Historical Overview

The fall of 1862 found the Confederate and Union armies moving their forces into the state of Kentucky (Figure 1).  The Confederate forces were under the command of General Braxton Bragg and General Edmund Kirby Smith.  The Union forces were under the command of Major General Don Carlos Buell.  The tentative plan of the Confederates was for Gen. Bragg and Gen. Smith to meet in central Kentucky and perform an offensive against Louisville or meet the Union army in Kentucky and engage in battle.  Meanwhile, Maj. Gen. Buell found his army limited to racing Gen. Bragg and his army into Kentucky.

Bragg’s hopes for taking Louisville appeared to be in jeopardy while camped in the Bardstown area.  A severe drought gripped the state of Kentucky that summer making food and water sources tough to come by.  This factor played a large role in Bragg’s decision to abandon the objective of Louisville.  The drought caused a lack of resources in the region, which became an overwhelming factor for the Confederates. This dictated that Bragg move towards central Kentucky and better resources.  Moving into central Kentucky would also allow him to unite with Smith and continue their offensive.  In the meantime Buell had won the race to Louisville.  He restocked his troops and marched out after the Confederates.

Facing a lack of resources Bragg’s Confederate army wandered around central Kentucky foraging for food, while Smith’s army was dispersed throughout the region. The  Confederates were in desperate need of water, and they happened to find it at Doctor's Creek, near the town of Perryville.  While the Confederates looked to secure this water source the Union army began to bear down on them.  As the Union army caught up to the Confederates the Battle of Perryville began.  From the night of October 7 through October 8, 1862 these two armies fought.

The night of the 8th found the Confederates forces facing overwhelming odds and they decided to retreat.  Initially a retreat was ordered to Harrodsburg and Danville, from there they could unite and formulate a plan.  From this position the Confederates decided to leave the state of Kentucky.  They followed a retreat route from central Kentucky  southward through the Cumberland Gap,  thus ending the Confederate invasion of Kentucky.

Geological Overview
 
To understand the role that geology played in the 1862 Invasion of Kentucky you need to become familiar with the geological background of the state.  Kentucky is divided into six primary physiographic provinces : Bluegrass, Knobs, Pennyroyal, Eastern Kentucky Coal Field, Western Kentucky Coal Field, and the Jackson Purchase Regions (Figure 2).  Each of these six regions reflects the underlying geology of that particular area.  The Bluegrass, Knobs, and Pennyroyal Regions were of particular significance to the Invasion of Kentucky.

The Bluegrass region is located in central Kentucky and is underlain by Ordovician limestones that are about 475 To 440 million years old.  This region is characterized by gently rolling hills, sinkholes, and springs, which are typical of karstic areas.  Due to the rolling terrain, the Bluegrass region was easy for armies to traverse, and thus control of the region was very desirable for both the Confederates and the Union armies.  This and other factors that initiated the Invasion of Kentucky came to a head at The Battle of Perryville, which was fought in the southern portion of this physiographic province.

The Knobs Region forms a horseshoe shaped band around the Bluegrass Province.  It is underlain by shales, sandstones, and limestones from the Silurian, Mississippian, and Pennsylvanian Systems, which means they were deposited approximately 440 to 330 million years ago.  This region’s topography is quite different from the gently rolling hills of the Bluegrass.  The Knobs is typically a very rugged region, marked by steep conical shaped hills.  These hills formed by differential erosion of the overlying, more competent, cap rocks and the underlying weaker shales.  This region formed a formidable barrier to troop movements, but also served  as an excellent way to protect an armies flank.  It was well used by the Union army to protect its southern flank as they swung out of Louisville to search through the Bluegrass for the Confederate army.

The bulk of both armies entered Kentucky via the Pennyroyal Region.  The Pennyroyal Region is located in south central and western Kentucky.  Like the Bluegrass region, the Pennyroyal is underlain by limestones which leads to similar topographies between the two regions.  This similarity meant that the Pennyroyal was also easily passable by both armies.  The limestones of the Pennyroyal are younger Mississippian rocks, which were deposited about 360 to 320 million years ago.  This region provided an easy means by which armies could move from Nashville to Louisville.  The tactical significance of this railroad corridor, which provided easy access to central Tennessee, was not overlooked by either side.  To control the Pennyroyal Region essentially meant controlling the heart of Tennessee, because of the easy access provided Louisville and Nashville Railroad.

To understand why these provinces are situated in the manner that they are you need to examine the Cincinnati Arch, which is the key geologic structure in the central portion of Kentucky.  The arch is basically a structural high where older rocks are exposed along its axis and younger rocks are exposed at the surface as you move away from the structure.  A dome (high point) along the arch in central Kentucky results in Ordovician rocks outcropping at the surface.  This outcrop zone is called the Bluegrass physiographic province.  As you move away from the dome in the Bluegrass region you enter a zone known as the Knobs physiographic province.  The knobs form a thin zone of steep hills that rings the outer Bluegrass.  Moving further to the south along the axis of the arch towards Nashville we cross the Cumberland Saddle, which is a low point for the arch.  This low point results in the Pennyrile Physiographic province.  Mississippian rocks are exposed at the surface in this region as would be expected since we are near the low point of the Cincinnati Arch.  To the south, in the Nashville area, the arch reaches another high point and Ordovician rocks again outcrop at the surface.