The deepest and oldest rocks that we know anything about under Eureka Springs and the rest of the Ozark Plateaus province are layers of the deep continent forming rhyolitic and granitic sheets of the Spavinaw Terrane.  The Spavinaw Terrane is located about 2000 to 3000 feet underground in some test wells that have been dug in Southern Missouri and Northwest Arkansas, but I haven’t been able to determine a depth under Eureka Springs itself.  These volcanic and igneous rocks, also known as the Spavinaw Granite, are extremely old, having been laid down between 1.48 and 1.35 billion years ago during the Proterozoic Eon of the Precambrian.  These earliest rock layers only penetrate to the surface in a few areas, most notably, the San Francisco Mountains of Southern Missouri.  

After these layers of granite were formed, the area was thrust above the surface of the ocean for a period of about 900 million years.  In general, rocks accumulate when a surface is below sea level, and are eroded away when the surface is exposed above water.  We refer to this period of time when the surface of the ground was above the ocean as an unconformity.  An unconformity is a gap in the geological record, where layers of rock were either not laid down, or were subsequently removed by erosion before younger rocks accumulated.  

The sedimentary rocks exposed on the surface of the Ozark Plateaus, including the portions exposed in Missouri, Kansas, and Oklahoma as well as those in Arkansas, are the oldest Sedimentary rocks in North America.  They range in age from sandstones of almost a billion years old to more recent limestone and dolomite deposits that range from about 300 million years old to about 500 million years old.  All of the exposed rocks underlying Eureka Springs were formed in the more recent part of this time span, between about 490 and 340 million years ago. 

All of these rocks were deposited during times when Northwest Arkansas was situated beneath the waters of shallow oceans or along their sandy shorelines.

[Detail of old geo map.]

The exposed rocks visible in the hills around the town represent about 700 feet of stratigraphic succession (stacked layers).  This sequence of layers reaches across an unusually broad and scientifically interesting span of time, lasting about 150 million years.  The oldest rocks are at the bottom of the valleys, and the youngest are on the tops of the hills.  The rocks in the streambeds at the lowest spots around town formed in the earliest parts of the Ordovician Period, before even the earliest creatures had crawled out of the oceans and on to land, and before the continents had begun to turn green with plant life.  It is these rocks that were quarried, just outside of town, to build the large, old stone buildings   

From this basement, the rock layers climb through one of the largest extinctions in global history, in the Devonian, when something like 90% of every species in the oceans went extinct.  Going upward geography and forward in time, life giving springs pour out of the sides of the hills, from the St Joe formation, deposited during the Mississippian period, when life flourished so prolifically that the period is called the Carboniferous, for the thick beds of coal laid down from the lasting remains of giant forests that covered the earth as crawling things spread in teeming numbers across the land.  

A short distance to the South, in the Boston Mountains, younger Carboniferous rocks from the Pennsylvanian period (300-311 million years old) are still sitting on top of the older rocks.  These Pennsylvanian layers are the youngest rocks in the region.  In the town of Eureka Springs, these most recent rock layers, which were never very thick, have long since been removed by erosion.  The chapters on structural geology and ‘stratigraphy’, a word which means the study of rock layers, their contents, and their origins, will go into much greater detail concerning both the ages of the rocks under Eureka Springs and their descriptions.

The youngest rocks in the immediate vicinity of Eureka Springs are exposed at the top of Pond Mountain, just south of town, and along a few adjacent ridge tops west of 23 in that area.  These rocks are a little over 300 million years old.