Caves are hollow spaces carved into hillsides or underground by
the action of water and the wind. On coasts, pounding waves
sometimes hollow out caves in the bases of cliffs.
HOW ARE CAVES FORMED?
Caves are most common in limestone rock. Rainwater,
which contains a weak acid, dissolves the soft
limestone and seeps into cracks, eventually carving
out channels that widen into tunnels and caves.
Caves are also found in glaciers and in cooled
volcanic lava.HOW DO LAKES FORM?
Small lakes called tarns form in mountains where
glaciers gouge out bowl-shaped hollows. Water
collects in the craters of inactive volcanoes to form
volcanic lakes. The world’s largest lake, the Caspian
Sea, lies in a hollow created by geological upheaval.
Artificial lakes called reservoirs are created by dams.WHAT IS AN ESTUARY?
As a river flows into the sea, it often widens and
forms a broad inlet called an estuary. The sea’s tides
carry salty sea water upriver to mix with the fresh
river water. The salt makes tiny particles of clay in the
fresh water clump together and sink, often causing
sediment to build up at the river’s mouth.HOW ARE DELTAS FORMED?
A delta is an area of flat, fertile land at a river mouth.
Deltas form when a slow-moving river deposits its
load of sediment as it reaches the ocean. The sediment
slowly builds up and dries out, forcing the river to
split into separate channels.A lake is an expanse of water that forms
inland where water collects in a hollow
in the ground and cannot drain away
through the rock below. Most lakes are fed
by rivers and, to a lesser extent, rainfall.4 STALACTITES AND STALAGMITES
Water dripping from the ceiling of limestone caves contains dissolved
minerals, such as calcite. Over thousands of years, tiny deposits of
calcite build up on cave roofs to form hanging spiky columns called
stalactites, and on the ground below to form stalagmites.LIFE OF A LAKE 3
This lake lies in a lush, green
valley and is topped up by river
water from the surrounding hills.
Lakes like this do not last forever
and eventually dry out. They are
slowly filled in by sediment
dumped by a river, or disappear
when there is less rainfall. What
is left of the lake then forms a
swamp, marsh, or bog.FIND OUT MORE. Early Farming 364–365 • Erosion 55 • Oceans 40–41 • Rocks 46–47
MISSISSIPPI DELTA 3
A false-colour satellite image of the Mississippi
Delta in the USA shows land in light blue, water
in dark blue, and sediment in green. The delta
has a ragged coastline shaped like a bird’s foot.
Other deltas have curved, fan-shaped coastlines.1 RIVER MOUTH
Marshes and swamps often form
at river mouths, where the river
flows over low-lying land. These
wetland habitats provide a haven
for wildlife, especially birds.LAKES
Point bar built
up by sediment
deposited on an
inside bend, where
the river flows
more slowly and
drops its load
Caves
FIND OUT MORE. Ice 58 • Rocks 46–47 • Volcanoes 44
rivers
caves