Coasts & Sea-level Changes (Cape Cod & Acadia)

The cod has been an extremely valuable resource for several centuries in Massachusetts. Its extensive use as a food dates back to the earliest period of European settlement in coastal New England. In colonial times, it was deemed so important that in 1693 the General Court of the Massachusetts Bay Colony ordered that farmers could no longer use cod as fertilizer. This action was one of the first recorded attempts at natural resource conservation and management on this continent.

Although one of the earliest fisheries resources to be broadly utilized after European settlement in New England, cod populations along the US coast proved to be very resilient. Cod apparently withstood more than 3 centuries of harvest without displaying major, long-term regulations in abundance. However, mid-twentieth century advances in fishing technology and the introduction into the northwest Atlantic of distant-water foreign fishing fleets during the late 1950's led to a period of reduced abundance and major annual fluctuations in population size. During the mid-1980s commercial vessels captured mostly 3 to 5 year old fish, indicating that few larger, older individuals remain along the North American coast.

Will cod fishing continue to be valuable in the future, feeding people and supporting jobs? We don’t know… human decisions on climate change and energy, and on rules governing fishing, are more important for the future of cod than anything else.

Will Cape Cod be there in the future? Looking out a few millennia, the answer is probably "no." Beaches, like rivers, are controlled by the interactions of water and sediment. Sand is supplied, and sand is lost. If these processes are interrupted, the coast or the river must change. And in the distant future, Cape Cod is likely to be the new Georges Bank, a shallow underwater place that could be home to great masses of fish, if we leave enough fish to populate it and we leave the cool waters that these fish need.

Learning Objectives

• Explain how coasts are highly complex environments that can change rapidly.
• Understand that some coastal changes were started by the ice age or other long-ago events, while other coastal changes are being caused by humans now.
• Explain how human actions to modify coasts can achieve their goals but also can fail greatly.

What to do for Module 8?

You will have one week to complete Module 8. See the course calendar for specific due dates.

• Take the RockOn #8 Quiz
• Take the StudentsSpeak #8 Survey
• Submit Exercise #4
• Begin Exercise #5

Questions?

If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.

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Overview of the main topics you will encounter in Module 8

Just for fun!

Traditional sea shanties are work songs used by sailors to help them coordinate their physical labor while making the work more enjoyable. This one was used by sailors from Cape Cod:

Cape Cod boys don't have any sleds
Look away, look away,
They slide down dunes on codfish heads
We're bound for Australia.
Cape Cod girls don't have any combs
Look away, look away
They comb their hair with codfish bones
We're bound for Australia.
Cape Cod cats don’t have any tails
Look away, look away
They lost them all in Cape Cod gales
We're bound for Australia.

Getting the Most from the Coast: Cape Cod, Acadia, and Friends

• There are many "types" of coasts (beaches, reefs, mud flats, cliffs, deltas, etc.); here, we’ll focus especially on beaches.
• Waves move LOTS of sand, mostly in, out, in, out…, which sorts by size to give sandy beaches.
• Waves move a little extra sand out during winter storms (breaking waves come in through the air without sand, and go out along the surface with sand).
• And waves move a little extra sand in during summers (a wave surges up the beach a bit faster than the water flows back out, and thus the wave moves a bit more sand in than out).

The Coast Is Friendly—the Ocean Waves

• Waves go slower in shallower water.
• The first part of a wave to approach the coast slows and waits for the rest of the wave to catch up, so the wave "piles up" to break, and turns to come almost straight in.
• But "almost" is not "completely" straight in, and this slight angle drives longshore drift of sand and water along the shore.
• Eventually, some sand is lost to deep water below the reach of waves.
• A beach thus needs sediment supply to balance this loss, or else the beach is eroded.

The Coast Is Friendly—Buoy Meets Gull

• New beach sand is supplied by longshore drift from river-fed deltas, or by erosion of the coast behind the beach.
• Erosion of land behind the beach, and loss of houses, roads, and other things there, are promoted by:
  • dams on rivers, which trap sand in reservoirs so the sand cannot reach beaches (for example, the Elwha River dams in Olympic National Park) caused beach loss,
  • “dams” along the coast (jetties or groins) that stick out will block longshore drift, trapping sand and letting clean water pass to erode beyond,
  • past sea-level rise, which flooded river valleys so river sediment now is trapped where rivers enter bays and do not reach outer beaches (e.g., Chesapeake Bay),
  • past deposition by glaciers or other processes that formed coastal land (e.g., Cape Cod) where there are no big rivers to supply sand,
  • ongoing sea-level rise or sinking of the land surface along the coast, flooding beaches and washing sand into deeper water. 

But Many Coastal Residents are Crabby

• Most U.S. coasts are retreating (about 75%)
• Beaches are retreating because of the processes mentioned in the previous section ("The Coast Is Friendly—Buoy Meets Gull")
• Beaches are especially retreating because humans are raising sea level
  • Human-caused global warming is melting glaciers, releasing water that reaches the sea
  • And global warming is causing the ocean water to expand
  • We also are adding a little more water to the ocean by pumping it out of the ground faster than nature replaces it
• We also contribute to beach retreats by pumping groundwater, oil, and gas from beneath beaches, allowing the beach to compact and sink
• Upward or downward motion of the land from natural mountain-building or from ongoing response to the changes in weight from the melting of the ice-age ice sheets is causing beaches to advance or retreat in different areas.

Coasts and Cape Cod

Your tour guide, Dr. Alley, is a lucky fellow. Through a fortuitous sequence of events, which involved marrying the right woman who had the right grandfather who was related to some people who have roots in the right place and worked hard to preserve those roots, our family has been able to stay for a week or two during many summers in a wonderfully historic house, built in the 1700s, in Eastham on Cape Cod. The land has been given to the National Park Service as part of the Cape Cod National Seashore. The house has access to the bicycle trail to the Coast Guard Beach, the Salt Pond Visitor Center, and the great Nauset Marsh. Dr. Alley has visited most of the National Parks, but he has spent more time at the Cape Cod National Seashore than any other. A few pictures from his family visits are given in the following slideshow.

Take a Tour of Cape Cod National Seashore

Credit: R. B. Alley © Penn State is
licensed under CC BY-NC-SA 4.0

We’ll start here to tell you some background about the Cape in case you want to visit, and to help you understand the geological setting that we will discuss in more detail soon.  

Cape Cod is a glacial moraine, a deposit that outlines the former extent of the ice-age ice sheet that flowed south from Canada. The part of Cape Cod that attaches to the mainland marks the end of a lobe of the Canadian ice sheet. The “forearm” where the Cape points north is an interlobate moraine—while one tongue or lobe of the ice sheet filled Cape Cod Bay between the Cape and Boston, a second lobe lay farther east in what is now the Atlantic Ocean, and built a moraine that is now the fertile fishing grounds of the Grand Banks. (You will see this when you reach the geologic map later in this section.) The northward-pointing “forearm” of the Cape is composed mostly of outwash, the sand and gravel that were transported and deposited by meltwater rivers flowing off these two ice lobes into the narrow space between them. Till, the deposit made from pieces of all different sizes dropped directly from the glacier ice, is found more commonly along the part of the Cape that attaches to the mainland.  

The forearm part of the Cape thus is an outwash plain, and most of the numerous freshwater ponds of the Cape began their lives as ice blocks buried in outwash sand and gravel. Melting of the ice later allowed the collapse of the outwash that had been deposited on top of the ice blocks, forming kettle ponds. Soon after it formed, the Cape had more ponds than we see today, but many have been filled with logs, sticks, leaves, peat, and other organic material. The logs and other materials in these former kettle ponds can often be seen eroding out on the bluffs along the Atlantic beach, including along the Coast Guard Beach that is so easily reached from the Salt Pond Visitor Center of the National Seashore. Radiocarbon dates on such deposits, together with other information, show that the ice was retreating from the Cape Cod region roughly 15,000 years ago. 

The sandy soils of the Cape, and its numerous kettle ponds, are home to cranberries, blueberries, and wild orchids. Beach plums produce their small but sweet fruit late each summer, and blackberries and dewberries vine across old dunes. Deer and grouse still inhabit the uplands, and turkey are multiplying rapidly as oaks spread across regions that were logged by humans but are being allowed to regrow. 

Much of the interest at the Cape focuses on the sea. Nauset Marsh, for example, is a wonderful place to visit for bird-watching, fishing and kayaking in protected waters that are miles long and a good chunk of a mile wide. The protection for the marsh comes from the “outer beach,” a long sand bar between marsh and ocean, which is split by one inlet (or occasionally more inlets, depending on what year you visit) allowing water to flow into and out of the marsh with the tides. Deep channels in the marsh are home to crabs, scallops, starfish and striped bass, often pursued by seals coming in from the open ocean. Between the channels, great flats of marsh grass flood during high tides and dry as the tide falls. Flocks of wading birds and shorebirds, from large herons and egrets to tiny plovers and sandpipers, breed there or stop to feed during their migrations. Osprey survey the marsh from high nests on perches constructed by the Park Service, as well as nests in trees along the shore. 

One year, a hurricane had driven unusually warm waters to the Cape, bringing ctenophores from the south. Ctenophores are also called comb jellies, and look something like jellyfish. In the sun these creatures often appear as if they have small rainbows down their sides, because the light is broken on the cilia or hairs that the creatures beat to move themselves around. At night, this particular type (Leidy’s comb jelly) is bioluminescent, glowing with a beautiful blue-green light when the waters around them are disturbed. Imagine, if you can, kayaking into the marsh after dark, with golf-ball-sized “fireflies of the ocean” glowing and swirling with every wave. Another year, tiny phosphorescent plankton (dinoflagellates) were washed into the marsh, lighting up at night with every drip from the kayak paddle and every little fish eating the plankton. Then, fast-moving striped bass slashed through, eating the little fish. (This is a likely reason why the small plankton have evolved to generate phosphorescence, protecting themselves from being eaten by the small fish by notifying the larger, predatory fish.) Indeed, the Cape is a wonderful place. 

The Cape is changing rapidly, however. The large Nauset Marsh has been narrowing as the outer beach moves steadily into the marsh. Two of the Cape’s lighthouses were moved during the summer of 1996, barely in time to avoid collapsing over the bluffs into the sea as the bluffs have been eroded away, and the day will come when the light houses must be moved again or else lost. 

At the ends of the Cape, to the north and south, new land is being formed, or has formed recently, from the deposition of sand (yellow on the map below). But, more than twice as much land is lost each year as is formed, and the Cape is slowly but surely disappearing. The next ice age might rebuild the Cape, but naturally that ice age is still far in the future, and our global warming is probably already so large and likely to last so long that the ice age will not occur, so the Cape appears destined to become islands and then an undersea bank over the next few thousand years, unless we humans spend a whole lot of money and effort to stabilize it somehow. The long-term retreat rate of the Outer Beach in Eastham has been about 3 feet per year over more than a century, with fluctuations and perhaps with a little recent acceleration. 

This map from the USGS Woods Hole Coastal and Marine Science Center shows the geology of Cape Cod. Magenta indicates glacial moraines containing till, and green shows other deposits formed in contact with ice. Oranges and red are outwash plains. Blue marks deposits from a lake that formed when the ice began melting back from the Cape but still plugged Cape Cod Bay and kept the ocean out. Brown shows marsh deposits, and yellow shows new sand deposits. We added the black arrows to show where the ice flowed in the past.

Credit: US Geological Survey Woods Hole Coastal and Marine Science Center (Public Domain), generalized from detailed mapping by K. F. Mather, R. P. Goldthwait, L. R. Theismeyer, J. H. Hartshorn, Carl Koteff, and R. N. Oldale

There are many types of coasts. North of Cape Cod at Acadia National Park in Maine, strong igneous and metamorphic rocks make sea cliffs. To the south, Virgin Islands National Park is famous for coral reefs, although they are now endangered by human-caused climate change and other impacts; the reefs are composed of the skeletons of trillions of tiny animals that have built upward from the sea bottom in shallow, clear, sunlit, oxygenated waters far from sediment that would bury and choke them. In Louisiana, we visited the Delta National Wildlife Refuge with its waterfowl-filled wetlands developed on mud delivered by the Mississippi River. At Cape Cod, we find sandy beaches. 

The type of coast depends on many things: the amount and type of sediment the coast receives, how energetic the waves are that hit it, how much the tide goes up and down, the type of rocks, how warm or cold the climate is, and many other factors. We discussed some issues with the muddy delta of the Mississippi River earlier. The biological challenge of saving coral reefs from overheating, pollution and other damage is large and important, but a little beyond the scope of this class. Here, we will concentrate on sandy beaches such as those at Cape Cod, to learn about them and to gain some insights to other types of beaches.  

If you watch the waves on Cape Cod beaches or any other sandy beach, you will see that those waves move a lot of water, and a lot of sand. Dig a hole just above the water level during a rising tide, and within a few minutes the hole will be mostly or completely filled with wave-carried sand. Go to the beach during a big storm and you will see immense amounts of sand moved. Hundred-foot-high bluffs may be eroded back several feet during a single storm, or layers of sand many feet thick may be added to the beach or eroded from it in hours. A movie long shown at the Salt Pond Visitors Center includes a series of photos taken of one section of beach before and after a string of storms one winter. The summer beach is an unbroken expanse of sand, but boulders many feet across (more than a meter) are buried in it, and were completely uncovered and then buried again several times during that one winter as the sand was moved off the beach into slightly deeper water and then carried back onto the beach as shown in the short video clip below. 

Video: Beach Changes (0:45 seconds)

The energy for moving all of this sand is mostly supplied by the wind, which drives the waves, and to a lesser extent by the tides, which are bulges of water raised by the gravity of the moon and sun and following them in their orbits around the Earth. (We saw earlier that earthquakes and other phenomena can cause tsunamis, which also can move a lot of sand, but significant tsunamis are very rare compared to wind-driven waves, and not a big issue for Cape Cod almost all the time.) Most of the sand transported by waves is simply moved onshore—from the ocean toward the beach—and offshore—from the beach toward the ocean—as each wave comes in and goes out. Most transport is into and out from the beach, rather than along the beach, because most waves turn so that their crests are almost parallel to the beach, and their water motion is almost directly towards and away from the beach. The turning happens because waves go slower in shallower water. If a wave approaches a beach at an angle, the first part to get close to the beach will slow down, allowing the rest of the wave still in deep water to nearly catch up, as shown in the diagram below. 

Video: Waves turning toward the beach (1:53 minutes)

Every wave moves sand up and down the beach. On even rather quiet days, if you sit down on a Cape Cod beach in shallow water, you soon will find that sand is piling up in places around you, and being eroded in other places, and that you have sand in your swimsuit, and possibly even in your hair. This in-and-out movement of the sand with every wave dominates the sand transport, and allows for very efficient sorting of the sand by size, taking away pieces smaller than sand pieces, leaving pieces larger than sand in other places, and eventually giving almost all sand on the beach (although sometimes with buried boulders in the sand, or some fist-sized rocks just offshore where you might step on them if you wade into the water). This repeated movement of pieces in waves also knocks the sharp edges off sand grains, sea shells, old bottles, and other material on the beach, making the rocks and sand and shells rounded, and giving pretty “sea glass”.

As described in the video below, if you look out to sea during a winter storm, you’ll see high-energy breakers coming at you. The white caps of the waves rise high, curl over, and crash down, so that some of the water arrives on the beach after coming in through the air rather than washing along the sand. But the water then rushes back toward the ocean along the sand, carrying some sand seaward. Storms, which frequently occur during winter, often move some sand from the beach into slightly deeper water. This transport may remove enough sand to lower the height of the beach surface by many feet or tens of feet during the winter, exposing buried boulders as described earlier. The waves of summer are on average lower in energy, and don’t break and travel through the air as much (occasional hurricanes change this story, but most of the time the story is fairly accurate). The surge of summer waves up the beach is slightly faster than the return flow down the beach, and may carry a tiny bit more sand up than back; the net effect is to bring sand from just offshore back to the beach, burying any beach boulders that were exposed during the winter. 

Video: Eroding Winter Beach (0:54 seconds)

Recall from earlier, though, that the waves and the sand come almost but not quite straight in and straight out—there is still some angle. If you are playing in the waves at the beach, and ride a wave in, swim out, ride in, swim out, ride in... after a while you may find you are drifting down the beach away from where you left your towel—even though you were mostly going toward the beach and back out to sea, the waves also were pushing you sideways. In such a situation, we say that you are experiencing longshore drift—you, and the water, and sand, are moving along the shore. Eventually, when the water and sand (but we hope not you!) reach the end of the Cape (at the Provincelands to the north, or Monomoy to the south), some of the sand carried by the longshore drift builds a spit or extension of land, but some of the sand is dumped off into deeper water beyond the reach of waves. This sand is then lost from the above-water part of the Cape, and the Cape has gotten a little smaller. Most references say that the great beach facing the Atlantic is retreating at about 3 feet (1 m) per year, although it may have been a little faster recently, and the panicked rescues of light-houses before they fell into the sea were needed because retreat for a few years was much faster. We’ll look at these issues, and what might be done, after visiting Acadia.

As mentioned above, waves move immense amounts of sand, primarily up and down the beach, but also with a little motion along the beach and eventually off into deep water. In this vintage video, Dr. Alley gets cold feet on Coast Guard Beach, Cape Cod National Seashore, to show you moving sand.

Vintage Video: The Feet (1:17 minutes)

Coasting Down the Coast Slideshow

Come take a trip with us to see a bit on sea-level change, some disasters, and some coastal processes, in some beautiful places. We will discuss these more when we visit Acadia, next.

Acadia National Park and Sea-Level Rise

Caption: Left: A lighthouse sitting on the hill of a rocky coastline. Right: A Map of the United States indicating Acadia National Park in Maine in gray.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

The subduction and collision with Europe during the closing of the proto-Atlantic made great granite bodies draped in metamorphosed rocks that started as sediments. Erosion over long times has exposed these rocks that once were deep in the mountain range, and we find them at the surface in places including along the Maine coast. The ice-age glaciers scoured those rocks and left the beautiful, bald mountain tops of what is now Acadia National Park, staring out at the storm-tossed North Atlantic across Somes Sound, the only fjord on the east coast of the U.S. 

The Wabanaki tribe of Native Americans probably reversed the modern tourist pattern, summering inland and then moving to the relatively more moderate coast of Acadia in the winter. Nasty winter storms do run up the coast, but the wintertime temperatures plunge far lower inland than they do on the coast. Thick piles of discarded shells from nutritious sea creatures dating back 6000 years attest to the importance of the sea to these early people. 

On September 5, 1604, the French explorer Samuel de Champlain landed on Acadia’s island. Impressed by the bare, rocky, deserted appearance of the glacially scoured granite mountain peaks of the island, he named it Isles de Monts Desert, “Island of the Bare Mountains.” 

French influence was important until the end of the French and Indian War, after which English and then U.S. activity came to dominate. In the mid-1800s, Mount Desert Island attracted the art world, and was featured in paintings by many artists including Frederic Church, Thomas Cole, and others of the Hudson River School. For example, see Sanford Robinson Gifford’s "The Artist Sketching at Mount Desert, Maine, 1864-1865", now in the National Gallery of Art and reproduced just below.

The Artist Sketching at Mount Desert, Maine, 1864-1865.

Credit: Sanford Robinson Gifford, CC0, via Wikimedia Commons.

The artists, in turn, attracted the “rusticators,” tourists who gradually were replaced by much wealthier tourists who built summer “cottages”—the Rockefellers, Fords, Vanderbilts, Carnegies, etc. These wealthy patrons in turn invested resources and political capital in preserving most of the island as a national park. 

Today, over 4 million visitors per year flock to Mount Desert Island, with most of them getting out of the gift shops and into the national park. The visitors enjoy the history, the beauty, the rather chilly ocean waters (based on personal observation by the author, even on hot summer days, more people are sitting by the sea than swimming in it!), the outstanding network of paths for bicycling, superb kayaking on lakes and sea, and much more. 

Take a tour of Acadia National Park

Experts on events happening near coasts often say that “Change is the only constant”. The Sea Grant Program at the Woods Hole Oceanographic Institution, on Cape Cod, reported that about 75% of the U.S. coastline is eroding, with only about 25% stable or advancing. For Massachusetts, 68% of the coastline was listed as eroding, 30% advancing, and a mere 2% stable. As we will discuss soon, much of the retreat is being driven by sea-level rise, which is being driven by human-caused global warming. But, the rising sea level is interacting with a very complex coastal system, and we’ll look at a little of that complexity, too, with land still moving vertically because of the ice age, coasts responding to natural and human-caused changes in sediment supply, and more.  

Up in Maine, the rocky coasts of Mt. Desert Island are among the few places that would be classified as “stable,” although very slow erosion is occurring as the sea pounds the granite headlands. But if you look further back in time, the size of the changes becomes evident. Glacial ice overran the highest peaks in the park during the ice age. Sea level was lowered 300-400 feet (100 m or a bit more) at that time to supply the water that grew the ice sheets, but the land of Mt. Desert Island was pushed down 600-700 feet (roughly 200 m) or even more by the weight of the ice. 

Ice-age ice extended south of Maine, beyond Cape Cod, and that southern ice began melting before the ice left Maine, so the sea began rising, and then loss of ice on Maine caused the rocks of Mt. Desert Island to begin rising faster than the sea; these rocks are still rising slowly today. Thus, as the ice retreated, the already much-raised sea first flooded in across broad regions of Maine and adjacent parts of the east coast. Beaches and sea caves formed along the edge of the sea, and deltas were deposited. Then, these coastal features were raised out of the ocean as the land rebounded. Such coastal features can be found today in Acadia to almost 300 feet (almost 100 m) above the modern sea level, and similar features occur all along coastal Maine, often extending well inland. We include a video of similar features from Greenland; the features in Maine are covered with blueberry fields, or trees, houses, roads and such, and although the features are quite easily identified by experienced geologists, the features are not as clear as those in Greenland to the beginning geologist. 

Video: Raised Deltas and Beaches (2:48 minutes)

You can see and hear the story of raised deltas and beaches in Maine, Greenland and elsewhere in this short video.

Regions that were slightly beyond the reach of the ice-age glaciers were pushed up during the ice age to form a forebulge, where the hot, soft, deep rocks pushed out from beneath the sinking ice sheets bulged up the land just before the ice. In those forebulge regions, the land now is sinking, as the deep, hot rocks flow back to their starting point; where forebulge sinking has combined with rising sea level as the ice melted, the total sea level rise has been especially large. Far from the ice sheets, sea-level rise has been about what you would expect based on the amount of water returned to the ocean by the melting ice sheets. (If you took a more-advanced course, you would learn that the entire surface of the Earth was warped by shifting water from the oceans to the ice sheets and back during the ice-age cycle, so the changes are all a bit more complicated than you might expect, just as a wine glass balanced anywhere on a cheap air mattress or water bed may tip over if you sit anywhere on the bed). 

Video: Forebulge (2:09 minutes)

To see the description of the Earth’s “water bed” responding to the ice age, watch this very short video.

By now, you may be getting the idea that what happens to a particular coast is fairly complex. If mountain-building is pushing the coast up, it rises; if mountain-building is pushing the coast down, it sinks. Where plates meet, when the edges lock together and build toward an earthquake, the motion may drag one side down and push the other side up; the earthquake that follows will suddenly reverse the offset—in the great Tohoku earthquake of Japan in 2011, parts of the Japanese coast moved as much as 8 feet toward North America, and offshore the largest motions of the sea floor were more than 150 feet horizontally and more than 20 feet vertically. Where cities are built on deltas, as in New Orleans, the compaction of the mud causes sinking. Much additional sinking is caused by pumping water or oil or gas out of the ground; as the fluids are removed, the ground compacts. This is happening a little on Cape Cod, and is quite dramatic in some places. Such pumping may have contributed to problems in and near New Orleans, in Venice, and elsewhere. (Pumping fluids back into the ground can partially offset this problem, and is being used in some places, but generally does not completely fix the problem.) 

So, coasts may be going underwater, or rising out of the water, because of sea-level changes as described below, and because of the land going up or down. But, coasts also may advance or retreat because of issues related to the waves moving sand and other sediment.

Beaches inevitably lose a little sediment to deep water, somewhat like losing socks behind a clothes dryer, because it is easy to drop something that falls way down there, and hard to get it back. Waves can pick up sand from below the ocean’s surface and carry that sand to the beach, but waves cannot reach sand in very deep water (no deeper than roughly half the distance between a wave’s crest and the next one). If sediment happens to slide or bounce deeper than that, then that sediment cannot be brought back easily. (The sediment can go into a subduction zone, make new mountains, and be eroded to make new sand that reaches a beach by longshore drift, but that takes millions of years or longer.)

Thus, a “happy” beach requires a supply of sediment to balance the loss to deep water. Normally, that supply comes from the material delivered to deltas by rivers, and carried to the beach by longshore drift. But if there is not enough sediment coming this way, the beach will narrow as it loses sediment to the deep ocean, and the waves will crash across the sand to erode the material behind it, gaining sediment in this way.

In some cases such as Acadia, longshore drift does not supply enough sand to sustain a beach, and the rocks are too hard for the waves to break rapidly to supply a beach. Then, the little bit of sand produced by the waves ends up in deep water, and many of the cliffs have no beaches. (There are a few small “pocket” beaches at Acadia in protected places, but most of the coast doesn’t have beaches, with the waves pounding directly on granite.) In other cases such as Cape Cod, the waves crossing the beach hit sand and gravel left by the glaciers, easily eroding the loose material to supply beaches.

In some places, dams on rivers have greatly reduced the delivery of sediment to the longshore drift, so the nearby coasts are eroding. You may recall that the dams on the Elwha River, draining Olympic National Park, caused the loss of beaches along the nearby coast. At Cape Cod, there really aren’t any rivers that humans could dam. The glaciers made a big pile of sediment in a place where rivers are not supplying much sediment to deltas, and so the Cape eventually will be lost to deep water.

Video: Sea Rise (2:26 minutes)

Coasts change for many different reasons, and in many different ways. But, recently, most of the U.S. (and world) coasts have been retreating because sea level is rising, and that rise is accelerating. The total size of the ocean is increasing, as water that had been stored on land in glaciers and ice sheets and in the ground is transferred to the ocean, and as warming of the ocean causes the water already there to expand and take up more space.

The rate of rise is now more than 3 mm/year (a bit over an inch per decade), and has been accelerating. That isn’t much if you’re at the top of a cliff in Acadia, but if you are on a sandy beach that slopes very gradually, the inch of sea-level rise may cause the coast to retreat by many feet or even a few tens of feet. That in turn means that a whole lot of houses and property can be lost in a single human lifetime.

This ongoing sea-level rise is being caused primarily by the rising temperature of the Earth’s climate (“global warming”), which is being driven primarily by human activities. (We’ll return to this later in the course, but we have very high scientific confidence that it is correct.) Most of the world’s small glaciers have been melting, Greenland’s ice sheet has been melting and flowing faster into the ocean, and Antarctica’s ice sheet has been flowing faster into the ocean, adding water to the oceans. Also, as the ocean itself warms, the water expands and takes up more room.

We also build dams on rivers, and the water that fills the reservoirs is taken out of the ocean and stored on land, causing sea-level fall. But, we “mine” groundwater by pumping it out of the ground faster than nature puts it back, and that water eventually reaches the ocean to raise sea level. Today, groundwater pumping is probably more important than dam building, contributing a little to sea-level rise.

The polar ice sheets contain a huge amount of water—if they melted, they would raise sea level nearly 250 feet (roughly 70-80 m). Philadelphia and the other great port cities of the world would become undersea hazards to shipping but really great places for fish to hide out, and the southern coast of Florida would be somewhere up in Georgia. We do not expect such a fate, but we cannot rule out the possibility that a dynamic collapse of the West Antarctic ice sheet could raise sea level more than 10 feet (3 m) in a human lifetime or two. If we don’t change our behavior, Greenland and its 24 feet of sea level is also looking shaky, although it would take centries or longer to melt completely. (Melting all of the remaining mountain glaciers would raise sea level only 1 foot or so, less than 0.5 m.) Drs. Anandakrishnan and Alley spend a lot of their research trying to reduce our uncertainty about the future of the great ice sheets, a fascinating and important topic.

Policy Implications of Sea-level Rise

Even if we humans stopped warming the climate, sea level will rise at least somewhat more, because much of the ocean has not yet fully warmed from the atmospheric warming we have already caused but will continue warming to “catch up” with the warmer air, and more ice will melt before reaching equilibrium with the warming of the air we have already caused. (If you come into a cold house in the winter and turn up the heat, it will take a while until everything feels warm; we have turned up the heat in the air, and it will take centuries for the ocean and the glaciers to fully catch up.) The near-certainty of continuing sea-level rise has some policy implications. For example, disaster aid following hurricanes that allows people to rebuild in vulnerable places will simply create the need for more disaster aid in the future. Many people believe that those who wish to build on the coasts should be required to carry insurance or to otherwise demonstrate that they have sufficient resources to cover their coming losses. Similar arguments apply to those who wish to build on earthquake faults, landslide deposits, and floodplains. Many people living in relatively safe but less-scenic places object to paying for others to live in dangerous but beautiful places.

Engineering Solutions

A groin is a human-made “dam” that sticks out into the ocean. Sand is often trapped “upstream” of the groin (on the side that the longshore drift reaches first), but erosion may occur on the “downstream” side of the groin, as shown.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Because people love the coasts so much, and wish to live near them, all sorts of engineering solutions have been tried. These have had some success, but many failures, and they often lead to legal and political difficulties.

One approach is to build “dams” that stick out into the water and block the longshore transport of sand. These dams are usually called “groins” if they are small, and “jetties” if they are larger. (See the figure above). By making the coast rougher, and slowing waves, the plan is to trap sediment along the coast in much the same way that a dam traps sediment along a river. This plan sometimes works. However, recall that when a dam is built on a sand-bedded river, sediment is trapped upstream of the dam but eroded downstream. The same often happens along a coast; sediment is trapped “upstream” of the groin (on the side from which the longshore drift comes), but sediment is eroded on the “downstream” side (the side to which the longshore drift goes), where the sand-free waves attack the beach to pick up more sand. Saving someone’s beach while destroying the beach of a neighbor is a good way to generate lawsuits.

Video: Groins (3:14 minutes)

People also spend millions of dollars to go out to sea, find sand that has fallen off into deep water, and bring the sand back to the beach. This sand usually lasts a single year or a very few years before being washed back to deep water, and in one recent case was mostly washed away in less than a week, but in some especially popular tourist destinations the investment may pay off.

We saw earlier that people do other things, such as building giant walls to keep the sea out. In the case of New Orleans, huge amounts of money were spent building a levee system, and then people built their houses and businesses in the supposed safety behind the levees. When hurricane Katrina broke the walls, the losses included the cost of building the levee system, the valuable things that the levees were built to protect, and the new valuable things that people built after the levees were erected, as well as the lives of so many people.

Geologists often look at such past events and then take a “natural” view of the coasts—we should figure out where the coast wants to go and build there rather than trying to stop the coast. But many people just don’t like that, and a lot of construction is likely to occur—and be destroyed—over the coming years, especially if we drive more warming.

Wishing for Water - When Salt and Fresh Mix

Near the coast and in some other places, pumping groundwater out of wells for our use can cause saltwater intrusion, eventually filling the wells with water we cannot use. Freshwater has a lower density than salt water, and so floats on salt water in much the same way that an iceberg floats on water or a mountain range floats on denser rocks of the mantle. (Salty water and freshwater can mix to make less-salty water, but if the freshwater is renewed by rainfall, the mixed waters will be forced out through the beach to the ocean, and there will continue to be nearly pure freshwater sitting on salty ocean water.) If the water table is lowered by pumping fresh water for human use, the interface between salt and fresh water will rise in the same way that the bottom of an iceberg or a mountain range rises if the top is eroded. The difference in density between salt and fresh water is small; an iceberg floating in the ocean has 9/10 of its thickness below the surface, but the fresh groundwater lens of Cape Cod floating on ocean water has 39/40 below sea level. So if enough water is pumped out of the well to lower the water table by 1 m, the salt water will have risen 39 m! If the freshwater table is lowered to sea level, the salt water will rise to sea level, and there will be no fresh water left for the well to pump. Many wells on the very low land of Cape Cod were drilled below sea level into fresh water, but are starting to pump up salt water, causing large problems.

Video: Saltwater Intrusion (1:40 minutes)

Here’s a video explaining the problem, and then a single diagram if you prefer shorter explanations.

Three more Cape Cod stories

Enjoy these optional vintage videos.

The Can Video (1:15 minutes)

Human impacts on the land are easy to see. We have changed the oceans greatly, but the water covers our tracks. In "The Can," Dr. Alley briefly reflects on some issues of the oceans, as he watches one of the less-beautiful pieces of the Cape Cod National Seashore.

The Marsh Video (0:43 seconds)

Many of the ocean’s big fish, and other denizens of the deep, rely on salt marshes as nurseries and in other ways. But, we are losing salt marshes in many places, as sea-level rise forces the “outer beach” toward the shore, but humans don’t allow the inner side of the marsh to expand into our yards or parking lots. Obvious answers are not easily available, but Dr. Alley frames the question in this next short film clip as he paddles one of the family kayaks on the Nauset Marsh of the Cape Cod National Seashore.

Meet Peat! Video (1:05 minutes)

Cape Cod is a gift of the glaciers. The numerous kettle ponds left by the ice contribute to the biodiversity of the Cape, but are slowly filling in with sand, peat, and other things. Many of the ponds have already filled, and a walk along the rapidly eroding outer beach often reveals where the sea has cut into one of these filled ponds. In this next clip, Dr. Alley shows one such exposed, filled kettle pond, just below the old Coast Guard station in the Nauset region of the Cape Cod National Seashore.

Yellow Submarine

The Beatles' Yellow Submarine surely must be very high on any list of songs likely to get stuck in your brain and play over and over and over and... But, watching our beaches, with perhaps three-fourths of the US coastline retreating inland in response to rising sea level and other changes, maybe a really "sticky" song is what we need down by the shore! Anyway, we hope you enjoy this parody, which really does review a lot of the key points that are likely to show up on the RockOn quiz. "We need more sand or the beach will wash away..."

Video: Submarine Beaches, a parody of the Beatles' "Yellow Submarine" (3:54 minutes)

Review Module Requirements

You have reached the end of Module 8! Double-check the list of requirements on the Welcome to Module 8 page and the Course Calendar to be sure you have completed all the activities required for this module.

Supplemental Materials

Following are some supplementary materials for Module 8. While you are not required to review these, you may find them interesting and helpful in preparing for the quiz!

• Website: Cape Cod National Seashore
• Website: Acadia National Park

Comments or Questions?

If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.