Gaps in Our Knowledge of Geologic History

There’s a new way of thinking about the geologic history of the Southern Appalachian Mountains. Read most summaries about their origins and you’ll learn the Appalachians arose during the last third of the Paleozoic Era. The authors will likely explain that the mountains have been worn down to their current form during the 251 million years since. This model for the origins of the modern southeastern landscape has been accepted for many decades. However, there is new evidence of a recent phase of “uplift,” or rising of the earth’s crust, and some of this evidence has been hidden in plain sight.

No one disputes the Paleozoic origins of the Appalachians, but new findings suggests there was an additional, more recent phase of mountain building in the Southern Appalachians, probably in the Miocene Epoch (23–5 million years ago). Some of this evidence comes from geological studies of how the pathways of the Appalachians’ streams weave through the mountains.

Mountains form as continents collide for two reasons – the uplifting of the earth’s crust as it is squeezed from either side and the accumulation of debris at the site of collision. These processes caused the formation of the Appalachians in the Paleozoic Era.

Streams in the region tend to flow away from an uplifted area. However, throughout Alabama’s mountains are places where streams cut through the mountains. These breaks in the mountains – known as water gaps – suggest that the paths of these streams were in place before the uplift of the mountains. As the earth’s crust was forced upward, the streams eroded into the rising rock to maintain their path across the landscape. Turkey Creek at Turkey Creek Nature Preserve is one such gap.

Other gaps in the mountains are known as wind gaps, named for how the wind speeds up as surface winds are funneled through them. Many wind gaps are also thought to have been formed by streams that existed before uplift in the Miocene Epoch. However, these streams were small and unable to erode the rising rock quickly enough to survive. Grace’s Gap, at the northeastern tip of Red Mountain Park, is a wind gap and the perfect path for the Birmingham Mineral Railroad.(Look for Grace’s Gap on the map of the Birmingham Mineral Railroad in 1930).  Another is found just to the northeast where Green Springs Highway crosses Red Mountain.

Some wind gaps were caused by fracturing or “faulting” through a mountain, and they may or may not be associated with the Miocene uplift. Red Gap, at the southwestern end of Ruffner Mountain Nature Preserve, is one of these. This gap was also an ideal location for heavy gauge rail lines for transporting red ore from local mines (On the Birmingham Mineral Railroad map Red Gap is near Gate City). A large fault (a large fracture that cuts through many rock layers) crosses through the mountain at this point. A small section of the mountain was pushed northward along the fault, and erosion widened the resulting notch into a gap. Some of these faults are still occasionally active. In 1916, the Red Gap Fault was the source of a significant earthquake that shook the city and was felt as far away as Atlanta.

Alone, observations of the water and wind gaps wouldn’t be enough to challenge the Paleozoic-only model of Appalachian uplift. However, together with other lines of evidence, they take on special significance.

The biggest “gap” in our knowledge is what caused the Miocene uplift. By the time the Miocene Epoch arrived, the Appalachians hadn’t been a “crunch zone” of colliding continents for over 200 million years. Thus, geologists are trying to figure out what else could have caused the Miocene uplift. They have several ideas. The one favored most right now is that the North American continent, which is slowly travelling westward, overran a large fragment of the earth’s crust. As that fragment passed beneath the Southern Appalachians, it forced the landscape upward, not unlike a car hitting a speed bump.

-R.Scot Duncan