Backyard Geology
Scientists can tell a lot about the geologic history of Spirit and
Opportunity's landing sites by examining the rocks they find. The same
applies to your backyard. The rocks you see around you offer a record
of ancient happenings in your patch of planet Earth. Over the next
several months, Athena Student Interns from across the United States
will present a glimpse of the geology that is native to their regions.
What's
in YOUR backyard?
Ramsay High School – Birmingham, Alabama
It is impossible to mention the geology of Birmingham, Alabama without
acknowledging Birmingham’s industrial history. The two are very
tightly intertwined. Indeed the existence of the city is due to the
geology of the area. The region was known to have the raw materials for
iron making years before it’s founding, as attested by various
small foundries that dotted the region in the mid 1800’s. Most of
these operated for short and interrupted spans, as they produced cannon
balls and other wartime implements for the Confederate Army, and were
usually prime targets for Union troops.
Birmingham began its journey to the status of “the Pittsburgh of
the South” when more complete surveys of the area determined that
all the natural elements needed for iron and steel production —
iron ore, limestone, and coal — all existed in large quantities to
meet the needs of early Industrial Age America. And so, in 1871, the
city of Birmingham came into existence. Railroads connected the various
mines and foundries together and for a number of decades iron and steel
production grew Birmingham rapidly into a large Southern city. Changes
occurred so rapidly in those days that a reporter once dubbed Birmingham
“The Magic City”.
Naturally times change, and with it steel production would be changed.
Eventually local steel mills began to depend on imported cheaper raw
materials and, one of the earliest recycling programs, scrap steel. By
the time Birmingham reached 100 years old, a number of those early
companies were shutting down, as imported steel took its toll. A few
major steel plants still remain, but the mining of natural resources for
steel making has long since gone the way of the steam locomotive.
For our survey, we went a bit further to explore than our back yard.
Our small back yard is made up of fill dirt from other locations, but
some evidences of what used to be occasionally emerge. A relative who
lives on the street, about a block away, with a much older lot has found
fossilized coral in his back yard. We have occasionally stumbled on
fossilized remains of ancient sea life, such as crinoid stems, in our
yard. The field behind our house is composed of sandstone, but like
most things in Birmingham, it is built on a sloped and terraced hill
side, and we do not know if the sandstone is original or brought in.
The hillside surrounding the field on one side is composed of brown or
orange clay.
We knew of a better place to look for Birmingham’s geological
history, and it is very close to our home. It is known as the
Ruffner
Mountain Nature Center. Though you may have never heard of Ruffner
Mountain Nature Center, what sounds like a small petting zoo is actually
one of the largest urban forests in the nation, with acreage that
exceeds that of New York’s Central Park. Ruffner Mountain is a
part of the Appalachian chain, and Birmingham is at its very beginning
— or end, depending on how you look at it.
Ruffner has naturally exposed boulders, but early miners dug both mines
and pits on both sides of the mountain. The western side has a man made
crater several hundred feet across, where limestone was mined. The
eastern side of Ruffner is Swiss-cheesed with many dark mine shafts
where reddish-purple iron ore was mined until the 1950s. So right away
we can see that seams of iron ore and limestone pushed up to form our
mountain many eons ago. (By the way, the other element of iron making,
coal, is located in the opposite end of the county, and is not in our
immediate vicinity.)
The seam of iron ore that graces the eastern side of Ruffner is known as
the “Irondale” seam. (The seam is named for the small town
of Irondale, which I include for trivia’s sake. The famed
Irondale Café was owned by Fannie Flagg’s aunt, and was the
inspiration for the movie “Fried Green Tomatoes”.) The
Irondale seam is the smaller of two main seams of iron ore that time and
the elements forced upward.
A larger seam is near Birmingham city center, Red Mountain. The Red
mountain seam was heavily mined in the early days of the city. In the
mid 1960s, a major road-building project produced a massive cut in Red
Mountain, exposing not only the iron ore seam, but great walls of eons
of geologic history in many layers of rock. At the time the great cut
was a record setter. Today, however, if you want to study the geologic
history of this cut-away mountain, you must do so at 60 mph. This
massive cut is very red, as it is saturated with iron ore.
Limestone also runs beneath our feet here, and is still being formed by
underground rivers that run underneath the city. Near to where we did
our survey, one of our earliest malls once pumped up the water from one
of those underground rivers for use in the mall’s air-conditioning
system.
After walking through several miles of woodland trails photographing
outcroppings and mine ruins, we went to a nearby mall and did some
further geological surveys in the parking lot. The mall is built into a
hillside, and the parking lot features walls of natural rock, dominated
by the deep red Irondale seam, along smaller seams composed of
sedimentary rocks such as shale, slate, and limestone. One spot has the
iron ore seam running above a seam that is suddenly folded. This
indicates a fault line, now mostly dormant. (Just on the other side of
the mall parking lot is another mountain cut for I-20. Today trees have
overgrown the excavated rock walls, but before vegetation overgrew the
area, a better view of this ancient fault could be seen.)
They call this a geologic puzzle. But it is a puzzle that is without a
reference picture, and the pieces are indeed scattered and scrambled.
What we know just by simple investigation is that there are evidences of
an ancient ocean, yet somehow many of these evidences have wound up in
hill top iron ore pits. This speaks of many eras of change and many
upheavals that disturbed the now dry seabed. Below is a very brief
geologic history of our area, as compiled by the Alabama Mineral and
Lapidary Society (From “Geology of the Birmingham Area”,
copyright 1993).
Our area is considered to lie within the Paleozoic Area. Local rocks
are folded and faulted, ranging in age from the Pennsylvanian Period
(Approximately 250 million years ago) to the Cambrian Period
(Approximately 400 million years ago). Deeper seams are mostly composed
of much older and complex igneous and metamorphic in nature. These
however, are not exposed.
The Cambrian rocks are abundant in fossilized life, such as trilobites,
snails, sponges, oystoids, and early algae. The ancient ocean that
covered much of the eastern United States deposited these. This ocean
was an inlet from the Atlantic that came inward up to the Appalachian
Mountains. At this time, by comparison, there was no Gulf of Mexico or
Caribbean, and the continental land mass extended to the area of the
Bahamas.
In time the ancient ocean subsided but in time would once more cover the
southern states. Later, in the Mississippian Period, the land was
uplifted slowly. For a time, great swamps covered the land, eventually
leading to the formation of coal. The later Pennsylvanian period
brought the development of the Appalachian Mountains, and helped to
drain the remnants of the early sea, swamps, and deltas that deposited
the early layers of rock we see today in the Birmingham area. During
this period, the Earth’s crust was actually shrinking, and this
led to the bunching up of areas of crust, what we call mountains. This
bunching up also lifted the land up, allowing definite areas of land to
form, verses the large shallow seas that had once covered much of
ancient America. After this period, no more marine sediments were
deposited here, leaving a time-gap in the geologic time table of some
150 to 200 million years.
Put in its simplest terms, geologists today speak of a much higher
mountain that once dominated the area, which lifted up multiple seams of
sediment into a great dome. In time, the main dome was worn away,
leaving a landscape dominated by great lines of mountains and wide
valleys. The great seams of rock, such as iron ore, shale, dolomite and
limestone slant in one direction near downtown, and in the opposite as
one travels to the south.
As the Mars Exploration Rover missions are ongoing at the time of this
writing, what parallels can we draw? We are looking mostly at hematite,
contained in martian rocks and soil for signs of water. Water, we
expect, could be an indicator of the possible formation of life there,
if indeed martian life might be based on what led to life on Earth. One
thing is certain, though, about a comparison between martian hematite
and our local iron ore. If martian ore looked anything like ours, there
would be no more questions as to whether or not Mars supported life long
ago. Ours is full of fossilized shells.
Our sedimentary rocks would tell a similar story, as compared to those
of Meridiani Planum, as they contain traces of fossils, such as shells,
corals, and trilobites. Our sedimentary rocks also contain many
different minerals deposited by water over the eons. On Mars, as we
have just discovered, the sediment at the rock outcrop was also borne of
water.
In time, perhaps we will also solve the geologic puzzle of Mars and
become as familiar with it as we are with our own backyard.
