IF YOU鈥橰E lucky enough to find yourself stretched out on the baking white
sands of Biarritz in France this year, roll over and take a look back up the
beach. You may spot a geologist or two lurking in the shadow of the cliffs.
They鈥檙e easy to spot: Biarritz is renowned for nude sunbathing and the
geologists are the ones wearing clothes.
The beaches of Biarritz, like those of a number of other top holiday resorts
round the world, have a story to tell. Sand, shells and the debris left below
cliffs by pounding waves tell tales about geological events that took place
millions of years ago and events that are in progress now. They even hint at
events to come. Some beaches, for example, reveal secrets from the age when
dinosaurs lived, while others show evidence of glaciation, volcanic eruptions
and even of the movement of entire islands.
Biarritz tells of the time just after the dinosaurs. Broad, sandy beaches
have been cut by the Atlantic Ocean from cliffs up to 30 metres high. The rocks
here formed offshore 65 million years ago, just after the dinosaurs became
extinct, and are rich in marine fossils. Sea urchins the size of golf balls show
up green in the brownish marl and whitish limestone layers that alternate in the
cliffs.
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Animals that lived on the seabed also left behind tracks, called trace
fossils, marking their passage through the mud. 鈥淭he rock is absolutely full of
evidence of life,鈥� says Peter Ward, a geologist from the University of
Washington in Seattle. Rapid erosion means that every year he visits he finds a
new set of fossils.
The real treasure for scientists are rocks that formed at the very end of the
age of the dinosaurs, at what is called the Cretaceous-Tertiary (K-T)
boundary. A few years ago, Ward found such rocks about 3 kilometres south of the
town. His goal was to find the last of the ammonites, extinct spiral-shelled
relatives of the nautilus and squid. He climbed the cliffs and tracked them
right up to the K-T boundary, but no further. This revealed clearly that
ammonites went extinct at the same time as the
dinosaurs.
For all the fun of fossil hunting on the beaches of Biarritz, it鈥檚 a job that
can leave you feeling like a fish out of water. People will walk around naked,
while geologists have to wear clothes, Ward insists. 鈥淲hen you鈥檙e banging away
on the rocks, you get rock chips flying. They鈥檙e sharp. The rock can cut the
hell out of you, so clothes are a necessity.鈥� And even when geologists are not
chipping away at the cliffs, they need their boots for climbing. 鈥淵ou look
really stupid naked with a pair of boots on,鈥� he says. Once the day鈥檚 work is
done, however, Ward admits that he too slips out of his clothes鈥攁nd
boots鈥攖o relax in the ocean.
Beckoning finger
On the other side of the Atlantic, the Cape Cod peninsula juts out of New
England like a finger bent upwards beckoning Europe to come closer. The Cape鈥檚
beaches are a popular holiday spot. In summer the ocean keeps the temperature
pleasant, but the winters can be harsh.
Cape Cod鈥檚 story is one of glaciation. Stroll along its eastern beaches and
you find odd boulders poking out of the cliffs. Drive about the Cape and you
find boulders as large as cars standing alone in the sandy landscape. Until the
middle of the last century, 鈥淐ape Cod was one great big geological anomaly鈥�,
says geologist Stephen Leatherman, the new head of the hurricane centre at
Florida International University in Miami. Geologists could not imagine how
gigantic chunks of green schist came to rest a couple of hundred kilometres from
their nearest possible source鈥攖he volcanic rocks of New Hampshire.
The answer came from the Swiss geologist Louis Agassiz, who did much of the
founding work on glaciation and coined the term 鈥渋ce age鈥�. He explored the Cape
after arriving at nearby Harvard in 1846. The sandy peninsula seemed a world
apart from the mountains and valleys of his homeland鈥攗ntil he spotted the
boulders. He knew that large rocks mark the melted edges of mountain glaciers,
and reasoned that the Cape鈥檚 boulders were a sign that it too had been covered
by ice. 鈥淚t was a key to understanding glaciation,鈥� says Leatherman.
We now know that the crooked shape of the Cape is the work of two lobes of
the North American ice sheet. One lobe extended down across Cape Cod Bay and at
its southernmost edge deposited the east-west portion of the peninsula. The
second lobe covered the continental shelf to the east, which is now under the
Atlantic. The north-south part of the Cape emerged from the boundary between
these two lobes.
Look carefully and you can see other evidence of the vanished ice sheet. As
it melted about 15 000 years ago, the ice deposited sand, gravel and silt on the
land below. The finest particles washed away, but the sand and
gravel鈥攇lacial 鈥渢ill鈥濃�攆ormed layers that are still visible in the
cliffs. You can also find small, deep ponds called kettle holes. The retreating
glacier left large chunks of ice embedded in the till. When these melted, they
left behind the large holes.
The landscape of the Cape is still changing. Ocean currents cut the cliffs
back by about 0.7 metres a year, carrying the sand north and south to create new
land at the Cape鈥檚 fingertip and on islands just south of its middle knuckle.
The ocean will eventually cut the Cape in two, leaving an island to the north
and a peninsula to the south.
Farther south still, the story is not so much of shifting sands as of moving
islands. Much of the eastern seaboard of the US is lined with barrier
beaches鈥攍ong, thin, sandy islands that form where strong waves break over
gradual slopes. Rare in Europe and Asia, the barrier islands run all the way
from New Jersey to southern Florida. These islands have become the sites of a
number of popular resorts from Atlantic City to Miami Beach.
Approach a barrier island from the mainland and you encounter salt marshes
and shallow, saltwater lagoons. Typically, a bridge arches over the water to the
island. An unspoilt barrier island has a backbone of rolling dunes covered with
shore grass, shrubs and small trees. The waves form a smooth beach on the side
facing the ocean.
Since the last ice age the sea has risen and pushed the barrier islands
inland ahead of it. This story is told by the oyster shells that you find along
an island鈥檚 ocean beach. Oysters live only in the quiet lagoon behind the
island, and it is not the shells that have moved, but the island.
As the ocean pushes the sand inland, the dunes tumble into the old lagoon.
This process continues until the bed of the ancient lagoon lies beneath the
beach on the ocean side. At this point, waves expose the layers of lagoon
sediment containing the shells and wash them onto the beach.
So how can you be sure that what you鈥檙e looking at is really a fossil? After
all, clams and whelks live round the island, and the shells may have been
inhabited until recently. For one thing, if an island has rolled over a shell,
the chances are that it will have been buried in mud and so will be stained
black. But it鈥檚 not always as easy as this. 鈥淵ou can鈥檛 really tell whether it鈥檚
a fossil, unless the shell obviously doesn鈥檛 belong,鈥� says Orrin Pilkey, a
coastal geologist at Duke University in North Carolina.
Some of the shells are definitely ancient. Geologists and the Army Corps of
Engineers have carbon-dated shells from some islands and found them to be
thousands of years old. In North Carolina, between 75 and 80 per cent of the
shells on the beaches are fossils, says Stan Riggs, a geologist at East Carolina
University in Greenville. Clam shells the size of dinner plates clock in at a
million years old, he says. And off Topsail Island and Onslow Beach in North
Carolina, researchers have found oyster shells more than half a metre across
dating back 30 million years.
Elsewhere in the world, it鈥檚 not what鈥檚 buried in the sand that tells a
story, but the sand itself. And perhaps the most bizarre tales come from the
Hawaiian islands, where sand can be black or green. Most sands around the world
are plain old quartz, but not Hawaii鈥檚, says David Clague of the Monterey Bay
Aquarium Research Institute in California. They鈥檙e created by lava flowing into
the ocean. As a glob of hot lava hits the seawater, it explodes into millions of
tiny fragments of volcanic glass which are then washed ashore.
Iron in the lava colours the glassy grains a deep brown, and a whole beach of
them appears black. The eruption of Kilauea on the big island of Hawaii, which
is still going on, 鈥渉as generated black sand beaches that cover 30 to 40 miles
of beachline鈥�, says Clague. Green beaches form in the same way, but the lava in
these cases is up to 30 per cent olivine鈥攖he green-tinged mineral
magnesium iron silicate.
When it hits the water, the lava鈥檚 steamy explosions generate not only black
volcanic glass but also fragments of olivine, which is tougher than the glass
and stands up better to abrasion. So as ocean waves grind up and carry off the
black glass, the beaches take on their greenish hue.
Dark beaches form only after an eruption and typically last about fifty
years. Storms wash the sands offshore, where they slide down the steep slopes
that surround the big island. Elsewhere the beaches are white, but not from
quartz. This sand is made of debris from fast-growing coral reefs. Although it
too slips down the offshore slopes, the white beaches do not disappear, however.
鈥淭he coral keeps growing,鈥� says Clague. 鈥淔ish and other animals eat coral,
constantly generating a supply of white sand.鈥�
Dinosaur teeth
Back round the other side of the world in Europe, just off the south coast of
England, is another island famous for its coloured sands. Souvenir shops on the
Isle of Wight sell gaudy bottles filled with coloured sands. Iron minerals tint
the island鈥檚 rocks different shades, producing sands that range in colour from
pure white to rusty red.
But this popular holiday haunt holds a much bigger attraction for geologists.
It happens to be 鈥淓urope鈥檚 premier dinosaur locality鈥�, says David Martill from
the nearby University of Portsmouth.
The island鈥檚 rocks date from the Cretaceous, up to 140 million years ago,
when the area was a flat coastal plain inhabited by dinosaurs and pterosaurs.
During the last ice age, the Isle of Wight and the rest of England were attached
to the European mainland. Then, about 7000 years ago, rising seas cut England
off from Europe and the Isle of Wight from England.
Today, winter storms batter the southern coastline of the Isle of Wight,
eroding a couple of metres a year from its crumbly rocks and uncovering their
ancient burden. The worst damage reveals fossil trees, dinosaur foot prints and
dinosaur and pterosaur bones to over-wintering palaeontologists and
residents.
A few fossils remain for summer visitors. But not everyone goes away
satisfied, according to Stan Hutt, curator of the Museum of Isle of Wight
Geology in Sandown. 鈥淔inger-like flint nodules are mistaken for the teeth of
meat-eating dinosaurs, and odd bits of rusty metal with beach pebbles adhering
are occasionally thrust under my nose by eager finders who hope they have a
dinosaur femur.鈥� Nevertheless, some excellent finds have been made.
So next time you鈥檙e stretched out on the sand鈥攚hether it鈥檚 the Isle of
Wight, Biarritz or a green beach in Hawaii鈥攕top sunning yourself for a few
moments and take a look around. William Blake saw a world in a grain of sand.
That might be asking a bit much, but you may be truly surprised by what you
find.