The Science Book

128

THE GLACIER

WAS GOD’S

GREAT PLOUGH

LOUIS AGASSIZ (1807–1873)

W

hen glaciers sweep

across a landscape,

they leave signature

features behind them. Glaciers

can scour rocks flat or leave them

smoothly rounded, often with

striations (scratch marks) showing

the direction in which the ice

moved. They also leave behind

erratics—boulders that have been

carried long distances by the ice.

These can usually be identified

because their composition is

different from the rocks on which

they lie. Many erratics are too large

to have been moved by rivers,

which is the usual way that rocks

are carried across a landscape. A

rock of a different kind from rocks

around it, therefore, is a telltale

sign that a glacier once passed

by. Another is the presence of

moraines in valleys. These are piles

of boulders that were pushed aside

when the glacier was growing, and

left behind when it retreated.

Riddle of the rocks

Geologists in the 19th century

recognized such features as

striations, erratics, and moraines

as evidence of glaciers. What they

IN CONTEXT

BRANCH

Earth science

BEFORE

1824 Norwegian Jens Esmark

suggests that glaciers are

responsible for the creation of

fjords, erratics, and moraines.

1830 Charles Lyell argues that

the laws of nature have always

been the same, so the clues to

the past lie in the present.

1835 Swiss geologist Jean

de Charpentier argues that

erratics near Lake Geneva

were transported by ice from

the Mont Blanc area in an

“A l p i n e g l a c i a t i o n .”

AFTER

1875 Scottish scientist James

Croll argues that variations in

Earth’s orbit could explain the

temperature changes that

cause an ice age.

1938 Serbian physicist Milutin

Milankovic relates changes in

climate to periodic changes

in Earth’s orbit.

There must have been glaciers in these

places some time in the past.

Retreating glaciers leave particular features behind them

in the landscape.

These features are found in areas where there are no glaciers.