Apparently during the middle Cretaceous, Australia, while still attached to
Antarctica, wandered near the Antarctic Circle and acquired a thick mantle of ice.
Large, out-of-place boulders strewn across the great central desert suggest that ice
existed there even during the warm Cretaceous about 100 million years ago. At
this time,Australia was still attached to Antarctica and straddled the Antarctic Cir-
cle, inside of which winters were sunless and cold. Maintaining wintertime tem-
peratures above freezing in the interiors of large continents at high latitudes is
difficult because they do not receive warmth from the ocean.
Like most continents of the Cretaceous, the interior of Australia contained
a large inland sea.The continents were generally flatter, and sea levels were hun-
dreds of feet higher than today. Sediments settling on the floor of the basin lithi-
fied into sandstone and shale. They were later exposed when the land uplifted
and the sea departed at the end of the period. After Australia had drifted into
the subtropics, the central portion of the continent became a large desert. Lying
in the middle of the sedimentary deposits are curious-looking boulders of
exotic rock called dropstones.They measure as much as 10 feet across and came
from a great distance away. Rivers or mudslides could not have carried the boul-
ders into the middle of the basin because such torrents would have disturbed
the smooth sediments composed of fine-grained sandstone and shale.
The appearance of these strange boulders out in the middle of the desert
suggests they rafted out to sea on slabs of drift ice. When the ice melted, the
huge rocks simply dropped to the ocean floor, where their impacts disturbed
the underlying sediment layers.The boulders apparently were not dropped by
permanent glaciers but by seasonal ice packs that formed in winter. During
the cold winters, portions of the interior coastline froze into pack ice. Rivers
of broken ice then flowed into the inland sea, carrying with them embedded
boulders dropped more than 60 miles from shore.
Evidence of ice rafting of boulders during the Cretaceous also exists in
glacial soils in other areas of the world such as the Canadian Arctic and
Siberia.This suggests that the high latitudes still had cold climates, in which
ice formed easily even during one of the warmest periods in Earth history.
Boulders were also found in sediments from other warm periods as well.The
same ice-rafting process is occurring even today in the Hudson Bay.
When the Cretaceous ended, the seas regressed from the land due to low-
ering sea levels, and the climate grew colder. The last stage of the Cretaceous,
called the Maestrichtian, was the coldest interval of the period. The decreasing
global temperatures and increasing seasonal variation in the weather made the
world stormier, with powerful gusty winds that wreaked havoc over Earth.
No clear evidence for significant glaciation during this time has been
found.Yet most warmth-loving species, especially many of those living in the
Tethys Sea, disappeared when the Cretaceous came to an end.The extinctions
appear to have been gradual, occurring over a period of 1 to 2 million years.Historical Geology