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stages that have been documented for the Pleistocene epoch? The first question deals with long-
term trends in temperature on a scale of millions of years, but this second question relates to
much shorter-term changes.
Although the literature of science contains many hypotheses relating to the possible
causes of glacial periods, probably the most attractive proposal for explaining the fact that
extensive glaciations have occurred only a few times in the geologic past comes from the theory
of plate tectonics.
Because glaciers can form only on land, we know that landmasses must exist somewhere
in the higher latitudes before an ice age can commence. Many scientists suggest that ice ages
have occurred only when Earth’s shifting crustal plates have carried the continents from tropical
latitudes to more poleward positions.
Today scientists understand that the areas containing these ancient glacial features were
joined together as a single supercontinent (Pangaea) located at latitudes far to the south of their
present positions. Later this landmass broke apart, and its pieces, each moving on a different
plate, migrated toward their present locations. Now we know that during the geologic past, plate
movements accounted for many dramatic climatic changes as landmasses shifted in relation to
one another and moved to different latitudinal positions. Changes in oceanic circulation also
must have occurred, altering the transport of heat and moisture and consequently the climate too.
Because the rate of plate movement is very slow—a few centimetres per year—appreciable
changes in the positions of the continents occur only over great spans of geologic time. Thus,
climate changes brought about by shifting plates are extremely gradual and happen on a scale of
millions of years.

Task 2. Look at Figure 8.1. A shows the supercontinent Pangaea with the area
covered by glacial ice about 300 million years ago. B shows the continents as they are
today. The white areas indicate where evidence of the old ice sheets exists. Using the figure
and the information from the text talk about the theory of plate tectonics as one of the
popular hypotheses relating to the possible causes of glacial periods.

Task 3. Build up a glossary to the most important terms used in the text.

Task 4. Answer the questions:
1. What is the name of earlier glaciations?
2. When were two Precambrian glacial episodes identified in the geologic record?

Task 5. Give a short report about some other hypotheses relating to the possible
causes of glacial periods, for example, variations in Earth’s orbit.

TEST YOURSELF

Task 1. Tell whether the sentences are true or false. Correct the false ones.
1. A reservoir rock is a rock that can both store and transmit fluids.
2. Saturation is the percent volume of the rock that is not occupied by solids.
3. Porosimeter is a cylinder of rock that is drilled from the well.
4. Porosity cutoff is a minimum porosity value.
5. Permeability is measured in units of darcys (D) or millidarcys.
6. Porosity and permeability in a single sedimentary rock layer are not related.
7. Oil field brine is very salty water that shared the pores with the oil.
8. Sandstones usually have water in the center of the pore, and oil is on the outside of the pore
in contact with the sand grains.
9. Earlier glaciations are indicated by a sedimentary rock formed when glacial till becomes
lithified.