tens of millions to a few hundred million years are required to process organic
material into oil, mainly depending on the temperature and pressure condi-
tions with the sedimentary basin.
CARBONIFEROUS GLACIATION
Terrestrial flora first appearing some 450 million years ago was plentiful
and varied during the Carboniferous. Forests of seed ferns and true trees
quickly spread across Gondwana and Laurasia in the early Carboniferous.
Primitive amphibians inhabited the swampy forests. These forests were
buzzing with hundreds of species of insects, including large cockroaches
and giant dragonflies.When the climate grew colder and widespread glacia-
tion enveloped the southern continents at the end of the period, the first
reptiles emerged. They began displacing the amphibians as the dominant
land vertebrates.
During the latter part of the Carboniferous around 290 million years
ago, Gondwana was in the south polar regions. Glacial centers expanded across
the southern continents, as evidenced by glacial deposits of tillites along with
striations in ancient rocks. Rocks heavily grooved by the advancing glaciers
show lines of ice flow away from the equator and toward the poles.This would
not have been possible if the continents were situated where they are today.
Furthermore, the ice would have had to flow from the sea onto the land in
many areas, which is highly unlikely. Instead, the southern continents drifted
en masse over the South Pole, and huge ice sheets crossed the present conti-
nental boundaries.
Glacial deposits were interbedded with marine sediments. Deposits of
boulders distorted the finer sediment in which they lie. This indicates they
fell onto the ocean’s bottom muds from rafts of ice. Apparently, floating ice
sheets extended outward from the land like they do today at Antarctica’s huge
ice shelves (Fig. 113). As the icebergs drifted away from the ice sheet and
melted,debris embedded in the ice dropped into the sediment on the ocean
floor over a wide area.
Strange, out-of-place boulders called erratics, which were composed of
rock types not found elsewhere on one continent, matched rocks on the
opposing continent. The glacial deposits were overlain by thick sequences of
terrestrial sediments. In turn, these were covered by massive outpourings of
basalt lava flows. Overlying these volcanic rocks were coal beds containing
similar fossilized plant material.
Glacial deposits in present-day equatorial areas showthat in the past
these regions were much colder. Fossil coral reefs and coal deposits in the
north polar regions suggest a tropical climate existed there at one time. More-
CARBONIFEROUS AMPHIBIANS