Geology Reference
In-Depth Information
During the cooler climate of the middle Cenozoic, layers of ice began to cover the poles
periodically. In the southern hemisphere, the Antarctic ice sheet was established by
about 10 million years ago. In the northern hemisphere, ice didn't become a geologic
feature of the Cenozoic until the beginning of the Quaternary period, around 2.8 million
years ago. Periodically over the last two and a half million years, large ice sheets have
extended southward over the continents of Europe and North America.
Each glacial stage (period of glaciation) during the Quaternary period lasted for thou-
sands of years. In between the glaciations were periods called interglacials when the ice
cover shrank. The alternation of glacial stages and interglacials over hundreds of thou-
sands of years has been linked by scientists to patterns of the earth's orbit, rotation,
and distance from the sun. (I describe these cycles and other features of glaciers and
glacial geology in Chapter 13.)
The modern landscapes of North America and Europe were shaped by the repeated
growing and shrinking of ice sheets. As the mass of ice grew, it moved farther south,
eroded rocks and sediments from the surface of the earth, pushed these earth materials
southward, and deposited them as moraines and other glacial features I describe in
Chapter 13.
The cycle of Neogene and Quaternary glacials and interglacials shaped the recent evolu-
tionary story of mammals, including man.
Entering the Age of Mammals
Mammals did not suddenly appear in the Cenozoic era. By 65 million years ago, mam-
mals had been living side by side with the reptiles and dinosaurs for almost 150 million
years. At the beginning of the Cenozoic, however, conditions were right for mammals to
take over as the dominant animals.
Mammals, like reptiles, are vertebrate animals, with an internal skeletal structure. But
mammals have certain characteristics that separate them from reptiles. I list a few of the
most easily observed characteristics here:
Mammals have differentiated teeth. This means that within the jawbone of a mam-
mal, the teeth are shaped differently to perform different tasks. For example, in hu-
mans the front teeth (the incisors ) are for cutting, while the back teeth (the mol-
ars ) are for grinding.
Mammals are endothermic. Being endothermic means an animal can regulate its
own body temperature, to warm up or cool down as needed. Reptiles do not have
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