2003 September 11  updated 2008

Why is Erla so stable?    the Cincinnati Arch

Composited below are three graphic images provided at the web site of the Kentucky Geological Survey, together depicting 4-dimensional representation of Kentucky geology.  In Kentucky, the older Cambrian layer forms the basement rock on which newer geologic layers have assembled.  Of these "newer-than-Cambrian", the oldest, deepest bedrock layer, Ordovician, is paradoxically exposed in Northern Kentucky, around which newer layers have subsided.  These areas of subsidence, or dips in the bedrock, regionally are:  (1) the Illinois Basin at the intersection of western Ky, Indiana, and Illinois; (2)  the interior Michigan Basin; and (3) Appalachian Basin in eastern Ky.  Paradoxically, the Appalachian mountains are founded in a geologic depression.  Between these 3 areas of depression is a now stable table of rock pushed up during earlier, vast geologic time.  This table is named the Cincinnati Arch.

The fourth dimension, geologic time, allows resolution of all paradoxes.  As described later, in pre-Cambrian times, the Ordovician plateau was at the bottom of the old ocean -- accounting for plentiful fossils around Greater Cincinnati.





These illustrations provide context for the Report by local geologist describing the geological context supporting Erla in Northern Kentucky,


Cincinnati Arch

Expanding our focus, we discover the Cincinnati Arch is "footed" in the vicinity of the KY-TN border, some say rising and falling in formations named the Cumberland Saddle and Jessamine Dome south of Lexington, and, further north, surfaces near the source of the Licking River, where it is broken by the transverse Lexington fault. Continuing north, it widely spans across (beneath) (is cut by)  the Ohio River, where it can be followed NW and NE in 2 divergent axes:  (1) along the long axis of Indiana, to the region of the Fortville Fault north of Indianapolis; (2) North and slightly East aligning with the Findlay Arch.  It's width extends well past Hamilton County, Ohio.  Geologically, "arch" (anticline) is the name for high ground relative to "basin" (syncline) representing low point of old bedrock that, over time, collect layers of newer geological depositions.  The basins on three sides of the Cincinnati Arch are epicentered (1) in middle Michigan; (2) at the Il-In-Ky intersect, and (3) along the Appalachian mountain range.

Following are various characterizations of this mid-continent rock of stability epicentered in Northern Kentucky.

Best illustration of the Ohio-Kentucky-Indiana Cincinnati Arch - by Professor Yacobucci at Bowling Green U., Ohio.


The Cincinnati Arch is a broad uplift in the sedimentary rock strata that traverses southwest Ohio in a north- south orientation before turning northwest where it joins the Kankakee Arch in east-central Indiana (Fig. 3).

Bedrock layers lie horizontally along the axis of the arch, but dip 5 to 10 ft/mi (feet per mile) along the flanks of the arch. The crest of the arch is approximately 75 miles wide. During the Paleozoic Era (230 to 700 million years ago), the Arch was an area of emergent land in shallow seas, and its flanks were areas of sediment deposition. Near the end of the Paleozoic Era, the seas receded and the arch was  subjected to erosion forming a flat plain that was subsequently dissected by streams. Because of this period of erosion, older rocks are found in the center of the arch, and younger rocks outcrop along the flanks


The Cincinnati Arch, a north-south-oriented, positive structural feature in southwestern Ohio and adjacent areas to the south, began to form in the Late Ordovician, perhaps initiated by the Taconic Orogeny. The axis of the arch is east of Cincinnati and continues northward until it splits into the Findlay Arch to the north and the Kankakee Arch to the west.

Text Figure 2
Generalized paleogeography of Ohio and adjacent areas during the Late Ordovician. The Taconic Mountains to the southeast were the source of mud carried far out into the shallow sea that covered Ohio. Modified from Coogan (1996).

The broad area formed by the three arches is called the Indiana-Ohio Platform.

The Cincinnati Arch was first discovered by John Locke during his work with the First Geological Survey of Ohio in 1838. It was viewed in a classical interpretation as an anticline, that is, the oldest rocks are exposed along the central axis and progressively younger rocks lie on the flanks. A cursory glance at the geologic map of Ohio implies an anticlinal feature: the oldest rocks exposed in Ohio--Ordovician--crop out along the axis and younger Paleozoic rocks dip eastward into the Appalachian Basin, westward into the Illinois Basin, and northward into the Michigan Basin.

Recent revelations about the geology of the deeply buried Precambrian rocks of the state (see Ohio Geology, Winter 1996) indicate that the Cincinnati Arch overlies a sediment-filled rift basin that formed in Late Precambrian time. The area underlain by the rift basin appears to be a comparatively stable crustal block surrounded by subsiding basins. The Cincinnati Arch, therefore, may be viewed not as an anticlinal structure but as a positive feature around which the crust has subsided. This development may have been initiated by the Taconic Orogeny in the Late Ordovician and further accentuated by later Paleozoic orogenic events.

Addressing Steve Greb of the Kentucky Geological Society:

Regarding differential vertical movement at the Lexington fault system, is it more accurate to say:

  1. The Cincinnati Arch is rising, or

  2. The Cincinnati Arch is stable as newer layers subside southerly.

Steve Greb:

Movement on the arch is mostly ancient. So it is no longer rising. 

Likewise, movement in the basins is ancient. They are no longer sinking.

Kentucky is eroding, rather than having new layers being deposited.

The arch was uplifted during the early to middle Paleozoic, and was

reactivated at several times later in the Paleozoic during various

mountain-building events. There were times when there was true uplift, but

compared to rates of subsidence in the basins on either side, most of the

relative movement was due to subsidence in the basins, alongside a mostly

stable arch.


Thus regionally, the exposed Cincinnati Arch in Northern Kentucky is the crest of the strongest, oldest, least-tattered geologic structure in the United States, representing the high, old ground of stability protected within the larger North American continent (craton).

Formation of the Cincinnati Arch   -- brief excerpts from Geologic History

Additional References

http://igs.indiana.edu/geology/structure/tectonicfeatures/index.cfm  Arch with MI and IL Basins

http://igs.indiana.edu/geology/structure/bedrockgeology/index.cfm  view of the (green) Ordovician  exposure (purple above in KY) into SE Indiana, named Maquoketa Group

http://www.ohiodnr.com/geosurvey/pdf/geologic.pdf view of the (pink) Ordovician (purple above in KY) exposure into SW Ohio

http://www.ohiodnr.com/geosurvey/pdf/mappg23.pdf Acrobat doc by Thomas Baranoski.  On page 2 (8 of 27) is depicted the spine of the Cincinnati Arch as it intersects in "Y" formation very near the Ohio River at the "nipple" of Northern Kentucky.  A closer, similar view, not yet available for web reproduction, is obtained from  KGS Pub. 1519, Exploring the geology of Cincinnati/northern Kentucky region, by U.Cincinnati Professor Emeritus Paul E. Potter.


Editor's Note:  The author of this compilation, john schmidt, humbly wishes to express joyful gratitude for the patient and generous offerings of time, conversation, and references by geologists contributing to the guidance and authentication of this report, which i hope is subject to continuing review and edification toward perfect understanding.  Noteworthy is the caution every expert advised in representing the accuracy of sketched linear depictions of the "spine" of the Cincinnati Arch.  Also noteworthy is that study of this geologic feature and its larger national context continues aggressively at USGS and within each state, only limited by funding constraints.  Various motivations include the desire to protect innocent victims of earthquakes that could be predicted in time to mobilize away from active sites.  Thank you for your suggestions for improvement.   john@ncad.net

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The bonus value for properly understanding this magnificent foundation for building, is to represent this unique, national treasure, enhancing local appreciation of Earth, while inviting those contemplating new, secure, large, tall, buildings -- to locate building plans near the spine of the Great Cincinnati Arch, where building costs are not so encumbered by counter-seismic considerations.