The Knobs is a physiographic region located in central Kentucky, which looks like a horseshoe south of Lexington with the ends of the horseshoe reaching up and touching the Ohio River. The Knobs are quite distinctive, including hundreds of knobby hills forming a circular pattern.
These hills are actually monodnocks, which were formed through erosion over thousands of years. The original Mississippi Plateau had ancient rivers that carved the shales, including the Borden Formation dating back to the Mississipian epoch deposited about 330 to 360 million years ago. The capstone for these hills can be either limestone like Mississipian Harrodsburg or sandstone. Since these rocks resisted erosion better than the shales, the knobs were formed, much like the Grand Canyon, but on a much smaller scale.
However, one question still remains: why was this region circular? In other words, if erosion were the only force at work on this region, what formed the circle? Certainly, streams meander and erode irregularly. Erosion came after the creation of the circular pattern, so what was the original cause for the circular feature?
There has been little study of a theory that the circular part of the Knobs originally carried up across the Ohio River into southeastern Indiana and southwestern Ohio, forming a complete circle. The glaciers removed the evidence of that northern formation of the Knobs. The scouring of the land by glaciers stopped very close to the Ohio River.
So, what happened somewhere between about 350 million years ago and the ice ages? And what could have created a gargantuan circular depression? Since we don’t find any evidence of igneous formations, we can rule out volcanic eruptions. This leaves us to ponder whether there is any evidence of a huge meteorite crashing into this area millions of years ago? An astrobleme would be circular.
If such a meteorite hit this area, it would have caused a mass extinction. There are three mass extinctions that could be possibilities during this period, and they were the Permian, the Triassic-Jurassic, and the Cretaceous-Tertiary extinctions. We know about the mass extinction during the Cretaceous-Tertiary time, which wiped out the dinosaurs. There is a large crater in Mexico that may be the evidence for such a meteorite impact. This Knobs crater might have been created by a meteorite or comet that could have caused one of the largest extinctions like the Permian, sometimes called “The Great Dying.” However, this is only speculation.
We need more evidence. We need to study this potential crater to see if there is any evidence to support this theory. There could be iridium or some other elements associated with meteorites in the area. As far as I know, geologists have not studied this area to determine if there is any evidence of this being an impact area.
We do know that smaller meteorites have struck this part of the state, and those impacts have created similar upheavals that cannot be explained by typical mountain building processes, but on a much smaller scale.
There is sufficient evidence to make us think that something huge may have caused a circular pattern in the north-central part of Kentucky with the northern part of that circle in Indiana and Ohio being erased by subsequent ice ages. We need to treat this meteorite theory as a new hypothesis that should be tested.