo Seco Ranch. You can pull over here to the right and 

look down the valley to where it widens. The flat surface in the foreground 

is a broad terrace that is mapped as Qfai3 (fig. 4) and is approximately 

35,000 to 40,000 years old (fig. 13C) (IRSL from Taylor and Sweetkind, 

2014). Geologic mapping units Qfai3 and Qfai4 are the predominant 

terrace surfaces in Arroyo Seco. Combined, they extend the full length of 

Arroyo Seco and mark former extensive broad flood plains about 80 to 

100 feet above the stream. The soil developed on this surface is characterized 

by an argillic B horizon with 25–35 percent clay.

3.1 Albertola Estates is a private development north of the stream that extends 

to the northern ridges of Arroyo Seco. House lots take advantage of the 

terrace remnants up to 1,000 ft above Arroyo Seco (fig. 4).

3.2 Coelho-Ford Fire Station and Arroyo Seco Community Center. Look to your 

right at the dramatic sequence of terraces in the valley. The lower, gently 

sloping surface to your right is typical of a terrace that has been buried by 

colluvium derived from the adjacent slopes.

3.4 Country Store. Sometimes it is open.

3.7 Miller’s Ranch, in its heyday after World War II, was a family resort with 

Quonset huts and a dance hall. It is developed on one of the youngest 

terraces (Qfy1) and is identified by the presence of sycamore trees.

3.8 Tilted Monterey Formation, with a well-preserved bedding plane, is exposed 

in the road cut.

4.0 Continuation of Arroyo Seco Ranch is on the right.

4.1 On the right, just before you cross the bridge over Piney Creek, there is a 

terrace remnant capped by rounded gravel.

4.4 Intersection with East Carmel Valley Road. The valley broadens downvalley 

from this point where Piney Creek merges with Arroyo Seco.

4.8 A small, legal pullout allows you to look at the broad open valley and the 

set of terraces along Arroyo Seco that are not visible from the road. The 

road cut on the left is a good example of weathered Monterey Formation 

buried by colluvium.
Road Log Map 39

Stop 6

6.7 Pull over on your right at the intersection of Sycamore Flat Road. You are 

parked on an extensive Qfai3 terrace, the same one seen at Stop 5. This terrace 

surface is often used for agriculture, including grapes. Terrace remnants of 

Qfai1 are above the road to the north. The steep eroded bluffs across Arroyo 

Seco to the south are good exposures of the Monterey Formation. Sycamore 

Road allows you access to the Arroyo Seco and a look at the youngest, gravelcovered terraces (Qfy1). Houses are developed among the sycamore trees on 

the terrace about 6 ft above Arroyo Seco. 

Stop 7

8.4 Pull over on your right and onto a paved road to a gravel quarry. From here, 

you can look down to one of the youngest surfaces (Qfy1) that has been 

quarried for sand and gravel. Up valley is the Qfai3 terrace, and immediately 

above the road is the Qfai2 terrace. The soil developed on the terrace has an 

argillic horizon with at least 35 percent clay, and the soil is about 50,000 years 

old (fig. 13C) (IRSL from Taylor and Sweetkind, 2014). Across the valley you 

can see a Qfai3 above Qfai4 which extends downvalley. Both terraces are 

correlative with alluvial fans at the intersection of the Salinas River valley 

(fig. 4). 

8.5 Pullout on the right—“No Dumping Subject to $100 fine” sign. Ideal view of 

lower terraces. Colluvium eroded into and burying Monterey Formation in 

road cut.

8.6 Pullout on the right—Look at the exposure in the road cut. There is a thin 

deposit of colluvium above a bed of rounded gravel (Qfai2o) which in turn lies 

on Monterey Formation exposed in the base of the road cut. The top of the 

alluvial sediment was dated to estimate the time of abandonment of the ancient 

flood plain and the maximum age of the colluvium (>65,000 years old by IRSL

from Taylor and Sweetkind, 2014).

8.7 Pullout on right

8.8 Buried terrace gravel in road cut. Look for rounded gravel on Monterey 

Formation in road cuts as you drive down the valley. 

11 Bridge crossing Arroyo Seco. End of field guide.
40 Investigating the Landscape of Arroyo Seco—Decoding the Past

Selected References

Faulting History

Jennings, C.W., and Bryant, W.A., comps., 2010, Fault activity 

map of California: California Geological Survey Geologic 

Data Map No. 6, map scale 1:750,000.

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42 Investigating the Landscape of Arroyo Seco—Decoding the Past

Glossary of Geologic Terms

A

Alluvial deposits Sediments composed of unconsolidated gravel, sand, silt, and clay deposited by a river or stream.

Alluvial fans River deposits adjacent to mountain fronts, typically in a fan shape.

B

Base level The altitude below which a stream cannot downcut. Sea level is the ultimate base 

level.

Basement rock A complex of pre-Cenozoic rocks below the Monterey Formation in Arroyo 

Seco. They are igneous and metamorphic rocks. Rounded granitic rocks derived from this unit 

occur in the soils and at the surface of the strath terraces.

Basin aggradation The process by which a basin or valley is filled by sediments.

C

Clast An individual weathered fragment of stone or rounded gravel.

Colluvium Sediment moved by gravity, generally down slopes. It is unsorted, made up of a 

mixture of sizes.

F

Flood plain The smooth land adjacent to a river channel that is covered by water when the 

river overflows its bank.

G

Geomorphic features A general term to describe a configuration of the Earth’s surface that is 

specifically used in this report to describe landforms produced by erosion or deposition.

Glacial cycle An interval of time marked by one or more major ice advances and separated 

by an interglacial.

Global climate change The fluctuation of temperature and precipitation that is felt globally.

Global eustatic record The record of global sea level independent of local factors, and resulting from fluctuations of the Earth’s surface elevation where land subsides beneath the weight of 

ice sheets and the subsequent uplift and rebound when ice sheets melt.

H

Hydrologic cycle The constant circulation of water from the sea to the atmosphere, returning 

to land and plants, and eventually back to the sea.

M

Map scale The relationship between a measurement on the map and a corresponding distance 

on the surface being mapped. For example, a map scale of 1:50,000 means 1 inch on the map 

equals 50,000 in. (about 0.8 mile) on the Earth’s surface.

Marine isotope stage (MIS) Alternating warm and cool periods in the Earth’s paleoclimate, 

deduced from oxygen isotope data measured from deep sea sediment core samples.
Glossary of Geologic Terms 43

Marine sediments Sediments deposited in an ocean.

Marine terraces Eroded wave-cut platforms that record sea level in the geologic past.

Meander sweep One of a series of curves, bends, loops, turns, or windings naturally 

developed in the course of a stream.

O

Oxbow lake A crescent-shaped lake in an abandoned channel of a meander.

P

Paleoclimate events Climates that have occurred in the past.

Perennial drainage A stream drainage that flows year round as opposed to an intermittent 

drainage that only flows seasonally.

S

Sedimentary basin A depression filled over time with sediment that is often transported by 

water.

Soil horizons Layers within a soil that are distinguishable from adjacent layers by characteristic physical properties and are often horizontal at varying depths below land surface.

Strath or erosional terrace A bedrock surface once eroded by an active river and capped with 

stream gravel that was deposited by the stream as it cut the surface. It lies above the present 

flood plain.

Stream equilibrium In terms of stream dynamics, a state of neither erosion nor deposition 

where river discharge and sediment load are in balance.

T

Tectonics Large-scale processes affecting the structure and properties of the Earth’s crust and 

its evolution through time. Examples of results are mountains and basins.

Terrestrial sediments Sediments shed off eroding mountains and generally deposited by 

streams in nonmarine environments. Terrestrial sediments also include slope wash and colluvium. Other common terrestrial sediments are those deposited in lakes or as sand dunes.

Tributary A stream that joins the main drainage.
44 Investigating the Landscape of Arroyo Seco—Decoding the Past

Notes
Notes 45
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