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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.