Coastal Landforms and Processes at the Cape Cod 

National Seashore, Massachusetts

A Primer

Circular 1417

U.S. Department of the Interior

U.S. Geological Survey
Front cover. Illustration by Mark Adams.
i

Coastal Landforms and Processes at the Cape Cod 

National Seashore, Massachusetts

A Prim er

By Graham S. Giese, S. Jeffress Williams, and Mark Adams

U.S. Geological Survey Circular 1417

U.S. Department of the Interior

U.S. Geological Survey
ii Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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

Earth, its natural and living resources, natural hazards, and the environment—

visit http://www.usgs.gov/ or call 1–888–ASK–USGS (1–888–275–8747).

For an overview of USGS information products, including maps, imagery, and 

publications, visit http://www.usgs.gov/pubprod/.

Any use of trade, firm, or product names is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

Although this information product, for the most part, is in the public domain, 

it also may contain copyrighted materials as noted in the text. Permission to 

reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Giese, G.S., Williams, S.J., and Adams, Mark, 2015, Coastal landforms and 

processes at the Cape Cod National Seashore, Massachusetts—A primer: 

U.S. Geological Survey Circular 1417, 86 p., http://dx.doi.org/10.3133/cir1417.

Library of Congress Cataloging-in-Publication Data

Names: Giese, G. S., author. | Williams, S. Jeffress, author. | Adams, Mark,

 1953 Nov. 13-

Title: Coastal landforms and processes at the Cape Cod National Seashore,

 Massachusetts : a primer / by Graham S. Giese, S. Jeffress Williams, and

 Mark Adams.

Description: Reston, Virginia : U.S. Geological Survey, [2015] | Series: U.S.

 Geological Survey circular ; 1417 | Includes bibliographical references.

Identifiers: LCCN 2015044260 | ISBN 9781411339941

Subjects: LCSH: Coasts--Massachusetts--Cape Cod National Seashore. | Coast

 changes--Massachusetts--Cape Cod National Seashore. | Cape Cod National

 Seashore (Mass.)

Classification: LCC GB459.4 .G46 2015 | DDC 551.45/809744--dc23

LC record available at http://lccn.loc.gov/2015044260
iii

Contents

Chapter 1. How the Earth System Works ...............................................................................................7

Chapter 2. Cape Cod’s Coastal Landforms: Works in Progress ........................................................15

Chapter 3. Global Climate: Glaciers and Sea-Level Change .............................................................37

Chapter 4. Wind and Weather................................................................................................................57

Chapter 5. Waves .....................................................................................................................................63

Chapter 6. Tides ........................................................................................................................................73

Chapter 7. Summary.................................................................................................................................83

Suggested References................................................................................................................................85
iv Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Acknowledgments

This report, intended as an educational resource for the general public, was made possible with 

the cooperation of the National Park Service (NPS). Andrea Toran, Anna Glover, Teri Losano, and 

Chris Mendlesohn of the U.S. Geological Survey (USGS), provided valuable assistance for the 

publication of this report. We very much appreciate the careful and insightful scientific reviews 

by Richard S. Williams, senior scientist, USGS, and John Portnoy, coastal ecologist, NPS. 

Special appreciation is extended to Walter Barnhardt, director of the USGS/Woods Hole Coastal 

and Marine Geology Science Center, and George Price, superintendent of the Cape Cod National 

Seashore. The use of the photograph collection of Mary M.T. Wiant is greatly appreciated. The 

text of this report developed from a request in 2004 to the first author by Nancy Finley at the 

Cape Cod National Seashore, and would not have developed without the support provided over 

many years by the Woods Hole Oceanographic Institution and the Center for Coastal Studies in 

Provincetown. This circular would not have been possible without the tireless help and support 

of Barbara Baker, Rebecca Upton, and Richard Adams.
Guide to Coastal Landforms and Processes at the Cape 

Cod National Seashore, Massachusetts

A Primer

By Graham S. Giese,1 S. Jeffress Williams,2 and Mark Adams3

Circular 1417

1

Center for Coastal Studies.

2

U.S. Geological Survey.

3

National Park Service.
2 Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

A surfer sits on his board on the sloping foreshore, scanning 

the southeast swell, waiting for the tide to fall just enough for 

the waves to break over an offshore bar.

A child teeters on the cobble step at low tide, erupting in 

laughter as a shore-breaking wave tumbles her in the swash.

A surfcaster deftly places a lure in dark water aided by an 

offshore wind.

A surveyor paces off the distance between someone’s cliff 

top dream house and the eroding bluff edge, launching a river of 

pebbles down over layers of soil horizons, stacked like layer cake 

above the smooth beach below.

Between sand ripples in a dune hollow, a solitary walker 

finds bits of coal and a rusty key, scattered remains of a squatter 

shack built of driftwood decades ago by a fisherman-poet.

Modern-day rangers in dress-whites of another era tamp a 

cannon to launch a rescue reenactment responding to a common 

fate of schooners navigating the parallel bars of Peaked Hill.

Photographs courtesy of Mary M.T. Wiant, used with permission
3

Kayakers distract a poised heron in the calm shallow flats 

of a marshy lagoon as the nearshore breakers send a salty mist 

over the barrier dune.

Storm chasers squint into the wind as bullets of sand fill 

the air and whipped foam overtops a grassy dune.

Binoculars trained on a line of dry seaweed reveal a half 

dozen rusty looking knots and plovers turning the drift for 

sand shrimp.

The morning after a storm, walkers puzzle over the 

hardened muddy impression of cartwheels and horses hooves 

exposed after many decades on a scoured beach.

Lifeguards slouching on their towers, maintain a laser-like 

gaze on rip currents that channel mountains of water, return 

to the sea.

A beachcomber notices an almost military line of fishing 

boats crowding against the edge of an invisible underwater shelf 

about 400 yards from shore.
4 Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Anyone who spends more than a few days on 

Cape Cod (the Cape) quickly becomes a coastal 

geologist, quickly learning the rhythms of daily tides 

and the seasonal cycles of beaches growing and being 

swept away by storms; swimmers and surfers track 

how the breakers appear, and dog-walkers notice the 

hard-packed sand blanketed overnight by an airy layer 

that leaves deep labored tracks.

Careful observers whose paths wander to the 

ocean’s edge will observe many of the landforms and coastal processes described in this 

book and if we have done our job well, the stories told here will seem familiar. Watchful 

experience brings insights; indeed, this is how scientists and perhaps how artists work, 

describing patterns that explain and predict. When is the next high tide? What will the 

winter bring? Where do we build, fish, swim? How do wind and waves offshore in the 

North Atlantic help arrange the plants and dunes and hollows on the beach? And most of 

all, as human animals drawn to live and play on the edge of the ocean, how do we get the 

benefits of this complex natural system of geology and biology? How do we affect coastal 

processes; how is the coast changing now and how is the coast likely to change in years 

ahead with climate warming and climate change?

This book is about the highly dynamic coastal landforms of Cape Cod—the beaches, 

bluffs, spits, dunes, barrier beaches, estuaries, and salt marshes. What they are, why they 

are where they are, how they behave with respect to the greater Cape Cod coastal system—

Careful observers whose paths 

wander to the ocean’s edge will 

observe many of the landforms 

and coastal processes described 

in this book.

Images copyright Google Earth, 2015
5

how the landforms respond to day-to-day and long-term geologic processes, such as waves 

and currents, change in sediment transport, relative sea-level rise, and meteorological 

processes such as hurricanes, nor’easters, and cold front passages. It is also about how the 

landforms got to be where they are and the way they are and where they are headed in the 

near future with the predicted effects of global climate warming and change.

Our objective is to provide a single source of understandable and readable scientific 

information for those who live, play, and work on outer Cape Cod and at the Cape Cod 

National Seashore, as well as to provide an introduction to Cape Cod’s coastal landforms 

for anyone with an interest in Earth science and nature who wants a better understanding 

of coastal systems and processes. Basic to an understanding of coastal landforms is the fact 

that they work together—they interact—as elements of many systems, and therefore our 

ultimate concern is not the individual landform 

itself but rather the geologic systems that make up 

Cape Cod and the Cape Cod National Seashore. 

Much of this discussion can be applied as well 

to Nantucket, Martha’s Vineyard, and other 

coastal regions.

The coast of outer Cape Cod, about 15,000 

years old and about 30 miles (mi; 50 kilometers 

[km]) long, is but a tiny piece of the global Earth 

system that operates within a much larger realm 

of space and time. Cape Cod’s coastal landforms 

Basic to an understanding of coastal 

landforms is the fact that they work 

together—they interact—as elements 

of many systems, and therefore our 

ultimate concern is not the individual 

landform itself but rather the geologic 

systems that make up Cape Cod.
6 Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

are temporary holding patterns within a continual interplay of land, sea, atmosphere, climate, 

ice, and life, including a variety of human activities that both affect and are affected by 

these processes. These interactions produce the landforms, and the landforms alter the 

interactions. The resulting landforms provide habitats for a wide variety of coastal plants 

and animals. The habitats along with their inhabitants and the interacting environmental 

factors controlling them constitute the Cape’s complex and varied ecosystems. But for now, 

we are here to enjoy it. We welcome you to delight and wonder at the perpetually changing 

handshake between the ocean and shore at New England’s Great Beach.

Our ultimate concern is not the individual landform itself but 

rather the geologic systems that make up Cape Cod.

Photograph by John Mullaney, U.S. Geological Survey
How the Earth System Works 7

Chapter 1.

How the Earth System Works

We start with the point-of-view that all coastal landforms are elements within coastal systems—systems into 

which energy and material flow, in which energy and material 

are processed, and out of which energy and material flow in 

altered form. More accurately, coasts are systems of systems, 

some better understood than others, but most are poorly 

understood. There are important coastal systems and processes 

yet to be fully understood or even recognized; all involve 

complex and often nonlinear, thus unpredictable, interactions 

as well as positive and negative feedback loops. This report 

will outline some of the long-term and large-scale processes 

that are responsible for the existence and characteristics of the 

Cape Cod (the Cape) coastal system as well as for those of the 

short-term and small-size constituent systems.

One major process that we will encounter in this 

guidebook is solar radiation, the engine that drives the Earth’s 

atmospheric and oceanic circulations. Together with the Earth’s 

orbit around the Sun and the current 23.5-degree (°) inclination 

Among the Earth system 

processes we will discuss are 

the planet’s daily rotation, 

systems of winds, waves and 

tides, and global climate.

of its axis of rotation to 

that orbit, solar radiation 

provides seasonality to our 

coastal system. Another major 

process is the Moon’s monthly 

elliptical orbit about the Earth, 

or more accurately, as we will 

show, the interacting orbits 

of the Moon and the Earth. 

Fundamental to all processes that will concern us, regardless 

of scale, is the force of gravity.

Among the Earth system processes we will discuss 

are the planet’s daily rotation, systems of winds, waves and 

tides, and global climate. But we will begin with the basic 

Earth process of mantle convection, which is the engine 

that drives plate tectonics. This process sets the stage for 

the eventual appearance of the Cape and the the Cape’s 

coastal system.
8 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Continental Margins

The Cape’s coastal system belongs to the global system 

of continental margins. Throughout geologic time, the Earth’s 

continents have grown and continue to grow at their margins. 

The changes that we see from day to day and from year to year 

along the shores of the Cape are part of that process, part of 

the growth of a continent or continental accretion as geologists say. But how can that be, you might reasonably ask, when 

much of the change that we see is commonly understood to be 

erosion? We see loss, not growth!

First, let’s define continental margins. Simply said, the 

margins of continents are where the Earth’s two types of crust, 

continental and oceanic, join one another. Continental crust is 

the bedrock that we terrestrial creatures live on, whereas oceanic 

crust is the rock of the deep ocean floor. Both types of crust 

essentially float on the mantle, the large segment of the planet 

that lies between the core and the crust. Most of Earth’s mantle 

has a plastic-like character and, due to high temperatures and 

extreme pressures, is in slow yet constant motion. This motion 

drives the plates that make up the lithosphere, the part of the 

planet that consists of the continental and oceanic crusts and the 

outermost part of the mantle.

Continental and Oceanic Crust

Continental crust is the lighter of the two crusts and 

therefore floats higher than the dense oceanic crust. The 

Earth’s crust, both continental and oceanic, and the rigid 

uppermost part of the mantle form the planet’s lithosphere. 

The lithosphere is segmented, rather like a poorly sewn soccer ball, into rigid plates that shift with the slow motions of 

the deeper mantle (fig. 1). The term plate tectonics refers 

to the interactive motions of the plates and the mantle. The 

16 or so major and minor plates vary in size, ranging from 

the enormous Pacific plate, the largest of the seven major 

plates, to the much smaller slabs of lithosphere such as the 

Caribbean plate.

Continental and oceanic crusts result from different 

processes. The process known as sea floor spreading creates 

oceanic crust as ocean basins are formed. Diverging flows 

within the mantle pull an overlying plate in opposing directions producing tension and eventually tearing or rifting the 

plate, causing it to separate, producing two plates where there 

had been one. As the new ocean widens, the original spreading zone (the midocean ridge system) remains at the center of 

the new ocean, creating new sea floor or oceanic crust.
How the Earth System Works 9

Figure 1. Continents, tectonic plates, and plate boundaries. The Burma plate (not shown) is a 

small plate between the Indian and Australian plates.
10 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Whenever new oceanic crust is being created at some 

places on the Earth, a similar amount of oceanic crust must be 

lost at places where ocean basins are closing due to converging mantle flows, or where plates converge. The process of 

ocean formation and closure may require hundreds of millions 

of years. In the process of closing, a plate with oceanic crust 

may collide into a plate with continental crust; the denser 

oceanic crust dives under the less dense continental crust, a 

process known as subduction. Earthquakes, faults, and volcanic activity are common along the margin of the continental 

plate as the subducting oceanic crust plunges beneath it into 

the mantle. The devastating tsunami of December 26, 2004, in 

Banda Aceh, Indonesia, resulted from such an earthquake off 

the western coast of northern Sumatra where the Indian and 

Australian plates have been diving beneath the Burma plate for 

many millions of years.

There is only a small probability that a tsunami could 

strike the Cape. Tsunamis are caused by the displacement of 

a large area of the sea floor caused by earthquakes, submarine 

landslides, and volcanic eruptions. None of these phenomena 

are in evidence near the Cape. However, an earthquake struck 

offshore of northeastern Massachusetts in 1755. In 1929, 

a damaging earthquake offshore of Newfoundland caused 

submarine landslides that severed submerged cables and caused 

a tsunami, killing 27 people along the Newfoundland coast.

Subduction and Continental Growth

During subduction, deep-sea sediments that have collected 

for millions of years on the old ocean floor covering the subducting plate are partly scraped off onto the continental crust 

and partly carried down beneath it. Other sediments—eroded 

sediments of continental origin carried by rivers and glaciers 

to the coast—are carried with the scraped off sediments. The 

material carried deep into the denser upper mantle along with 

the subducting oceanic plate is altered by high pressures and 

temperatures. Being less dense than surrounding material, the 

sediment tends to move upward and, as pressures drop, partially melts. Some of the melt may reach the surface in association with volcanoes and some will cool beneath the surface 

and become incorporated in the older crust. Either way, new 

continental crust is created.

At the end of the subduction process, when all of the seafloor has been recycled and the oceanic rift closes, the converging plates each carry only continental crust. Light continental 

crust collides with equally light continental crust. Neither dives 
How the Earth System Works 11

under the other; both are pushed up. Sediments carried to the 

margins of the two plates are subjected to great pressures and 

are also uplifted. A vast mountain range can be formed and two 

plates can become one. That is how the Appalachian Mountains were born about 250 million years ago and how mountain systems such as the Andes and Himalayan Mountains are 

forming today.

Active and Passive Margins

Two different types of crust meet at continental margins: 

light continental crust and dense oceanic crust. Margins associated with closing oceans are tectonically active and characterized by frequent earthquakes and volcanic activity produced 

by the subduction of oceanic crust beneath continental crust. 

These, reasonably enough, are known as active margins (for 

example, the coast of the Pacific Northwest). New continental 

crust is created at active margins.

However, margins associated with growing oceans 

are not tectonically active. Instead, in this case, tectonic 

activity—earthquakes, faulting, and volcanic activity—takes 

place at ancient midocean ridges where new oceanic crust is 

formed. As the ocean widens, the sea floor at the continental 

Figure 2. Passive continental margin.

margin becomes ever older, cooler, denser, and deeper. Such 

margins are said to be passive because there, ocean and continent rest passively, side-by-side, on the same plate (fig. 2).

At these tectonically passive margins some of the material that will contribute to future continental growth collects 

in thick sediment layers. Because the sea floor is oldest at the 
12 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

margin, marine sediment deposits blanketing the oceanic crust 

are thickest there. Similarly, continental sediments produced by 

weathering of upland rocks and transported seaward by flowing water and ice cease their travel at the margin, and deep 

continental sediment deposits form broad coastal plains and 

continental shelves. The sediments blanket the margin in thick 

wedge-shaped deposits that can be miles thick at the outer edge 

of the continental shelf.

The New England Passive Margin

The coastal plain and continental shelf of southern 

New England provide an excellent example of a passive margin. 

This margin lies quietly on the same plate, the North Atlantic 

plate, as the adjoining oceanic crust that extends eastward to the 

sea floor spreading center at the 

Mid-Atlantic Ridge. From the 

time that North America first 

began to drift away from Africa 

almost 200 million years ago, 

the Atlantic continental margin 

has received an enormous blanket of sedimentary deposits.

The coastal plain and 

continental shelf of southern 

New England provide an 

excellent example of a 

passive margin.
How the Earth System Works 13

The exposed bedrock nearest to the Cape is found at Brant 

Rock in Duxbury, Mass. At Brant Rock, ancient continental 

crust, bedrock that formed long before the Atlantic Ocean was 

born, is now being eroded by coastal waves. That same bedrock lies under the Cape as well, buried hundreds of feet deep 

below sediments. The farther seaward you travel, the deeper 

the bedrock is buried, first by hundreds then by thousands and 

tens of thousands of feet of sediment. By the time you reach 

the outer continental shelf beneath the enormous shoal that lies 

to the southeast of the Cape at the edge of the continental shelf 

known as Georges Bank, bedrock lies many miles below the 

sandy sea bed.

At the lowest level of this wedge of shelf sediments lie 

reef materials and shallow water deposits that were formed 

when the Atlantic Ocean was young and narrow. As the ocean 

broadened, rivers flowing down to the sea from the then-lofty 

Appalachian Mountain range left wide deltas of sediment 

above the shallow water sediments. During the past several 

million years, the process of sediment deposition has continued 

with the uppermost layer formed of materials supplied by the 

continental ice sheets of past ice ages.

The sea floor surrounding the Cape has a geologic character and sediment similar to areas onshore because of similar 

geologic origins. However, the sea floor has been highly 

reworked by waves and currents as the sea level has risen 

during the past 15,000 years. Knowing details of the offshore 

geology is important for linking onshore to offshore geology 

and processes. The U.S. Geological Survey Woods Hole 

Coastal and Marine Science Center has been actively mapping 

the Massachusetts nearshore, including Cape Cod, for the past 

several years.

The Role of Coastal Erosion

The shoreline changes that we see today along the shore 

of the outer Cape are part of the continuing growth and 

geologic evolution of the New England continental margin. 

Glacial ice eroded old continental crust that had been pushed 

high by former active margin events, and the sand, clay, gravel, 

and boulders were transported to the coast by the flowing ice. 

Today, we find these sediments towering up to 100 feet (ft; 

30 meters [m]) in bluffs above the sea. Tomorrow, coastal 

erosion will have transported these sediments seaward and 

incorporated them into the growing continental shelf.

Finally, these sediments are incorporated into shelf 

deposits by the sea through coastal erosion. Storm waves 
14 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

cut into the glacial deposits, and currents carry the eroded 

sediments offshore where they are deposited on the shelf. 

Eventually, the open Atlantic Ocean will close again. The miles 

of sediment now covering this passive continental margin will 

become part of a complex mix that will include igneous mantle 

material and ocean floor sediment and crust, and the mix will 

be forged into new continental crust. The cycle is long, but 

continuous. We experience a part of it each time we see a 

winter wave cut into the reddish sand of a Cape Cod bluff.
Cape Cod’s Coastal Landforms: Works in Progress 15

Chapter 2.

Cape Cod’s Coastal Landforms: 

Works in Progress

Cape Cod is a geologically new landform on a geologically old continent. It is sometimes said that it is a new and a 

temporary land, but that depends on your point-of-view and 

time frame. The North American continent is certainly old, 

having separated from Europe and Africa about 200 million 

years ago, but its margin—upon which the Cape rests—has 

built up since then. In fact, it is still building, and the Cape 

supplies raw material for its continued development. Glaciers, 

similar to rivers, are vehicles for transporting sediments for 

continental shelf construction. The Cape may disappear from 

the visible, subaerial world in 

several millennia as sea level 

continues to rise and erosion 

continues, but its contribution 

to the North American continent’s margin will remain.

Cape Cod is a geologically 

new landform on a 

geologically old continent.
16 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

“The Provincetown peninsula may be expected to 

outlast the Truro mainland; for as long as the latter 

exists, the former must receive contributions from 

it ... All these changes are rapid, as changes go on 

the Earth’s surface. The Truro mainland will soon 

be destroyed, and the sands of Provincetown will be 

swept away as the oceanic curtain falls on this little 

one-act geographical drama” (Davis, 1896).

So how new is Cape Cod to North America? Just as new 

as its native people—both came as a result of climate change, 

toward the end of the last ice age. About 15,000 years ago, 

lowered global sea levels provided a pathway for eastward and 

southward movement of Asiatic peoples along North America 

and South America. At about the same time, the continental ice 

sheet originating in eastern Canada left vast deposits of sand, 

gravel, boulders, silt, and clay along the east coast of presentday New York, Connecticut, Rhode Island, and Massachusetts. 

Long Island in New York, Block Island in Rhode Island, and 

Martha’s Vineyard and Nantucket in Massachusetts as well 

as the Cape were formed from the deposits left by three great 

arcuate lobes of the southeastern margin of the Wisconsinan 

ice sheet (fig. 3).

Following the last retreat of the ice sheets as the climate 

warmed, global seas rose about 400 ft (120 m), submerging 

much of the coastal glacial deposits and sculpting the edges of 

the rest through the action of waves and tides. Land managers 

use the term uplands to describe landforms that were created 

during the glacial deposition period and consist of glaciofluvial 

sediments virtually undisturbed since that time.

In contrast, wetlands refers to more recent landforms, such 

as beaches, dunes, and marshes—which may or may not be 

actually wet—created from sediments eroded from the original 

glacial deposits and transported by ocean currents and wind. 

The connection between uplands and wetlands, although sometimes direct, is often indirect. Storm waves undercutting an 

upland sea cliff may immediately carry the eroded sand to an 

adjacent beach for example, but some of the same sand grains 

may reach a distant dune field many years or centuries later 

through wind erosion of a beach far away from the original 

upland source.

Human activities affecting wetlands such as beaches, 

dunes, and marshes are controlled by State and Federal regulations to conserve the value of the wetlands as natural resources 

for two quite distinct albeit complementary reasons. Statelevel regulations, such as the Massachusetts coastal wetlands 
Cape Cod’s Coastal Landforms: Works in Progress 17

Figure 3. Landforms produced by the Wisconsinan glaciation in southeastern New England and New York. Redrawn partly from 

Gustavson and Boothroyd (1987) and Uchupi and others (1996).
18 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

regulations, attempt to reduce disturbances to wetlands to 

preserve for public benefit their function as buffers against 

storm waves and flooding that could otherwise damage upland 

development. Federal regulations by the National Park Service, 

U.S. Fish and Wildlife Service, and U.S. Environmental Protection Agency, on the other hand, tend to emphasize the value 

of wetlands as habitats for a wide variety of wildlife and, more 

generally, as naturally functioning ecosystems.

Beaches

Beaches are the most basic of the wetlands because they 

link all wetlands to their upland sources. When storm waves 

erode beaches that blanket glacial deposits on eroding upland 

coasts, they cut away at those long dormant sands, silts, and 

gravel and throw them into a powerful mixer.

Silts and clays, some of the finest sediments, will end up 

in a quiet tidal marsh or far offshore in deep water, whereas 

much of the fine sand will build up offshore sand bars. But 

breaking waves will gradually transport the sand and gravel 

along the shore as part of “a river of sand,” including the 

beaches of the outer Cape, a vast deposit broken only by a few 

tidal inlets, such as at Nauset, from Monomoy Point in Nantucket Sound to Long Point in Provincetown Harbor.

At places along the coast where barrier beaches such as 

Nauset Beach have formed, waves, storm-enhanced high tides, 

and very high spring tides will, at times, carry some of the 

sediment up and across the low dunes behind the beach to build 

delta-like lobes of sand within a lagoon or bay. At other places 

or other times, onshore winds will blow sand landward into the 

dunes, adding to the height and resistance of dunes to stormsurge wave overwash.

Zones of the beach.—An outer Cape Cod beach consists 

of a series of zones extending seaward from the dunes or bluffs 

out to deep water. Central to this series of bands that continue 

out of sight along the length of the shore is the foreshore, 

the steep seaward-facing slope that is alternately submerged 

and exposed as tides rise and fall. The swash and backwash 

of waves breaking on the foreshore move sand alternately 

landward and seaward. Because waves generally approach the 

beach at an angle, at the end of each swash-backwash cycle the 

sand is also transported along the shore in the direction dictated 

by the angle of the waves. The backshore, which lies behind 

the foreshore, has a more nearly horizontal surface, though it 

is not without significant features of its own. The noticeable 
Cape Cod’s Coastal Landforms: Works in Progress 19

Beaches nourish 

all wetlands.
20 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

change in slope where the two meet is called the berm of 

the beach. At high tide during periods of spring tides or storms, 

the swash frequently flows up and over the berm onto the near 

reaches of the backshore. Farther back, however, on the mostly 

dry parts of the backshore, beach vegetation often appears, first 

sparsely with scattered beachgrass and searocket volunteers but 

then becoming increasingly dense.

Seaward of the foreshore is the dynamic surf zone, the 

highest parts of which may be exposed at low tide, in particular 

the tops of the innermost sand bars. On the outer Cape, there 

are typically two bar systems parallel to the shore, the inner 

and outer bars, both long and mostly submerged ridges of 

sand. Usually out of sight during storms, the presence of bars 

is indicated by turbulent zones 

of offshore breaking waves. At 

these times, they are the most 

visible, audible, and dangerous 

parts of the entire beach.

Rip currents and 

nearshore circulation.—The 

largest breaking waves typically 

drive a distinct flow pattern 

in the surf zone. Even in fair 

weather during summer, incoming swell frequently produces 

currents near the shore that pose a hazard for swimmers. Simply 

put, a packet of high waves breaking on the inner bar throws an 

excess of water into the confined area between the bar and the 

beach, raising sea level there sufficiently to drive first a current 

that flows along the shore between the bar and the beach and 

then a more narrow but more rapid current that flows seaward, 

away from the beach (fig. 4). This seaward flow, the rip current, 

can be fatal to swimmers caught unaware in its grip.

Seasonal beach changes.—Beaches undergo distinct 

changes between the stormy winter season and the calmer, 

quieter summer. In autumn, the energetic high and steep storm 

waves erode the foreshore, producing a gentle foreshore slope 

and a steep escarpment, or beach scarp, between the foreshore 

and the backshore. Subsequent storms push the scarp further 

back until finally the waves cut directly into the upland bluff or 

sand dune lying landward of the backshore. Where upland bluffs 

lie behind the beach, glacial deposits—boulders or clay layers, 

for example—may be exposed where only the summer before 

they lay buried beneath 5 or even 10 ft (1.5 to 3 m) of backshore 

or foreshore sand. Although not exposed to view, storm wave 

erosion of glacial deposits continues offshore as well as in the 

surf zone and beyond—as much as 1 mi (2 km) from shore.

On outer Cape Cod, there are 

typically two bar systems 

parallel to the shore, the 

inner and outer bars, both 

long and mostly submerged 

ridges of sand.
Cape Cod’s Coastal Landforms: Works in Progress 21

Of course, not all areas erode together. Sand bars enlarge 

and shift as eroded foreshore and backshore sand is transported 

seaward at the height of a storm. As these offshore shoals grow, 

they help dissipate the force of the waves.

Figure 4. Nearshore circulation and rip currents.

As the storm center moves away and the chaotic storm 

waves subside, more orderly swell waves move the sand 

landward from the nearshore. With the advancement of the 

seasons, winter and spring storms wane and this landward 

return of nearshore sand becomes the dominant pattern. The 

foreshore steepens and the backshore widens until finally, in 

summer, high, wide beaches return.

The land behind the beach.—If we focus only on the 

beaches of the outer Cape Cod coast, we see a familiar form 

everywhere—sandy backshores and foreshores, only slightly 

different from each other, from Monomoy to Provincetown. 

But the same coast takes on a very different character when 

our view includes the landforms behind the beach: barrier 

beaches from Chatham to Eastham, glacial bluffs from Eastham to Head of the Meadow in Truro, and dunes and barrier 

beaches northward through Provincetown.

Sand bars enlarge and shift as eroded foreshore and 

backshore sand is transported seaward at the height 

of a storm.
22 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Figure 5. The bluff coast of Truro, Mass.

Bluffs

We began this review of coastal wetlands with a discussion of beaches, saying that they are the most basic landform 

because they link all wetlands to their upland sediment sources. 

We could just as well have begun with the outer Cape’s bluffs 

and said that these bluffs and their submerged sea floor extension are the most basic landform because they are the source of 

the sediments that make up all the others.

 We cannot overemphasize the fact that all the sand that 

makes up the Cape’s beaches is ultimately derived from eroding 

upland glacial deposits. Unlike many other coasts, there are no 

rivers supplying sediment to these shores. It has been calculated 

that, were the eroding glacial bluffs to be protected somehow 

from wave erosion, the beaches fronting the glacial bluffs from 

Eastham to Truro (fig. 5) would disappear in less than 100 years. 

Elsewhere, this erosion might 

be slowed by engineering 

structures, such as seawalls and 

revetments, intended to protect 

bluffs and houses. However, 

such structures usually result in 

increased erosion of the beach 

and adjacent shoreline because 

All the sand that makes up the 

Cape’s beaches is ultimately 

derived from eroding upland 

glacial deposits.
Cape Cod’s Coastal Landforms: Works in Progress 23

they block sediment supply to normal coastal processes and 

focus wave energy on the rigid seawall, which is reflected back 

offshore, transporting sand offshore as well.

Glacial history.—Based on geologic study and mapping of 

offshore regions, globally rising sea level reached the eastern 

coast of the outer Cape about 10,000 years ago, eventually 

submerging Stellwagen Bank and Georges Bank. This began 

the process of coastal erosion and bluff retreat that is still 

continuing today. The bluffs once extended as much as 4 mi 

(7 km) seaward of their present position. They were higher than 

now, because sea level was lower then and because the upland 

surface sloped upward toward the east. This slope indicates that 

glacial meltwater from an ice lobe to the east transported the 

future outer Cape sediments westward to form a vast delta in 

what is now known as the former Glacial Lake Cape Cod. Silt 

and clay deposited on the floor of this former lake are found 

today beneath the sandy sediments of the outer Cape.

Bluff form and retreat.—Sailors approaching the Cape 

and looking westward at the bluffs of the outer Cape see a 

long stretch of unvegetated or sparsely vegetated banks with 

an almost constant slope rising up to more than 150 ft (46 m) 

to the relatively flat, wooded upland surface. This view was so 

common to mariners arriving from sea that this area is commonly 

referred to as the tablelands of Cape Cod on older charts (fig. 6).

Sailors approaching Cape Cod and looking westward at the 

bluffs of outer Cape Cod see a long stretch of unvegetated 

or sparsely vegetated banks with an almost constant slope 

rising up to more than 150 feet to the relatively flat, wooded 

upland surface.
24 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

The bluffs once extended as 

much as 4 miles seaward of 

their present position.

A

Figure 6. Historical views of the tablelands of Cape Cod, Mass., from A, Des Barres (1779) and

B, U.S. Coast Survey (1872).
Cape Cod’s Coastal Landforms: Works in Progress 25

B

Figure 6.—Continued.
26 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Unlike the stable, vegetated upland surface, the glacial 

deposits exposed in these eroding bluffs have a sandy appearance, so much so that even some long-time Cape residents 

refer to them incorrectly as dunes. However, when they have 

been recently undercut by winter storm waves, bluff sediments 

slump, revealing a fascinating display of the varied sediment sizes, 

shapes, colors, and features left by glacial streams. The work 

of rain and wind eventually erases these distinctions. When a 

few years pass without erosion, beachgrass takes root, and subsequently a more complex plant community—including beach 

plum, seaside goldenrod, and searocket—can appear.

Bluff erosion events are episodic. They follow a fairly clear 

but irregular pattern that is intimately tied to the long-term beach 

changes that underlie the regular winter and summer profiles. Over 

many years, the beach at a particular location becomes wider or 

thinner as a result of changes in the alongshore transport of sand, 

referred to as littoral drift. In the thinning phase, practically all the 

backshore sand fronting the bluff toe can be cut away by winter 

storms, leading to storm waves at times of high tide undercutting 

the bluff and unstable sediments above slumping downward.

As this cutting and slumping process continues, storm after 

storm, sediment slumps extend to the bluff top. Many feet of the 

upland surface, on rare instances more than 20 ft (6 m), can be 

lost this way during or shortly after a single storm. Bluffs in 

this unstable condition pose a threat to passersby and children 

playing on the beach below. Such catastrophic sudden burial by 

slumping and burial of the bluff can prove fatal.

The phase of beach thinning and bluff erosion typically lasts 

several years, after which the bluff face becomes smooth once 

again, regaining the typical angle-of-repose of about 35°. Several 

points concerning the buff erosion process should be mentioned. 

First, at any one time there will be erosion hot spots along the bluff 

section of the outer Cape where most storm-related bluff erosion 

will occur. Second, a counterpart to the first, at any one place along 

the line of bluffs there will be periods of erosion separated by longer periods of no erosion. Many property managers have come to 

grief, because seeing no bluff erosion for an extended period, they 

forget about the long-term danger. Finally, although vegetation 

on the bluff face helps reduce 

bluff sediment transport by wind 

and rain, almost all bluff erosion 

and slumping on the outer Cape 

is the result of storm waves 

undercutting the base of the 

bluff; vegetation plays no role in 

stopping that process.

Slumping bluffs on Cape Cod 

are the result of storm waves 

undercutting the base of the 

bluff; vegetation plays no role 

in stopping that process.
Cape Cod’s Coastal Landforms: Works in Progress 27

Although there is a large 

variability in the rate of bluff 

erosion from year to year, 

over long periods of time, as 

much as a century, the entire 

bluff section of the Cape 

retreats at an average rate of 

about 3 feet per year (ft/yr; about 1 meter per year [m/yr]). It 

was thought for many years that this long-term average retreat 

held constant along the section from Truro to Eastham and 

that, as a result, the orientation of this coast was also constant. 

Results of recent study, however, indicate that the rate of bluff 

retreat doubles along the bluff section from north to south, 

from about 1.5 ft/yr (0.5 m/yr) in North Truro to a little less 

than 3.5 ft/yr (1 m/yr) at Coast Guard Beach in Eastham.

The discussion of bluff erosion would not be complete 

without extending our picture of bluff erosion as the sole 

source of sediment for Cape Cod beaches, dunes, and marshes. 

Glacial deposits also lie beneath temporary blankets of sand 

offshore of the bluffs. Storms erode these seafloor glacial 

deposits along with the bluff deposits. Sediments from both 

sources contribute to the growth of wetlands and beaches.

Dunes

Sand dunes typify Cape Cod to many people—the French 

explorer Samuel de Champlain, rounding Race Point in 1604, 

called the place Cap Blanc for its white hills of sand—and in 

fact, when viewed from the sea, dune sand and its vegetation 

is what one sees wherever glacial bluffs are absent. Unlike the 

bluffs of the Cape, which owe their form to erosion, coastal 

dunes are depositional landforms, built of sand washed by 

waves along the shore until, once at the surface of the backshore, either strong winds or storm waves carry the sand inland 

out of the reach of the sea.

Wind and dune vegetation are the primary factors in sand 

dune development. On a still day, sand grains lie quietly in justthe-right-size pockets provided by their neighbors, and even 

in light winds strong enough to roll them around on a table 

top, they stay in place on the rough dune surface. However, 

when the wind reaches a threshold velocity for those particular 

grains, they literally jump into action.

Saltation is a term geologists use to describe the motion 

of sand when it bounces and skips along a surface. When the 

wind is strong enough, grains pop up and away from the dune 

surface, fly in an arc above it, and slam down, pushing others to 

The entire bluff section of 

Cape Cod retreats at an 

average rate of about 3 feet 

per year.
28 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

follow suit. This could and sometimes does lead to mass movement of sand, but not if blades of beachgrass are in the way. 

The grass calms both the wind and the bouncing sand grains, 

and there they stay while sand grains that are not thus constrained fly wildly above sand that is restrained by grass. Thus 

it is that wind and grass together make dunes. Wind brings in 

the sand; grass keeps it in place.

Dunes of the Province Lands .—Under the current [2015] 

conditions of wave configurations and sea level, most of the sand 

eroded from the Cape’s glacial bluffs eventually makes its way 

to the Provincetown Hook, contributing to the development of 

the spit both above and below the surface of the sea. At this 

moment in the Cape’s geologic history, the major zone of growth 

lies between the Peaked Hill Bar and Race Point. Geologically 

speaking, the shore of the spit on the northern end of the Cape is 

young, having developed within the past several thousand years. 

Dry winter winds from the north and northwest (the prevailing 

wind direction in winter) move sand from the ever-widening 

beach to the dunes behind.

Dune zones.—Dunes form best when two conditions coincide—accreting sandy beaches and strong onshore dry winds—

and this combination is just what we find between the Peaked 

Hill Bar and Race Point on the spit at Provincetown Hook. 

As the beach grows wider year after year, waves reach the 

landwardmost part of the backshore less and less frequently 

and beach vegetation takes root. Seeds and roots concentrated 

in former storm wrack lines are major contributors, producing 

beachgrass (Ammophila breviligulata), searocket (Cakile edentula spp.), and beach pea (Lathyrus japonicus), stalks and vines 

that inhibit sand grains from bouncing inland with the northwest 

wind. Through time—and only a few years are needed—the 

newly accumulated foredune surface grows high enough to protect its grassy surface from the highest tides and all but the most 

extreme storm waves.

The large, primary dunes rise landward of the foredunes 

and may have a much more diverse suite of vegetation, often 

including beach plum (Prunus maritima), poison ivy (Toxicodendron radicans), and beach rose (Rosa rugosa). At places, 

however, particularly where some disturbance such as a footpath has destroyed the plant cover, a deep depression may have 

been cut by the wind, forming a blowout. Still further landward 

are the secondary dunes, the oldest and sometimes highest 

of all dunes. With their mature forests and intervening fresh 

water ponds, secondary dunes are as close to upland as is ever 

found in the Province Lands, and indeed the highest part of 

the village of Provincetown is built upon the secondary dunes. 
Cape Cod’s Coastal Landforms: Works in Progress 29

Wind and grass together 

make dunes.
30 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Northwest of High Head are several parabolic sand dunes on 

the northern side of Pilgrim Lake and U.S. Route 6. The movement of sand by wind to the west and south often encroaches 

on the highway as the dunes migrate. Periodically, the State 

highway department has to clear the road to keep the road open 

to traffic. This is but one example of natural processes affecting 

the human-built environment.

Barrier Beaches and Salt Marshes

Past Race Point to Herring Cove and beyond to Wood 

End and Long Point, barrier beaches become the dominant 

landform. Barrier beaches are the most common coastal feature 

along the Atlantic shore of the United States south of New 

England and also along the Gulf of Mexico, but they are less 

common north of Cape Cod.

Essentially a long beach and dune complex separating 

estuaries, bays, and marshes from the open sea, barrier beaches 

are often overwashed or even breached by storm waves and 

tides; therefore, land managers commonly limit pedestrian and 

vehicular traffic to protect the vegetative cover of the dunes. 

Although the importance of protecting dune vegetation cannot 

be overemphasized, it must also be remembered that overwash, 

Although the importance of protecting dune vegetation 

cannot be overemphasized, it must also be remembered 

that overwash, breaching, and even new inlet 

formation are all part of the natural functioning of 

barrier beach systems.
Cape Cod’s Coastal Landforms: Works in Progress 31

breaching, and even new inlet formation are all part of the 

natural functioning of barrier beach systems.

Herring Cove and Long Point.—Most of the sand that 

is being provided to the active beach by bluff erosion today 

ends up along the shore of the spit at Provincetown, and some 

of it gets around the corner of the spit at Race Point (fig. 7). 

The present spit that extends from Peaked Hill to Race Point 

is just the most recent of a series of spits that have built up 

outward from the Truro upland over many thousands of years. 

The outer ends of those former spits have eroded, producing 

a broad, shallow shelf between Herring Cove Beach and the 

steep submarine slope that descends to the 150- to 200-ft (45- 

to 60-m) water depths of the inner continental shelf.

Herring Cove Beach is an example of a barrier beach that 

is eroding and migrating landward. Although the beach itself 

is eroding, the dune behind the beach continues to grow or 

at least to maintain its volume as it retreats in the absence of 

human intervention, so the barrier beach generally is able to 

survive, albeit in a more landward location.

Salt marshes.—Standing between the open sea and the 

quiet waters of bays, lagoons, and estuaries, barrier beaches 

provide protection from ocean waves and currents; this is a setting in which salt marshes develop and thrive. Salt marshes are Figure 7. Race Point and Herring Cove, Mass.
32 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Standing between the open sea 

and the quiet waters of bays, 

lagoons, and estuaries, barrier 

beaches provide protection 

from ocean waves and currents; 

this is a setting in which salt 

marshes develop and thrive.
Cape Cod’s Coastal Landforms: Works in Progress 33

dominated by several types of salt-tolerant plants, such as Atlantic cordgrass (Spartina alterniflora), and saltmeadow cordgrass 

(Spartina patens), which are well known for their rapid growth 

and for being essential habitats for many wetlands animals.

Marsh plants differ in the degree to which they can withstand inundation by seawater, but none can survive at elevations 

much below mean tide level. As a result, marsh vegetation tends 

to form in bands bordering the topography of the marsh basin. 

The band that covers the inside slope of the protecting barrier 

beach is called the back-barrier marsh, and the band along the 

inner upland boundary is known as the fringing marsh. Mudflats with tidal channels typically lie between the two. The salt 

marsh that lies between the western end of Provincetown and 

Herring Cove Beach illustrates this marsh system.

The Nauset-Monomoy Barrier System.—Nauset Beach 

and the Monomoy spits and islands (fig. 8) comprise the major 

barrier beach system of the outer Cape and one of the most 

important in New England. The entire system stretches more than 

20 mi (32 km) from Coast Guard Beach in Eastham to Monomoy 

Point. The southern boundary of the Cape Cod National 

Seashore is defined as the southernmost extremity of Nauset 

Beach, and North and South Monomoy Islands are protected as 

national wildlife refuges by the U.S. Fish and Wildlife Service. Figure 8. Nauset Marsh, Mass., estuary and barrier beach system.
34 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Almost all of Nauset Beach, like Herring Cove Beach 

to the north, is sediment deficient; loss of beach sand 

exceeds supply, producing coastal erosion and shoreline retreat. 

The exception is the southernmost extremity of the barrier system 

at Monomoy Point on South Monomoy Island, which receives 

sand that is eroded from the coast to the north and continues to 

build southward toward Nantucket.

The entire Nauset barrier island complex is well-known 

for its rapidly changing shoreline and dynamic tidal inlets. At 

the northern end of this barrier system, the spits at Nauset Inlet 

together almost entirely enclose Nauset Harbor. Just to the south, 

the barrier beach has migrated landward to such an extent that it 

joins the upland and forms an upland-fringing dune and beach 

complex for a short distance. It is here that Orleans has established its popular town beach, 

and this is also the access 

point for off-road vehicles 

bound for points farther 

south on Nauset Beach. The 

uninterrupted barrier finally 

ends in North Chatham (fig. 9) 

at a recently [2007] formed 

tidal inlet. To the south are a 

The entire Nauset barrier 

island complex is well-known 

for its rapidly changing 

shoreline and dynamic tidal 

inlets. Figure 9. Looking south from Chatham to Monomoy, Mass., along 

the Nauset barrier beach.
Cape Cod’s Coastal Landforms: Works in Progress 35

barrier island, a second tidal inlet, a recently attached former 

barrier island, and North and South Monomoy Islands. North 

and South Monomoy Islands together made up a single spit until 

a storm in 1958 breached the spit, creating Monomoy Island. 

Then during the Blizzard of 1978, Monomoy Island separated 

into two islands. The geologic evolution of Monomoy is a 

classic example of the dynamic nature of the coastal landforms 

on the Cape.

Tidal inlets.—Tidal inlets are pathways that facilitate tidal 

flow between the open ocean and lagoons and estuaries. Inlets 

are critically important for coastal ecosystems that depend 

on tides for the supply and removal of water and nutrients. 

They are also of major importance to local communities that 

rely on them for providing access to the sea and regular flushing of inland water bodies.

Tidal inlets along the outer coast of Cape Cod typically 

shift in position, and it is not unusual for a storm-produced 

breach through a barrier beach to develop into a new tidal inlet. 

Newly formed tidal inlets often cause problems to people and 

communities in the short term by shifting channels and shoals 

and allowing larger waves to reach previously protected inland 

shores. Eventually, however, the new inlet is likely to replace 

its predecessor and provide inland estuaries and lagoons with 

the same or even an improved level of tidal flow.

Inlets are critically important for coastal ecosystems that depend 

on tides for the supply and removal of water and nutrients.
36 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer
Global Climate: Glaciers and Sea-Level Change 37

Chapter 3.

Global Climate: Glaciers and SeaLevel Change

As we have seen, both the past and future of the 

continental margin depend highly on forces interior to the 

planet that move vast plates of the Earth’s lithosphere, carrying 

oceanic and continental crust, crushing them together and 

destroying oceans and creating new mountain ranges or tearing 

them apart and creating new oceans. Such interior forces 

dominate active margin processes and are known as endogenic

forces. However, the passive margin on which the Cape is 

built is now at a different stage of the cycle, one dominated by 

exterior or exogenic forces.

Exogenic forces can take many forms. The gravitational 

interaction between the Earth and the Moon is an example; it 

produces astronomical tides. The primary exogenic processes 

on the Earth, however, are driven by radiation from the Sun. 

Solar radiation drives the continual interplay of Earth, sea, 

atmosphere, and life, sculpting passive margins and producing 

coastal landforms such as those here on the Cape. It produces 

the rivers and glaciers that erode mountain ranges and pile up 

the debris in thick wedges to cover the continental margins, the 

winds and waves that shift coastal sediment, and the climate 

and climatic changes that determine which regime, ice or 

water, will provide the sculpting tools.

Solar radiation drives the continual interplay of Earth, sea, 

atmosphere, and life, sculpting passive margins and producing 

coastal landforms such as those here on Cape Cod.
38 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Global Climate

The terms climate and weather are easily confused and 

often—but incorrectly— used interchangeably. Climate describes 

the long-term (decades to centuries) cumulative weather record 

based on observations and measurements and is the average of 

the highly variable weather cycles. Weather, on the other hand, 

is the short-term state of the atmosphere at a specific location.

Global climate has been highly variable throughout Earth’s 

history. The variability results from complex interactions between 

the land, ocean, and atmosphere as influenced by changes in the 

Earth’s orbital movements around the Sun and variations in solar 

output. Geologic and historical 

records show cycles in global 

sea levels associated with global 

climate change.

The Earth’s climate is 

determined by a delicate balance 

between the energy that the 

Earth absorbs from the Sun and 

the energy it radiates back out 

to space. Changes in climate are 

driven by disturbance to this 

balance at various scales of space and time. Although there are 

variations in the amount of energy radiated by the Sun, it is not 

the amount of energy radiated by the Sun that concerns us. For 

our purposes, we can consider that to be relatively constant.

What does concern us about our changing climate is 

the amount of incoming solar energy that the Earth absorbs, 

which varies as the result of many factors (fig. 10). Clouds, 

for example, reflect incoming sunlight and reduce the amount 

absorbed; the oceans absorb more sunlight than land; ice-free 

regions absorb more sunlight than ice-covered regions. 

Volcanic ash, aerosols, and gases in the upper atmosphere 

reflect sunlight and can lead to short-term global cooling. 

Because geologic and atmospheric changes influence the 

distribution of clouds, ash, water, and ice, they too lead 

to changes in the amount of solar radiation absorbed at 

the Earth’s surface.

What about the output side of the balance—the energy 

radiated by Earth back out to space? The output, or back radiation, 

depends on the Earth’s surface temperature. When the planet 

takes in more heat, it sends out more heat, bringing the system 

back into equilibrium, but at a warmer temperature. The reverse 

situation holds as well: when less heat is taken in, less is sent out. 

Again a balance is reached, but now at a cooler temperature.

Climate describes the longterm (decades to centuries) 

cumulative weather record 

based on observations and 

measurements and is the 

average of the highly variable 

weather cycles.
Global Climate: Glaciers and Sea-Level Change 39

Figure 10. Solar radiation and its effects on global climate.

Other factors in addition to the planet’s temperature affect 

the output. Chief among these are atmospheric greenhouse 

gasses such as water vapor, carbon dioxide, and methane that 

intercept some of the outgoing radiation and prevent its loss to 

outer space. As a result, temperatures rise until a new balance is 

reached. It is not incidental that several significant greenhouse 

gasses are produced by life forms, and without their presence, 

the planet’s temperature would be much less hospitable to 

those life forms!

We now know that humans, by burning fossil fuels (for 

example, coal, oil, methane gas), are producing carbon dioxide, 

a major greenhouse gas, at a rate that threatens to warm the 

planet to temperatures higher than in the past several million years and even above those temperatures that we would 

consider to be hospitable to humans and many other life forms 

both on land and in the oceans.

Continental Glaciation

The lobes of the ice sheet that formed Cape Cod resulted 

from a marked global cooling, the most recent of a series of 

climate fluctuations that the Earth has experienced over the 

past several million years, a period known as the Pleistocene 
40 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Epoch of geologic history. At least four major and ten or more 

minor cooling events, called glacial stages, occurred during the 

Pleistocene. The last glacier advance, known as the Wisconsinan 

glacial stage, began less than 100,000 years ago.

The source of the vast ice sheets that eventually blanketed 

all of New England lay far to the north in the Hudson Bay 

region of Canada. At the beginning, due to alterations in oceanic and atmospheric circulations and changes in the Earth’s 

orbit, snow began accumulating year by year. By its very 

existence, the spreading snow cover would have reflected and 

therefore reduced the total amount of absorbed solar energy 

and contributed to global cooling.

Flowing ice.—Due to the pressure exerted by its own 

weight, the thickening snow became increasingly dense until 

it started to become ice when it reached dozens of feet thick. 

When the ice sheet grew larger and thicker, gravity forced it to 

start flowing, spreading south, and further depressing the planet’s 

temperature. At the same time, the sheer weight of the ice sheets 

also depressed the land beneath them and caused the land surface 

hundreds of miles from the ice margin to bulge upward.

Advance and retreat of glacial margins are controlled 

by the balance between ice flow outward and ice melting. As 

long as the ice sheet remains thick enough, glacial ice continually moves outward from the ice cap, from colder regions to 

warmer ones. When the ice flows outward faster than it melts 

at the glacier’s margin, the ice front advances, but if it melts 

faster than it flows, the front retreats. At times, ice flow and ice 

front advance are in balance, and the ice sheet margin tends to 

remain in one general location as long as the balance continues.

The Wisconsinan glacial advance, which was actually marked by many intervals of advance and retreat, 

finally attained its most southerly position a little more than 

20,000 years ago when the Laurentide ice sheet reached its 

southernmost limit along Cape Cod and Long Island. The 

southernmost edge of the ice margin at that time followed 

approximately the present southern coasts of Long Island, Block 

Island, Martha’s Vineyard, and Nantucket (fig. 11). Retreating 

north after a few thousand years, the ice lobe margins next took 

a position along the present location of the Cape.

The Wisconsinan glacial advance, which was actually 

marked by many intervals of advance and retreat, finally 

attained its most southerly position a little more than 

20,000 years ago when the Laurentide ice sheet reached its 

southernmost limit along Cape Cod and Long Island.
Global Climate: Glaciers and Sea-Level Change 41

Figure 11. Glacial maximum in southeastern New England.
42 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

The ice margin was made up of a series of arcuate lobes 

that projected southward; three of these lobes, named the 

Buzzards Bay, Cape Cod Bay, and South Channel Lobes, are 

responsible for the original basic form of the Cape. During this 

time, sea level was about 400 ft lower than at present, and the 

shoreline was about 100 miles (160 km) east.

Glacial moraines.—As glacial ice advances by gravity 

flow, it erodes the surface beneath, incorporating the subsurface 

material—clay, silt, sand, rock fragments, and boulders—into 

its mass; the subsurface material and the ice then all flows 

outward together. When the ice melts, it releases this transported sediment load. Thick deposits of such glacial sediment 

are found in regions where the ice front stalled or reached its 

maximum extension because, in those locations, melt equaled 

flow for extended periods and large volumes of sediment were 

released in the same place and accumulated in ridges at the 

edge of the glacier. Such ridges, characterized by unsorted 

sediments of all sizes mixed together, are called moraines.

Moraines can also be formed of material literally shoved, 

bulldozer style, in front of advancing glacial ice and then left as 

a ridge at the location where the advance stopped. This process 

is most significant when the ice front first retreats temporarily 

and then advances again, pushing in front of it the carpet of 

loose debris that it had previously left behind.

The high ridges that form the backbone of the upper and 

mid-Cape are moraines that probably formed by a combination of these two processes. Their locations and orientations 

indicate that they were deposited by the Buzzards Bay and 

Cape Cod Bay Lobes of the glacier (fig. 12). To the east of the 

current shore of the lower Cape, a third moraine associated 

with the South Channel Lobe probably ran from north to south. 

Coastal erosion would have removed the moraine long ago, 

leaving an area of submarine shoals behind. The ice sheets 

often carried large boulders; as the ice melted, the boulders, 

called glacial erratics, would be deposited with the moraine. 

Doane Rock in Eastham in the Cape Cod National Seashore is 

famous and the largest on the Cape.

Outwash plains and kettles.—In contrast to the rough, 

boulder-strewn terrain of the 

moraines, most of the Cape

consists of gently rolling plains 

that slope seaward away from 

the moraine ridges toward 

Vineyard and Nantucket 

Sounds from the Buzzards Bay 

and Sandwich Moraines and 

toward Cape Cod Bay from the 

site of the former (and hypothetical) Lower Cape Moraine. 

In contrast to the rough, 

boulder-strewn terrain of 

the moraines, most of Cape 

Cod consists of gently rolling 

plains that slope seaward 

away from the moraine ridges.
Global Climate: Glaciers and Sea-Level Change 43
44 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

These are outwash plains and they consist mostly of sand, silt, 

and pebbles that were washed out and away from the stagnant 

ice margin by streams of glacial meltwater. Frequently they 

are traversed by small valleys that may have been formed by 

flowing streams of emerging ground water or by ordinary surface 

meltwater. These groundwater-fed stream valleys, although common on the Cape, are rare in other glaciated regions.

The Cape’s outwash plains are dotted with rounded ponds 

and sometimes similar, round depressions without water. These 

are kettles and kettle ponds, and they mark the sites at which 

large blocks of stagnant ice once lay as ice chunks left behind 

by the retreating glacier. Sediment forming the outwash plains 

washed around and over them, and when the ice blocks eventually melted with the warming climate, the plains were left with 

dimples where the blocks had been. Ponds were produced 

when rising groundwater flooded the kettle floors. Some kettle 

ponds are periodically wet and dry as the groundwater level 

fluctuates seasonally and annually. The water level in the ponds 

corresponds with the surface 

elevation of the water table.

Retreat of the ice 

lobes.—The vast lobes of glacial ice that formed the major 

moraines—the Buzzards Bay, 

Cape Cod Bay, and South Channel Lobes—retreated in that 

order, from west to east. They retreated haltingly, and at times 

they readvanced. As they retreated, lakes of meltwater were 

frequently left behind, trapped between the moraines and the 

ice margins.

One such lake, Glacial Lake Cape Cod, lay between the 

Sandwich Moraine and the Cape Cod Bay Lobe when it had 

retreated north of the present-day location of Provincetown 

(fig. 12). The South Channel Lobe, lying to the east, was still 

in place and supplying layer after layer of sandy sediment to its 

outwash plain. Those sediments were deposited in the shallow 

waters of Glacial Lake Cape Cod, and for this reason they are 

sometimes referred to as deltaic deposits. Most of the upland 

of the outer Cape consists of outwash plains formed in this 

manner in several stages, producing the characteristic sloped 

terrace-like forms of the Truro, Wellfleet, and Eastham Plains.

Glacial Lake Cape Cod, lay between the Sandwich Moraine 

and the Cape Cod Bay Lobe when it had retreated north of the 

present-day location of Provincetown

Ponds were produced when 

rising groundwater flooded 

the kettle hole floors.
Global Climate: Glaciers and Sea-Level Change 45

Figure 12. Glacial retreat (18,000 years ago) and Glacial Lake Cape Cod, Mass.; redrawn from 

Uchupi and others (1996).
46 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Sea-Level Change

Sea-level change results from several processes, including 

changes to global climate. The main causes of sea-level rise are 

warming and expansion of ocean waters and melting of onland 

ice sheets and glaciers. Sea-level rise is a major effect of global 

climate warming and a growing concern in the United States 

and worldwide. The causes of sea-level rise, however, are 

complex and the result of not only warming but also regional 

and local effects of geologic, oceanographic, and atmospheric 

conditions that are highly variable.

Sea level can be expressed as global sea level or relative sea 

level. Global mean sea level or eustatic sea level is the elevation 

of the ocean used in describing the effects of climate warming. 

Relative or local sea level is the elevation of the ocean relative 

to the land surface is used to describe the combination of global 

plus the various regional and local geologic and oceanographic 

factors. Some of the important regional factors are land subsidence 

and uplift of the crust, tectonic forces, sediment compaction 

and consolidation, gravitational changes, and changes in ocean 

circulation patterns and wind patterns. The distinction between 

global and relative sea level is important for understanding 

projections of future sea-level rise and how regional factors can 

increase or decrease effects of future rise on coasts. Although 

global sea level is used in discussing future sea-level rise 

scenarios, relative sea level is most important in understanding 

the likely regional and local effects on the coasts worldwide, 

including the coast of the Cape.

One of the most evident effects of sea-level rise is shoreline change from inundation, erosion, and transgression. On 

sandy coasts, shoreline change results from changes to the 

morphology of the beach-dune system and shoreface. These 

changes do not occur as a direct result of sea-level rise, but 

rather because coasts are in an almost continual state of change 

in response to waves and currents as well as sediment availability at the coast, sea-level rise leads to shoreline change by 

altering these processes and 

the coast’s response to them.

This is especially true for 

shoreline changes observed 

during the past century (fig. 13). 

During the past century, major 

storms, variations in sediment 

supply to the coast, and 

human activity have had direct 

effects on shoreline change. 

One of the most evident 

effects of sea-level rise 

is shoreline change from 

inundation, erosion, and 

marine transgression.
Global Climate: Glaciers and Sea-Level Change 47

Figure 13. Sea-level rise since the last glacial maximum, 22,000 

years ago. Information is from Fleming and others (1998) and Milne 

and others (2005).

Large storms coupled with elevated sea level can cause 

changes in shoreline position that persist for weeks to decades 

or even longer. Complex interactions (the mechanics of which 

are poorly understood) between nearshore sand bodies and 

the underlying geology also influence the behavior of beach 

morphology. In addition, human actions to control erosion, 

protect infrastructure, extract mineral and groundwater 

resources, and maintain navigation channels have altered many 

regions of the coast.

Human actions to control erosion, protect infrastructure, 

extract mineral and groundwater resources, and 

maintain navigation channels have altered many regions 

of the coast.
48 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Global Sea-Level Change

Sea level has varied throughout the Earth’s history due 

to processes that operate over a range of spatial and temporal scales. On a global scale, sea level varies as the volume 

and mass of ocean water and the volume of the ocean basins 

change. There are two primary contributors to ocean volume 

and mass, one from thermal expansion of ocean water through 

heat uptake from the atmosphere, and the other is addition of 

meltwater from land-based ice sheets and glaciers. Oxygen 

isotope records during the past 3 million years indicate that sea 

level has varied in response to shifts from glacial to interglacial periods. These records indicate that, during the past 

800,000 years, sea-level change has been on the order of 400 to 

460 ft (120 to 140 m) within 100,000-year cycles.

Since the end of the last ice 

age 21,000 years ago, sea level has 

risen about 400 ft (about 120 m). 

Evidence from analyses of coral 

reefs indicates that sea-level rise 

between 21,000 and 6,000 years 

ago averaged 0.4 inch per year 

(in/yr; 10 millimeters per 

year [mm/yr]) and was punctuated with two distinct meltwater pulses when rates may have reached as much as 12 in/yr 

(50 mm/yr). Sea-level rise then slowed to a rate of about 

0.02 in/yr (0.5 mm/yr) from 6,000 to 3,000 years ago and has 

slowed episodically, eventually reaching a near standstill of 0 to 

0.01 in/yr (0 to 0.2 mm/yr) 2,000 to 3,000 years ago.

Sea-level rise averaged 0.06 in/yr (1.7 mm/yr) on a global 

scale in the 20th century with fluctuations occurring throughout 

the century. From 1993 to 2014, observations from satellite 

altimeter and tidal gages indicate that the rate of eustatic 

sea-level rise increased to more than 0.13 in/yr (3 mm/yr), a 

50 percent increase compared with sea-level rise during the 

20th century (fig. 14). Given this short record, it is not yet 

possible to determine with certainty whether this is a natural 

variation averaged over a decade or a definitive acceleration 

in sea-level rise due to observed climate warming. Recent 

studies suggest that the increase is due to equal contributions 

from ocean thermal expansion and ice-sheet melting as a direct 

result of global climate warming. This finding is supported 

by mapping and assessments of Greenland, Antarctica, and 

most on-land glaciers that show global warming is causing 

widespread and rapid loss of ice with the meltwater feeding 

into the oceans.

Since the end of the last 

ice age 21,000 years ago, 

sea level has risen about 

400 feet.
Global Climate: Glaciers and Sea-Level Change 49

Figure 14. Worldwide sea-level rise in the past 130 years; 

adapted from Commonwealth Scientific Industrial Research 

Organization (n.d.).

Relative Sea Level

Long-term tidal gage data from many monitoring stations around the United States, including stations in Boston and 

Woods Hole, have been the primary source of long-term and 

reliable measurements of relative or local sea-level trends during 

the past century. The rate of relative sea-level rise measured by 

tidal gages at specific locations along the U.S. Atlantic Coast 

varies from 0.08 in/yr (1.8 mm/yr) to as much as 0.17 in/yr 

(4.4 mm/yr). The lower rates, which occur along northern New 

England and from Georgia to northern Florida, are close to the 

global rate of 0.06 in/yr (1.7 mm/yr). Along the coast of southern 

New England, tidal gages in Boston and New York measured a 

sea-level rise of about 0.11 in/yr (2.7 mm/yr). The highest rates 

are in the mid-Atlantic region between northern New Jersey and 

southern Virginia. The high rates of relative sea-level rise are 

attributed to subsidence of the land surface, which is due mainly 

to adjustments of the Earth’s crust in response to the melting of 

the Wisconsinan ice sheet and to the compaction of sediments 

due to freshwater withdrawal from coastal aquifers. In contrast, 

some regions, such as Alaska, are experiencing a drop in relative 

sea level due to uplift of the land in response to the melting of 

ice sheets and glaciers.
50 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer
Global Climate: Glaciers and Sea-Level Change 51

Effects on Coastal Regions and Islands

Accelerated global sea-level rise is already affecting 

coastal regions and low lying islands, such as Cape Cod and 

the islands from Buzzards Bay and Block Island Sound to the 

Nantucket Shoals, around the United States and worldwide, 

and the effects are very likely to increase in the near future. 

The effects of climate change on coasts are not uniform but 

vary considerably from region-to-region and over a range of 

temporal scales.

Coastal lands will not simply be inundated, but will be 

modified by a variety of dynamic processes and in ways that 

vary by location. The driving forces that influence the evolution of the coast in response to sea-level rise include geologic 

framework, oceanographic processes, sediment supply, and 

human activity. All these factors interact in complex ways, 

driving the response of coastal landforms to sea-level change.

The effects of climate change on coasts are not uniform but 

vary considerably from region-to-region and over a range of 

temporal scales.

Sea-Level Change and Cape Cod

Glaciers delivered the materials (sand, gravel, silt, clay) 

for the building of the Cape 15,000 years ago; rising sea level, 

wind and weather, and waves and tides have been sculpting it 

ever since. Let’s look at the role of rising seas first. Later, in 

following sections, we will explore the work of the other agents.

Global sea level, as we have seen, was more than 400 ft 

(120 m) below its present level during the last ice age, but the land 

surface of the early Cape was far below its present level also, the 

Earth’s crust in this region having been depressed by the enormous 

weight of glacial ice. As the ice retreated, the crust rebounded, and 

by about 12,000 years ago, falling relative sea level had exposed 

an enormous coastal landmass comprising glacial deposits and 

including almost all of present Cape Cod, Martha’s Vineyard, 

Nantucket, and Georges Bank (fig. 15). Most of the floor of Cape 

Cod Bay was part of that landmass, but it was a low-lying plain 

surrounded by higher land to the east, south, and west. The spit 

at Provincetown was not present—it had yet to form—but most 

of the lower Cape was much larger than it is today and extended 

eastward as much as 4 mi (6 km) seaward of the present shore.

As the rate of crustal uplift slowed, rapidly rising global 

sea levels resulting from enormous volumes of meltwater 
52 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Figure 15. The changing shape of Cape Cod, Mass., during the past 10,000 years and the 

development of the hook-shaped spit at Provincetown; redrawn from Uchupi and others (1996).

As the ice retreated, the 

crust rebounded, and by 

about 12,000 years ago, 

falling relative sea level had 

exposed an enormous coastal 

landmass comprising glacial 

deposits and including almost 

all of present Cape Cod, 

Martha’s Vineyard, Nantucket, 

and Georges Bank
Global Climate: Glaciers and Sea-Level Change 53

flowing into the oceans from the rapidly melting continental 

ice sheets became dominant and seas flooded the margins of 

the ancient glacial landmass. The rate of sea-level rise, fed by 

melting of the northern ice sheets in North America and northern Europe, was extremely rapid (fig. 13), and by 6,000 years 

ago, the present outlines of the Cape and its many islands were 

emerging. The outer Cape was still without Provincetown and 

was still wider than today on both the eastern and western 

shores. The coast of Cape Cod Bay had a convex form, sweeping southward from High Head, then gradually southwestward over the present location of Billingsgate Shoal, once the 

historical location of inhabited Billingsgate Island, with many 

houses, a school house, and a lighthouse.

The ocean coast was still several miles seaward of today’s 

shore and differed from it in two other important respects: 

ocean wave-energy was less than now, and sand moved southward along the entire length of the coast. Both differences were 

the result of protection provided to the outer coast of the Cape 

by an enormous shoal with scattered islands lying southeast of 

the Cape at the edge of the continental shelf. Today we know 

the submerged shoal as Georges Bank.

For thousands of years, Georges Bank had existed as a 

separate landmass between the open Atlantic Ocean and the 

Cape, but by about 6,000 years ago, rising seas had finally submerged almost all the landmass. Open ocean waves, previously 

blocked by the landmass from reaching the Cape, now began 

crossing the shoals. More and more frequently, waves from the 

southeast attacked the outer cape shore, moving sand northward along the coast. Sand spits grew northward and curved 

westward, each overlapping and extending farther than its 

predecessor, each contributing to development of the landform 

that would become the spit at Provincetown Hook.

The rate of global sea-level rise during most of the past 

6,000 years was low, averaging 0.02 in/yr (0.5 mm/yr) or less, 

but the regional subsidence rate was at least twice as fast so 

that the rate of relative sea-level rise was probably close to 

0.06 in/yr (1.5 mm/yr), enough to increase water depths over 

Georges Bank by almost 33 ft (10 m) since it first submerged 

6,000 years ago.

For thousands of years, Georges Bank had existed as a separate 

landmass between the open Atlantic Ocean and Cape Cod, but 

by about 6,000 years ago, rising seas had finally submerged 

almost all the landmass.
54 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

As a result of the increasing ocean wave energy reaching 

the outer coast of the Cape from the southeast, the coast continued to erode, probably at a rate of more than 1 ft/yr (0.3 m/yr) 

until recently. The eroded sand and gravel was transported 

northward, adding to the growth of the spit at Provincetown Hook.

Development of the spit at Provincetown Hook during the 

past 6,000 years had a striking effect on the bay shore of the 

outer Cape (fig. 15). At the beginning of the period, the western 

coast had extended southwestward from High Head to the present Billingsgate Shoal in Cape Cod Bay. Unimpeded northwesterly wind waves moved eroded sand southward all along 

the coast until the growing spit at Provincetown Hook began 

extending westward and blocking those waves.

Wave energy from the northwest diminished as the Provincetown Hook grew westward, and eventually southwesterly 

wind waves became dominant along the northern section of 

the bay shore, causing the direction of sand movement there to 

reverse from southward to northward. This lessened the sand 

supply to the southern part of the coast where Billingsgate 

Shoal lies today; that decrease coupled with the continuing 

rise of sea level led to the eventual eroding away of the former 

Billingsgate Island, which is why Billingsgate Island is now a 

submerged shoal.

Although the southern section of the earlier bay coast is 

eroding currently [2015], the northern section is growing rapidly 

from sediment carried northward by southwesterly wind waves. 

The new deposits have formed a barrier beach, known as Beach 

Point, which had almost reached the spit at Provincetown Hook 

by the 19th century when the two were artificially connected.

Projections of Future Sea-Level Rise

The most recent global climate change assessments of 

the Intergovernmental Panel on Climate Change (2007, 2014) 

include model-based projections of sea-level rise by the end of 

the 21st century. Projections in Intergovernmental Panel on Climate Change (2007) are that sea level could rise by 7 to 23 in. 

(18 to 59 cm), but these projections did not include acceleration 

in melting of the last two major land-based ice sheets in Greenland and Antarctica. Projections for sea-level rise in Intergovernmental Panel on Climate Change (2014) are somewhat larger 

but still have a wide range and considerable uncertainty due to 

inherent uncertainties about global emissions of greenhouse 

gases between now and the end of the century.

More recent studies suggest that sea-level rise may accelerate even more in the decades ahead as melting of the Greenland 
Global Climate: Glaciers and Sea-Level Change 55

ice sheet and breakup of the West Antarctic ice sheet continue 

at a faster rate than previously anticipated. Global sea-level rise 

is predicted in Melillo and others (2014) to be in the range of 

1.6 to 6.6 ft (0.5 to 2 m) by 2100, but rates of rise will be highly 

variable regionally. Although a rise of 1.6 ft (0.5 m) by 2100 

is very likely and 6.6 ft (2 m) is possible, it is improbable that 

sea-level rise will exceed 6.6 ft (2 m) by the end of this century. 

This includes the Cape Cod region where land subsidence is a 

major contributor to sea-level rise. Additional modeling studies 

conclude that gravitational effects and shifts in ocean currents 

are likely to result in non-uniform rise in sea level, possibly an 

additional 1.6 ft (0.5 m) rise along the northeastern coast of the 

United States, including the Cape Cod region.

On a longer time frame beyond 2100, global warming is 

expected to continue. Climate scientists have suggested that 

accelerated melting of the Greenland and Antarctica ice sheets 

and of glaciers could lead to global sea-level rise of 13 ft (4 m) 

or more over the next several hundred years, possibly reaching the 20-ft (6-m) levels attained during the last interglacial 

warm period. Continued warming and effects of such dramatic 

sea-level rise have huge implications for all coastal regions, 

including the Cape.

More recent studies suggest that sea-level rise may 

accelerate even more in the decades ahead as melting 

of the Greenland ice sheet and breakup of the West 

Antarctic ice sheet continue at a faster rate than previously 

anticipated.
56 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Photograph by the Cape Cod National Seashore, National Park Service
Wind and Weather 57

Chapter 4.

Wind and Weather

The continual and dynamic interplay of the Earth, sea, and 

atmosphere is vividly demonstrated by our planet’s response 

to solar heating. We discussed the delicate balance between the 

energy that the Earth absorbs from the Sun and the energy that 

it radiates back to space. Although balanced for the planet as a 

whole, the amount of heat absorbed and back-radiated varies 

markedly from place to place and over time.

Most important to global circulation is the temperature 

difference from the tropics to the poles with a surplus of incoming heat energy at low latitudes and a deficit at high latitudes. 

Also important to the energy balance are the differences 

between continents and oceans because land heats and cools 

more quickly compared with water, which retains heat much 

longer. Together, the atmosphere and oceans work to balance 

these differences, moving heat by atmospheric and ocean currents from the tropics to the poles and back and forth between 

land and sea. The winds and temperatures on the Cape, situated 

as it is between the tropics and the poles and between the 

continent and the ocean, reflect that continual exchange.

Prevailing Winds

During summer in the northern hemisphere, temperature 

gradients between tropical and polar zones are relatively small. 

In contrast, temperature differences between ocean and continent are pronounced and steady. As a result, the semipermanent 

high pressure system known as the Bermuda High forms over 

the relatively cool western North Atlantic and drives a clockwise 

circulation over the East Coast of the United States (fig. 16).

The winds and temperatures on Cape Cod, situated as it is 

between the tropics and the poles and between the continent 

and the ocean, reflect that continual exchange.
58 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Figure 16. The Bermuda high atmospheric pressure zone and associated winds.
Wind and Weather 59

On the Cape, that circulation produces the summer 

prevailing southwesterly winds, known as southwesterlies. 

Because these winds are directed offshore relative to the outer 

Cape, they tend to subdue the wave action and energy. These 

winds are seldom strong, however, and summer winds are often 

dominated by local land and sea breezes or by the circulation 

associated with passage of weak weather fronts.

Winter is a different matter. The dark polar region develops a huge heat deficit, and an intense temperature gradient 

develops between it and the tropics, producing a strong polar 

front that oscillates back and forth over the Cape with passing 

waves in the jet stream. In addition, a large semipermanent 

center of low pressure, the Icelandic low, develops over the 

relatively warm North Atlantic. The difference between the 

oceanic low pressure and high pressures over the winter-cooled 

continent, although not as constant as the reverse summer 

condition, is nevertheless 

analogous to it, in this case 

producing a counterclockwise 

circulation that drives cold 

air southward.

This winter circulation 

produces the northwesterlies, 

the prevailing winds for the 

season. Less regular but much 

stronger than the summer 

southwesterlies thanks to the 

stronger pressure gradients 

that produce them, these 

winds truly dominate the 

Cape’s winter weather when conditions are fair and play a 

major role in the dynamics of coastal dunes.

Storms

Two major types of storms affect the Cape Cod coast, the 

extratropical winter northeastern storms or nor’easters and summer and autumn tropical storms or hurricanes. Nor’easters have, 

by far, the greater long-term effect on the Cape’s outer coast. They 

are initiated by the intense winter temperature gradients that exist 

between the tropics and the poles. High-latitude cold air produced 

by winter cooling meets warmer and moister lower latitude air 

along the polar front, a zone of sharp meteorological change 

associated with the jet stream. The polar front is in fact a group 

of meteorological waves that travel from west to east around the 

globe, causing the front to undulate northward and southward.

On Cape Cod, that circulation 

produces the summer prevailing 

southwesterly winds, known as 

southwesterlies.

This winter circulation 

produces the northwesterlies, 

the prevailing winds for the 

season.
60 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

As the global waves propagate eastward across North 

America, dense cold and dry 

polar air slips beneath and elevates the warmer, moister air, 

causing intense low-pressure 

systems to develop along the leading edge of the front. These 

storms are carried eastward with the waves. As a result, a 

typical winter weather pattern on the Cape consists of a storm 

arriving from the west, followed by cold, clear, windy weather 

(as the front passes), followed by warmer, moister, more 

overcast days (as the high pressure following the front passes), 

followed by another storm (as the next front) approaches.

Nor’easters and southeasters.—When the storm 

approaches from the southwest and passes west and north of 

the Cape, we get strong winds from the southeast as the storm 

nears the Cape due to the counterclockwise circulation around 

low pressure systems. These are the common southeasters.

However, if the storm comes from the southwest and 

passes south and east of the Cape, strong winds blow from 

the northeast as the storm approaches. The northeasterly 

storm winds blow directly onto the outer shore of the Cape, 

Nor’easters have, by far, the 

greater long-term effect on 

the Cape’s outer coast.

and as a result we get storm waves as well as storm winds 

during nor’easters.

A powerful variation of this scenario occurs when the 

difference in temperatures of the ocean and continent in winter 

adds an additional kick. In a typical case, a low-pressure 

system moving eastward with the leading edge of a polar front 

initiates a secondary low-pressure system over the relatively 

warm coastal water. Such winter coastal storms can develop 

intense low-pressure systems and wreak havoc along the coast 

as they travel northeastward just offshore. They are especially 

potent here because of the long stretch of open water over the 

Gulf of Maine that lies northeastward of the Cape, giving a 

vast distance, or wave fetch, over the ocean over which storm 

winds generate waves. In the event that the storm stalls offshore the Cape due to some atmospheric blocking mechanism, 

storm waves on top of elevated storm surge can batter the 

coast for many days.

Storm surge.—An additional threat associated with these 

coastal storms are the sometimes-severe storm surges that can 

elevate sea levels many feet above normal high-tide levels and 

bring storm waves closer to vulnerable, easily erodible coastal 

landforms. Such surges are essentially long ocean waves 

characterized by a wave crest, or sea-level high, that develops 
Wind and Weather 61

beneath the low pressure center of the storm and that moves 

northeastward along the coast with it. Strong onshore winds 

also contribute to the elevation of storm surges. When large 

storm surges happen to occur at periods of exceptionally high 

astronomical tides, the combination can cause severe erosion 

and damage to coastal roads and buildings.

Hurricanes.—Unquestionably, severe tropical storms, such 

as the Great New England Hurricane of 1938 and Hurricane 

Bob in 1991, have no equal when it comes to extreme damage 

in New England. But our focus in this guide is on the outer 

coast of the Cape, which was not severely affected by either 

storm. Hurricanes only rarely produce major coastal changes 

on the outer Cape for a variety of reasons. First, the beaches 

tend to be wide and full during hurricane season, late summer to 

early fall, so they provide better protection than do the depleted 

winter and spring beaches. Second, hurricane-generated storm 

surges, a major cause of flood damage, seldom amount to more 

than several feet on the outer Cape. Hurricanes typically move 

fast by the time they reach the Cape, so they usually (although 

not always) cross the Gulf of Maine too quickly to build 

extremely large waves and large surges. Hurricanes get their 

strength from moving over warm ocean waters (greater than 

80° Fahrenheit [26.7 °Celsius]) and ocean temperatures off 

the Cape are generally too cool to sustain powerful hurricanes. 

Finally, although the powerful right wall of hurricanes can be a 

major threat to the western shore of the outer Cape when these 

storms cross Cape Cod Bay, the southerly winds that accompany 

it are not much of a threat to the eastern coast. Nevertheless, 

given the power and unpredictability of hurricanes, careful 

attention should be paid to storm warnings; with the added 

dimension of climate change, the power and frequency of hurricanes may increase in future years.

When large storm surges happen to occur at periods of 

exceptionally high astronomical tides, the combination can cause 

severe erosion and damage to coastal roads and buildings.
62 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Local Winds

Each season on the Cape brings its own special suite 

of small-scale meteorological processes: lake-effect (more 

accurately sea-effect) snow in late autumn, sea smoke in 

early winter, fog in the spring, and sea breezes in summer. 

The gentle sea breezes typically blow onshore during summer afternoons when the Sun-warmed land heats its blanket 

of surface air, causing the warmed surface air to rise and be 

replaced by the cooler sea air just offshore. Sea breezes can 

bring welcome relief on sweltering afternoons, but they can 

also induce an offshore fog bank to move in over a warm, 

sunny shore. The coming of the evening counter-part, a land 

breeze, causes local winds to subside toward sunset, sometimes stranding sailors who sailed out several hours earlier in 

a brisk afternoon sea breeze.
Waves 63

Chapter 5.

Waves

Waves and the longshore currents they generate are the 

workhorses of this coastal system. Wind moves sand from the 

backshore and dunes, tides dominate inlets, and currents have a 

lot to say about the final resting place of much of the sediment. 

But most of the hard, tough work is performed by waves, at 

work every minute of every day. Glaciers brought their load of 

sediment transported from erosion of ancient mountains this far 

and stopped. Waves complete the process, pulling the piles of 

glacial rubble into the sea. But what really are waves?

Water waves are a form of water motion that transfers energy 

through space and time. Toss a pebble 

into the center of a pool now, and a 

little later, a toy sailboat at the edge of 

the pool bobs up and down. The force 

that transfers energy from stone to boat 

is gravity, and thus all the waves we 

will discuss are termed gravity waves.

Water waves are a 

form of water motion 

that transfers energy 

through space and time.

Photograph by Mary M.T. Wiant, used with permission
64 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

A pebble disturbs the water surface, gravity acts to restore 

it to the original condition, and inertia causes the water to overcompensate. The upshot for a water particle is orbital motion, 

moving in the direction of wave travel under the crest and in 

the opposite direction under the trough (fig. 17).

So although the energy flows in the form of waves across 

the water surface, out and away from the original energy 

source, the water particles themselves move around and around 

in their orbits, but do not have net motion in the direction of 

wave travel.

Figure 17. Circulation of water particles associated with a simple progressive surface wave. Orbital motion of water particles 

beneath waves is distorted in shallow water.
Waves 65

Wind-Generated Waves

Waves carry energy; the higher the wave, the more energy 

it carries, so stronger winds create larger waves. But the height 

of wind-generated waves also depends upon the length of time 

that the wind blows, termed duration, and the distance of open 

water over which the wind blows, wave fetch. This too makes 

sense when we realize that more energy can be added to an 

initially wavy sea surface when there is more time and distance 

for the wind to add the energy.

However, there are limits. As wind-generated waves grow 

in height, they also grow in length (fig. 17). In fact, they must 

in order to survive because, when a wave’s height reaches oneseventh its length, it becomes unstable and breaks. So windgenerated waves become higher and longer as the wind continues to blow. But there’s a catch; the longer the length of a wave 

is, the faster it travels, and the faster the wave travels, the less 

energy it gains from the wind that is pushing from behind.

There is another limiting factor as well, one that is 

especially important for storm-generated sea waves. As the 

waves become higher and steeper, they break more frequently, 

thereby losing energy, so a balance will inevitably be reached 

between energy input increasing wave height and energy 

loss decreasing it. There is a 

term for the balance between 

wind strength and the maximum resulting waves—fully 

developed sea.

The word sea in this expression refers to the fact that when 

winds blow, especially storm winds, they produce a mix of 

waves with different heights and lengths. What changes with 

increased wind speed, duration, and fetch is the height and 

length of the waves in the mix that carry the most energy. 

So when we speak, for example, of the fully developed sea 

produced by a 20-mile-per-hour (mi/hr; 32 kilometer-per-hour 

[km/hr]) wind we refer to the most energetic waves that result 

from a 20-mi/hr (32-km/hr) wind that has blown for a sufficient duration and over a sufficient fetch such that more of 

either would not change the result.

As the waves become 

higher and steeper, they 

break more frequently, 

thereby losing energy.

Waves get higher and longer with wind. Longer waves are 

faster but when they reach wind speed, they no longer get 

energy from wind and reach a balance (a fully developed sea).
66 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Increases in the fully-developed seas associated with 

increasing wind speeds are dramatic. The fully developed sea for 

a 12-mi/hr (19-km/hr) wind has a wave height of 1 ft (0.3 m) and 

wave length of 35 ft (11 m), and for a 50-mi/hr (81-km/hr) wind, 

a wave height of 34 ft (10 m) and wave length of 514 ft (157 m)!

Ocean Swell

The seas produced by storm winds are chaotic. Within a 

short time, turbulent winds of widely varying speeds and directions roil the same sea surface and produce a remarkably varied 

mix of waves in terms of length, height, period, and direction. 

But the size of all storms is limited, and the waves, especially 

the longer waves, eventually outrun the storms that form them. 

Waves that have moved beyond the winds that produced them 

are referred to as swell.

In deep water—that is to say water that is deep compared 

with the length of the waves—swell waves have a smooth, 

almost sinusoidal form rather like a big version of those ripples 

that formed from the pebble-in-the-pond example described 

earlier in this chapter. As we have seen, the longer the length of 

a wave, the faster the wave goes. For example, a 30-ft (9-m)-

long wave travels about 8 mi/hr (13 km/hr); a 300-ft (91-m)-

long wave, about 27 mi/hr (44 km/hr).

In addition, long waves preserve their energy much better 

than short ones. For both reasons, only the longest waves may 

reach a distant shore; of course, the more distant the coast, 

the smaller the wave’s height because even the longest waves 

lose energy as they travel. As a consequence, a long, low swell 

may be the only indication along a Cape Cod shore of a distant 

powerful ocean storm.

Wind blows, storms occur, making waves on the 

continental shelf.
Waves 67

Photograph by Mary M.T. Wiant, used with permission

Waves that have moved 

beyond the winds that 

produced them are referred to 

as swell.
68 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Waves at the Coast

Shoaling and breaking waves.—Anyone who spends 

time at the beach, especially surfers, become keen observers 

of the behavior of swell. After a long time at sea with little 

change, the swell waves are rapidly transformed at the 

shoreline. As they reach water that is shallower than their 

length, the shoaling waves slow down, decreasing in length 

while increasing in height. No longer sinusoidal, they become 

steeper with higher crests and long, broad troughs. Eventually, the heights of the increasingly high wave crests approximate the decreasing water depths, the wave fronts collapse, 

and the long-traveled swell 

waves become breakers.

Wave refraction.—

Another important and 

wonderful-to-watch consequence of the slowing of 

wave travel in shallow water 

is the bending of wave crests 

termed refraction. When 

waves approach the shore at 

an angle (as they usually do) 

or when water depths vary 

alongshore (and they usually do, too), the shallower 

part of a single wave’s crest 

slows relative to the deeper 

parts, causing the wave 

crest as a whole to bend 

(fig. 18). Typically, the crests 

of refracting waves tend 

to more nearly parallel the 

shoreline as they shoal. However, waves traveling over offshore shoals and sandbars may refract away from the shore, 

sometimes producing complex patterns

Wave period.—Despite all these changes associated with 

shoaling, one major characteristic of incoming swell remains 

constant: the number of waves arriving per unit time, or 

wave frequency. At the coast, instead of measuring frequency 

directly we often measure its inverse, wave period, the time 

that elapses between arriving wave crests.

Because of its constancy, a wave’s period measured 

at the coast gives valuable clues about the origin and 

characteristics of the deep-water wave that produced it. 

For example, the length of a deep-water wave, in feet, is 

The heights of the 

increasingly high wave crests 

approximate the decreasing 

water depths, the wave 

fronts collapse, and the longtraveled swell waves become 

breakers.

Because of its constancy, 

a wave’s period measured 

at the coast gives valuable 

clues about the origin and 

characteristics of the deepwater wave that produced it.
Waves 69

approximately the square of the wave period multiplied by 

5.1. So, when on some calm August morning, for example, 

we note long, low, 14-second swell arriving at the Highlands 

Center wave observation station, we think immediately of 

tropical storms, because our calculation indicates deep-water 

wave lengths greater than 1,000 ft (300 m), and that requires 

an impressive combination of wind speed, duration, and fetch 

such as only an intense ocean-storm system can produce.

Figure 18. Wave refraction associated with shoals.
70 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Local Wind Waves, Nor’easters, and Hurricanes

Local onshore winds usually kick up small, short-period 

waves along the shore of the Cape, and when a swell is running 

as well, the wave trains cross and produce an interesting and 

complex sea surface. Usually, the local winds are relatively 

gentle, but when local storm winds blow, the Cape can experience the most extreme conditions of all.

As we have seen, nor’easters pass the Cape Cod coast 

multiple times every winter, and occasionally they cause 

considerable erosion and damage to coasts and infrastructure. 

Among the notorious nor’easters that have ravaged the coast 

include the Portland Gale of 1898, the Blizzard of 1978, 

and the Perfect Storm of 1991 (so named because it had the 

perfect path, duration, and strong winds to produce maximum 

wave conditions). Easterly to northeasterly winds send waves 

directly onto the shore, and the unsorted mix of waves in a 

chaotic, fully developed sea ensure turbulent surf at the shore 

that rapidly erodes beaches, bluffs, and dunes.

In contrast, hurricanes—though sometimes a scourge to 

the upper Cape and occasionally also the western shore of the 

lower Cape due to the large storm surges that can accompany 

them—travel through too fast to produce seas comparable to 

those of nor’easters. Nevertheless, the intense hurricane winds 

coupled with storm-surge flooding and wave action make a 

destructive combination that can cripple all of the Cape.

Shallow Water Waves and Storm Surges

In water that is deep in comparison to the length of the waves, 

wave length alone controls the speed of waves. But in shallow 

water where the depth of the water is less than the length of the 

waves, depth also plays a role in the speed of wave travel. In 

fact, the role of water depth increases as depth itself decreases 

in relation to the length of the traveling waves. A point is 

finally reached, when the depth is only about a 20th the length 

of the wave, where water depth alone determines the speed of 

the waves. Waves whose speeds are controlled only by water 

depth are known as shallow water waves (fig. 19).

Figure 19. Calculation for the speed of a shallow water wave.
Waves 71

Easterly to northeasterly 

winds send waves directly 

onto the shore, and the 

unsorted mix of waves in a 

chaotic, fully developed sea 

ensure turbulent surf at the 

shore that rapidly erodes 

beaches, bluffs, and dunes.

Photograph by the National Park Service
72 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

But the name of this class of waves is often a source of 

confusion because shallow refers to relative depth, which is 

water depth relative to wave length. The water may not be 

shallow to observers at the shore; just shallow in relation to 

the wave length. There is no confusion if we are talking about 

ordinary coastal wind waves or swell. For example, an incoming 

wave with a wave length of 100 ft (30 m) becomes fully a shallow 

water wave when it reaches a water depth of 5 ft (1.5 m).

But waves can be longer than that, much longer. Earlier 

we spoke of 1,000-ft (300-m)-long swell generated by powerful 

distant storms. These would be shallow water waves at a depth 

of 50 ft (15 m). However, tides are also waves, and they may 

have lengths of hundreds even thousands of miles. So, tides are 

shallow water waves, too, even in the deepest ocean basins.

Tides, that is to say, astronomical tides, will be discussed in 

detail in the next section, but storm tides, or more accurately storm 

surges, are weather generated, as are the other waves discussed in 

this section. These waves are 

produced at the ocean surface in 

response to an overlying area of 

low atmospheric pressure. They 

can be thought of as an upward 

bulge of the sea surface in the 

area of low atmospheric pressure produced by inflowing water 

from surrounding areas with high atmospheric pressure.

Imagine, for example, a typical winter storm system over 

the continental shelf off the North Atlantic coast. Under the lowpressure center of the storm, the sea surface might be elevated as 

much as 1 ft (0.3 m) and as such forms the crest of a wave, the 

length of which could be more than 100 mi (160.9 km), making it a 

shallow water wave. If the shelf depth were 100 ft (30 m), then the 

wave speed would be 5.7 × 10 or 57 feet per second (17 meters per 

second), which is equivalent to almost 40 mi/hr (64.4 km/hr).

Typically such winter storms move northeastward along 

the shelf, and if the storm we are imagining were also to move at 

about 40 mi/hr (64.4 km/hr), a condition of resonance between 

the storm and the shallow water 

wave would result, and more 

and more energy would be transferred to the wave, producing 

a potentially destructive storm 

surge. It is not unusual for storm 

surges generated in this manner 

to reach heights of as much as 

3 ft (0.9 m) when they reach the 

outer coast of the Cape.

Tides are shallow water 

waves, even in the deepest 

ocean basins.

It is not unusual for storm 

surges generated in this 

manner to reach heights 

of as much as 3 feet when 

they reach the outer coast of 

Cape Cod.
Tides 73

Chapter 6.

Tides

Of the major coastal processes—winds, waves, and 

tides—tides are the most predictable. We may have little idea 

of what the wind or wind-produced waves will be next week, 

but with little trouble, we can find what the tides will be like 

then or next year for that matter. At any one place, the tidal 

cycles repeat themselves regularly. But what about changes in 

tides from place to place? Those are not so regular. Take the 

tides around the Cape, for example.

On September 27, 2015, a high tide of 11 ft (3 m) at 

10:58 a.m. was predicted for Provincetown Harbor on Cape 

Cod Bay, whereas at Falmouth on Nantucket Sound, 50 mi 

(80 km) away as the crow flies, high tide was projected to reach 

only 1 ft (0.5 m) at 7:48 a.m. At nearby Monument Beach 

on Buzzards Bay, high tide was projected to be 6 ft (2 m) at 

7:45 a.m.! Why such wide discrepancies?

To understand such variations, we must recognize that 

tides are a global phenomenon. They result from gravitational 

interactions between the Earth, the Moon, and the Sun, so we 

must step back and look at a 

big picture. To get to the big 

picture, we will illustrate the 

four types of tides involved: 

Earth tides, which are tides 

in the solid Earth; theoretical 

ocean tides, which would be 

tides in an enormously deep ocean covering the entire Earth; 

real tides in real ocean basins; and coastal tides.

Earth Tides

For the discussion of Earth tides, we will ignore the 

oceans entirely. Let us consider the combined motions of the 

Earth and the Moon. We are accustomed to thinking of their 

motions separately, the rotation of the Earth about its axis—

completed every 24 hours relative to the Sun—and the revolution of the Moon around the Earth—29.5 days from new moon 

Of the major coastal 

processes—winds, waves, 

and tides—tides are the 

most predictable.
74 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

to new moon. We do not usually think about how the two affect 

one another, as indeed they do because each moves within 

the gravitational field of the other. Their mutual interaction 

keeps the pair together as each year they orbit the Sun. This 

interaction also distorts the shape of each.

Visualize two dancers whirling around while holding each 

other firmly by the hand. Though each pulls steadily on the 

other, they do not approach but only circle one another. Their 

two arms appear as one, strained by the mutual pulling, while 

their outer arms, their hair and the loose parts of their garments 

are flung out and away. Their bodies and clothing are distorted. 

Still they remain together because each pulls the other with 

exactly the force required to produce circular motion.

So it is with the Earth and the Moon and in fact with 

any two celestial bodies held together by mutual gravitational 

attraction. The Earth and the Moon, each being within the other’s 

gravitational field, are pulled toward each other; the center of 

each circles a point between their two centers (fig. 20). Since 

the Earth is by far the more massive of the two bodies, the midpoint of their mutual motion lies near the surface of the Earth.

Because the strength of the Moon’s gravitational field 

decreases with distance, the Earth is pulled more strongly at 

locations closer to the Moon and more weakly at points farther 

from the Moon (fig. 20). Each point on the Earth feels the pull, 

and all together, the total pull on the planet is exactly what is 

required to maintain the Earth-Moon system; the variations in 

the strength of the pulling force cause the Earth to be distorted. 

A surface bulge, a high, occurs on the side of the Earth closest 

to the Moon where the pull is greater than what is needed to 

Figure 20. Gravity between the Earth and the Moon and the 

tidal bulge.
Tides 75

maintain the circular motion. On the far side, we find another 

surface bulge (fig. 20). The pull of the Moon on the far side of 

the Earth is less than what is needed to maintain the circular 

motion, and the surface bulges out and away from the Moon.

Because the Earth, in addition to dancing with the Moon, 

is continually rotating about its axis, these highs—the areas 

closest and farthest from the Moon—are continually moving 

across the surface of the globe. These are Earth tides, or more 

accurately lunar semidiurnal Earth tides. As a result of such 

Earth tides, each point on the surface of the Earth, including 

you and me, rises and falls by about 1 ft (0.3 m) twice a day. 

We experience other types of Earth tides as well, but these are 

not pertinent to our discussion. What about tides in the sea? 

To get an idea of ocean tides let us theoretically cover the 

Earth with water.

Simple Ocean Tides

Lunar semidiurnal ocean tides.—Rather than one-third 

of the surface of the Earth covered in land and the rest by water, 

think of a theoretical Earth entirely covered by seas, vastly 

deeper than our actual oceans. Every particle of sea water, being 

within the Moon’s gravitational field, would be pulled moonward. But only those at the same distance from the Moon as 

the Earth’s center would have just the right pull to maintain the 

planet’s circular motion with the Moon. All other particles of 

the sea water would experience a little more or a little less pull 

depending on their distance from the Moon. Again, two surface 

bulges would result, this time bulges of water, or high tides, and 

these lunar semidiurnal ocean tides would pass again and again 

around the world, keeping pace with the Moon (if the ocean 

were sufficiently deep) as the Earth rotated beneath them.

Solar semidiurnal ocean tides.—But the lunar tides would 

not be alone. The Earth dances with its big partner, the Sun, too. 

Being within the Sun’s gravitational field, every water particle 

in our global sea would experience a sunward pull as well. Two 

more high tides would be produced, one on the side of the Earth 

nearest the Sun and the other on the side farthest from the Sun. 

Even though the Sun is vastly larger than the Moon, its distance 

Because the Earth, in addition to dancing with the 

Moon, is continually rotating about its axis, these 

highs—the areas closest and farthest from the Moon—

are continually moving across the surface of the globe.
76 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

from the Earth is so great that the strength of its gravitational 

field on the Earth is weaker than that of the Moon. As a result, 

these solar semidiurnal tides would be only about one-half the 

size of their lunar counterparts.

Although we refer to both the lunar and solar tides created 

in this manner as being semidiurnal, that is, occurring every half 

a day, in fact their time intervals are slightly different. In the 

case of the solar tide, there are two high tides and two low tides 

during the time it takes the Earth to complete a full revolution 

relative to the Sun. So for each full cycle, one high tide to 

the next requires exactly 12 hours. For lunar tides, however, 

each full cycle takes a little longer because, although the Earth 

rotates around its axis, the Moon revolves around the Earth in 

the same direction. Approximately 24 hours and 50 minutes, 

one lunar day, is required for one full rotation of Earth relative 

to the Moon. A single lunar semidiurnal cycle, therefore, takes 

about 12 hours and 25 minutes.

Spring and neap tides.—The solar tide, its cycle being 

faster, would catch up with the lunar tide about every 2 weeks, and 

the two tides added together would produce a single extra-large 

tide. This would happen when the Sun and the Moon are aligned 

with each other at periods of new and full moon (fig. 21). Such 

large-range tides are generally known as spring tides, though 

Figure 21. Alignment of the Earth and the Moon at spring 

and neap tides.
Tides 77

they are locally sometimes 

called moon tides due to their 

association with lunar phases. 

Note that spring tides have no 

relation to seasons. Similarly, 

at half-moon periods (first and 

last quarter moons), the lunar and solar tides would be opposed 

to one another producing a smaller tidal range—a neap tide.

Diurnal tides.—Observations reveal daily, or diurnal, 

tides as well as twice-daily ones. These might be expected from 

the fact that Earth’s axis of rotation is inclined at an angle to 

its path around the Sun and to the path of the Moon around it 

(fig. 22). As a result, twice each month, the Moon is far to the 

north or south of the Equator.

At such times, one of the two high-tide bulges would be in 

the Northern Hemisphere, and the other, in the Southern Hemisphere; an observer in either hemisphere would see just one 

high tide and one low tide daily—a lunar diurnal tide (fig. 20). 

Similarly, solar diurnal tides would be at their maximum during the summer and winter solstices when the Sun is farthest 

north or south of the Equator.

Perigean tides.—Because the magnitude of the tides 

depends upon the strength of the gravitational fields of the Sun Figure 22. Relative orbits of the Earth and the Moon.

A single lunar semidiurnal 

cycle, therefore, takes about 

12 hours and 25 minutes.
78 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

and the Moon, tide heights vary with changes in the distance 

of those bodies from the Earth. This is especially significant 

in the case of the Moon because its path around the Earth is 

highly elliptical. Approximately once each month, the Moon 

reaches perigee, its closest approach to the Earth. Increased 

lunar tides, perigean tides, occur at those times. When perigean 

tides coincide with ordinary spring tides, a perigean spring 

tide is produced. The perigean spring tide in combination with 

onshore storms can produce major coastal change and has been 

responsible for major damage to coastal structures in the past.

That is what we might expect if the Earth were entirely 

covered by very deep seas, and indeed recordings of tidal levels 

around the world provide excellent examples of all these types 

of tides. We find lunar and solar diurnal and semidiurnal tides 

and many more types besides. But these tides do not occur at 

the times and with the heights that our explanation suggests. 

Real tides in real ocean basins are not so simple.

The perigean spring tide in combination with onshore storms 

can produce major coastal change and has been responsible 

for major damage to coastal structures in the past.

Real Ocean Tides

Tides in actual oceans are made 

complex by three factors that we have 

ignored so far. First of all, ocean tides 

are, in fact, water waves and real 

ocean basins are not sufficiently deep for long water waves, 

like tides, to keep pace with the turning planet. Second, oceans 

do not cover the entire Earth, but rather occupy discrete basins 

separated by continents. And third, the rotation of the Earth 

causes the tide waves to turn and follow circular paths.

Long waves.—The first factor follows directly from our 

discussion of waves. Tides are shallow water waves because 

their wave lengths measure on the order of a thousand miles 

while the average ocean depth is less than 2.5 mi (4 km). 

Long waves in oceans at this depth travel at about 400 mi/hr, 

although higher speeds occur in deep-ocean basins. In contrast, 

the Earth rotates at about 1,000 mi/hr at the Equator; tidal 

waves simply cannot keep up.

Standing waves.—The second factor is more obvious. 

An actual ocean bulge following the Sun or Moon on a rotating Earth would find its progress blocked when it reached a 

coast. What happens then arises from the wave nature of tides. 

Tides are waves.
Tides 79

The tidal wave reflects from the coast and returns again to the 

ocean basin. Then, it reaches a far shore and reflects again. 

When waves reflect back and forth within closed basins, 

standing waves form from these seesaw-like motions, similar 

to sloshes in the soup bowl that you try to carry to the table 

without spilling.

Rotary waves.—But even this motion is altered by the 

third factor, the Earth’s rotation. The planet’s rotation about 

its axis causes water moving in the Northern Hemisphere to 

be deflected to the right of its course, a phenomenon known 

as the Coriolis effect. How do tidal waves react to this effect? 

Nature has come up with a neat answer; tidal waves in real 

ocean basins are rotary waves that move around their basins 

in circular patterns (fig. 23) like horizontal mill wheels. The 

technical name of these rotary tides is amphidromic systems, 

amphidromos being an ancient Greek circular dance form.

Let us look, for example, at the actual lunar semidiurnal 

tide in the North Atlantic (fig. 24). Here we see a tidal wave 

that rotates around the ocean basin every 12.5 hours. The tidal 

range is smallest at the center of the system, the amphidromic 

point, and largest at the coasts. When it is high tide off the 

coast of New England, it is low tide off the coast of Europe; 

but in 6 hours, the crest of the high tide, rotating counterclockwise, will pass by the Bahamas, North Africa, and finally reach 

the British Isles, and by this time, the trough of the wave (low 

tide) will have spun past Greenland and reached New England.

It will not quite have reached New England, though, 

because the major North Atlantic tidal amphidromic system is 

restricted to the deep waters of the ocean basin. It reaches the 

edge of the continental shelf about 100 mi (161 km) southeast of the Cape. There, the tidal range is about 3 ft (0.9 m), 

much less than we find generally in the harbors of northern 

New England.

Figure 23. The motion of 

waves in a basin, showing 

slosh and rotation.
80 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

Figure 24. Rotary tides in the world’s oceans. The dashed lines are the cotidal lines, which show the areas that have tides at the same 

time of day.
Tides 81

Coastal Tides

Standing waves (again).—As strange as it may seem, 

tides in the Gulf of Maine, like other coastal tides, are not 

caused directly by the gravitational fields of the Moon and 

the Sun. Rather, they are directly produced by the ocean tides 

swinging around in their deep basins. Here again in coastal 

waters we find standing waves, smaller in length than the ocean 

tides but often much greater in height. These shorter length 

waves are pushed by the ocean tide waves like a child in a 

swing being pushed by an adult.

With every rise of the tide at the edge of the continental 

shelf, water is set flowing across the shelf toward the coast. 

This water moves as a long 

wave like the deep ocean tide 

but slower because typical 

shelf depths are just hundreds of feet as opposed to 

the miles-deep ocean basins. 

In the Gulf of Maine, the 

incoming wave reflects back 

to the shelf edge, producing 

a standing wave. The coastal 

water sloshes back and forth, 

getting another push every 

approximately 12.5 hours 

from the ocean tide at the 

shelf edge.

Resonance.—The shape 

and depth of the Gulf of 

Maine give its waters a natural swing period very close 

to the 12-hour-and-25-minute lunar semidiurnal tidal period. 

Every time the ocean rotary wave at the shelf edge pushes the 

metaphorical swing, it does so at just the right time to make the 

kid on the swing go flying. That is resonance.

Big tides, small tides.—Because the Gulf of Maine is in 

near resonance with the ocean lunar semidiurnal tide, it has 

one of greatest tidal ranges on the Earth. Tides average 35 ft 

(10.7 m) in part of the Bay of Fundy, 11 ft (3.4 m) in Wellfleet, and 9 ft (2.7 m) in Provincetown. But most continental 

shelves, lacking the great length of the Gulf of Maine, have 

shorter natural periods and thus are not so close to resonance 

with the ocean tide.

An example of such a smaller system is Block Island 

Sound, just south of the Gulf of Maine. Here, a progressive 

As strange as it may seem, 

tides in the Gulf of Maine, 

like other coastal tides, are 

not caused directly by the 

gravitational fields of the 

Moon and the Sun.

Because the Gulf of Maine 

is in near resonance with the 

ocean lunar semidiurnal tide, 

it has one of greatest tidal 

ranges on the Earth.
82 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

wave forced by the ocean tide passes south of Nantucket 

and Martha’s Vineyard; part of it then travels north and west 

through Vineyard Sound and Nantucket Sound. This coastal 

tide is not in resonance with the ocean tide; it has a range of 

just a few feet in most places.

The range is especially small in Falmouth, a little more 

than 1 ft (0.3 m). There, the tidal wave moving north through 

Vineyard Sound meets another tidal wave coming from the 

east through Nantucket Sound, produced by the Gulf of 

Maine tide (fig. 25). These two tidal waves are not in phase 

with each other. The high tide of one occurs near the time 

of low tide of the other. Thus the two waves tend to cancel 

each other out, producing the low-range tides found along the 

southern side of the Cape.

Figure 25. Waves can combine to reinforce, interfere, or overlap each other.
Photograph by the National Park Service

Summary 83

Chapter 7.

Summary

The 31-mile (50-kilometer) long coast of outer Cape Cod 

began to be created about 15,000 years ago from the retreat 

of the glacial sheet at the end of the last ice age. The Cape is 

a geologically new landform on a geologically old continent. 

It is sometimes said that it is a new and a temporary land, at 

least in terms of geologic time. The North American continent 

separated from Europe and Africa 200 million years ago, but the 

continental margin upon which the Cape rests has built up since 

then. In fact, it is still building, and the Cape is raw material for 

the continued growth of the margin. Glaciers instead of rivers 

transported sediment for continental shelf construction, but the 

erosion and transport that applied to glacial processes are now 

being substituted by wind and wave erosion and transport. The 

Cape may disappear from the visible, subaerial world in several 

millennia as sea level continues to rise and erosion continues, 

but its contribution to the North American continental margin 

will remain.

The sea floor surrounding the Cape has sediment and a geologic character similar to areas onshore because of similar geologic 

origins to glacial deposits and outwash sediment; however, the sea 

floor has been highly reworked by waves and currents as the sea 

level has risen during the past 15,000 years. The all-too-apparent 

shoreline changes that we see today along the shore of the outer 

Cape are part of the continuing growth and geologic evolution 

of the New England continental margin. Glacial ice eroded old 

continental crust that had been pushed high by active geologic 

processes, and the sand, clay, gravel, and boulders were transported 

to the coast by the flowing ice and are found towering as much as 

100 feet (30 meters) in bluffs above the sea. In the future, coastal 

erosion will have transported this sediment seaward to be incorporated into the growing continental shelf. The processes of coastal 

change—erosion and accretion—are long, but continuous and 

make up a cycle we experience each time we see a winter wave 

cut into the reddish sand of a Cape Cod bluff.
84 Guide to Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer

The rising sea level from the warming and expansion of 

ocean waters and melting of onland ice sheets and glaciers can be 

seen on the Cape in the shoreline change from inundation, erosion, 

and transgression. Beaches have formed by transport of sand 

from sources inland and from other beaches and also have been 

consumed by waves. Barrier beaches are often overwashed or 

even breached by storm waves and tides. Almost the entire barrier 

island system between Nauset and Monomoy Beaches loses more 

sand than it gains, producing coastal erosion and shoreline retreat.

During the past century, major storms, variations in sediment supply to the coast, and human activity have had more 

direct effects on shoreline change. Large storms coupled with 

elevated sea levels can cause changes in shoreline position that 

persist for weeks to decades or even longer. Complex interactions between nearshore sand bodies and the underlying geology 

also affect the behavior of beach morphology. In addition, human 

actions to control erosion, protect infrastructure, and maintain 

navigation channels have altered many regions of the coast.

Accelerated global sea-level rise is already affecting coastal 

regions and low lying islands, such as Cape Cod and the islands 

from Buzzards Bay and Block Island Sound to the Nantucket 

Shoals, around the United States and worldwide, and the effects 

are very likely to increase in the near future. The effects of 

climate change on coasts are not uniform but vary considerably 

from region-to-region and over a range of time scales.

Coastal lands will not simply be inundated, but are 

expected to be modified by a variety of dynamic processes and 

in ways that vary by location. The forces that drive the evolution of the coast in response to sea-level rise include geologic 

framework, oceanographic processes, sediment supply, and 

human activity. All these factors interact in complex ways, driving the response of coastal landforms to sea-level change.

Winds, waves, and tides have also had a hand in the development of the Cape as we see it today and continually change 

the landscape at the Cape. The prevailing weather patterns of 

the Northeastern United States produce the nor’easters in the 

summers and move some of the tropical storms and hurricanes 

through New England, as well as the occasional storm surge 

that affects the Cape.

The short-term effects of winds, waves, and tides and the 

long-term effects of the geologic processes of tectonic activity, 

glaciation and deglaciation, and climate change are all seen and 

felt on the Cape. These forces formed the Cape Cod we know 

today and will continue to change the landscape of the Cape in 

the future, sometimes in small, incremental changes and other 

times in more drastic ways.
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All illustrations by Mark Adams, National Park 

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Editorial review by Anna Glover and 

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For more information, contact:

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Geology Science Center

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Williams—Coastal Landforms and Processes at the Cape Cod National Seashore, Massachusetts—A Primer—Circular 1417

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