Glaciers and Glaciation - The Changing Earth: Exploring Geology and Evolution - page 375

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Figure 14.17 Development of Features Associated with Past Continental Glaciation

Sediment accumulations
in low spots in ice

Emerging
subglacial
stream

Terminal
moraine

RETREATING ICE FRONT

Buried and isolated
ice blocks

Terminal moraine

This retreating continental glacier once covered a larger area as indicated by the terminal moraines.

a

Kames

Esker

Recessional
moraine

Drumlins

ICE GONE

Kettle ponds

Abandoned outflow channel

b Moraines, eskers, and drumlins form during glaciation, though eskers and drumlins originate under
the ice cover. Kames and kettles develop at the end of glaciation.

turbulence in these lakes were minimal; otherwise, clay and
organic matter would not have settled from suspension.
How then can we account for dropstones in a low-energy
environment? Most of them were probably carried into the
lakes by icebergs that eventually melted and released sedi-
ment contained in the ice.

snow and ice over the years. But this really does not address
the broader question of what causes ice ages—that is, times
of much more extensive glaciation. Actually, we need to ad-
dress not only what causes ice ages, but also why there have
been so few episodes of widespread glaciation in all of Earth
history. Only during the Late Proterozoic Eon, the Penn-
sylvanian and Permian periods, and the Pleistocene Epoch
has Earth had glaciers on a grand scale. Additionally, wide-
spread glaciation occurred at least four times during the
Pleistocene, with each glacial episode separated by a long in-
terglacial stage during which glaciers were restricted in their
distribution (see Chapter 23).

WHAT CAUSES ICE AGES?
We discussed the conditions necessary for a glacier to form
earlier in this chapter: More snow falls than melts during
the warm season, thus accounting for a net accumulation of