Geology Reference
In-Depth Information
many elegant trilobites from reef ecosystems tend to have large eyes that could scan the
full 360 degrees of their environment. Farther offshore, black shales accumulate slowly in
deep dark water; their fauna commonly include filter feeders and blind trilobites—animals
quite different from those of shallower photic zones.
If each outcrop paints a picture of a time and place, then a sequence of rock layers piled
one atop the next tells a rich story of change. Particularly dramatic sequences of layered
rock types often occur in association with economically valuable (and correspondingly
well-studied) coal deposits. Coal, which formed abundantly in swampy coastal zones three
hundred million years ago, commonly occurs sandwiched between layers of sandstone,
which are in turn bounded by shale. Such a sequence—shale, sandstone, coal, sandstone,
shale repeated over and over—implies significant shifts in sea level as it drops, then rises,
then drops again, perhaps in response to the retreat and advance of polar ice and glaciers.
An inescapable conclusion is that for hundreds of millions of years, ocean depths have re-
peatedly varied by hundreds of feet.
To modern humans, with our immense coastal cities and vast seaside infrastructure, the
height of the oceans (at least within the ebb and flow of tides) seems to be a fixed aspect
of the globe. It's hard to imagine a change of even 10 feet, much less hundreds of feet.
But the recent sedimentary record is unambiguous onthis point. Within the last few tens of
thousandsofyears,theoceanshavebeenatlevelsmorethan150feethigherandmorethan
300 feet lower than today. Without any possible doubt, such changes will come again to
radically alter the shapes of continental coastlines. Such is the story told by the rocks and
their fossil ecosystems.
Life on Land
The most dramatic terrestrial transformation in Earth history had to await the rise of land
plants—an innovation recorded as distinctive, sturdy microscopic fossil spores in rocks as
old as 475 million years. Though no body fossils of the delicate, easily decayed vegeta-
tion from that time have yet been found, those first true plants were probably not unlike
modern liverworts—rootless, ground-hugging descendants of green algae that could sur-
vive only in low, wet places. For a span of more than 40 million years, in terrestrial rock
formationsaroundtheworld,decay-resistantsporesaretheonlyphysicalevidenceforland
plants. Evolution of these hardy green pioneers seems to have been steady but slow.
About 430 million years ago, a significant change in the global diversity of spore fossils
points to a marked shift in the distribution of land plants. Over the next 30 million years,
the liverwortlike spores became less abundant, while those similar to modern mosses and
simple vascular plants assumed dominance. Rocks of this interval from Scotland, Bolivia,
China, and Australia also hold the oldest known unambiguous fossils of the plants them-
