Chapter 3, General Stratigraphy
Elevation and Topography
The city of Eureka Springs sprawls across a series of steep slopes. In general, the bottom of town near the train station is a little over 1100 feet above sea level, and the highest ridges and hilltops have an elevation of about 1500 feet. Several of the steeper streets reveal a vertical succession of 250 to 300 feet of rock layers within just a few blocks. Basin Park rests at a little over 1250 feet above sea level, and the Crescent Hotel sits at about 1450 feet.
To get printed copies of topographical maps of Eureka Springs or its surrounding areas, you can visit the Arkansas Geological Survey website at
https://www.geology.arkansas.gov/maps-and-data/topographic-maps.html
How Old Are The Rocks – The Quick Version
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 the geological record, 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 succession of rock layers, where period of time is missing, meaning where rocks were eroding away rather than forming.
The sedimentary surface rocks of the Ozark Plateaus, exposed in Missouri, Kansas, and Arkansas, are among the oldest sedimentary rocks in North America. (There are much older igneous rocks.) They range in age from sandstones of almost a billion years old to more recent limestone and dolomite deposits of around 300 to 500 million years old. These rocks were deposited during times when Northwest Arkansas was situated beneath the waters of shallow inland seas or along their sandy shorelines.
In an exposed vertical section of about 400 feet, the rocks around the town of Eureka Springs record an unusually broad and scientifically interesting period from 490 and 340 million years ago, a span of about 150 million years.
The rocks start 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. From this basement in the creek beds and valley bottoms, 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 in time past this extinction to the elevation, symbolically and literally, where the life giving springs pour out of the sides of the hills in the St Joe formation, the rocks climb to the Mississippian time period. During the Mississippian period, life flourished so prolifically that the period is called the Carboniferous, for the thick beds of coal laid down from the unperishing remains of giant forests that covered the earth as crawling things spread in teeming numbers across the land.
A short distance to the South of town and continuing down to the Boston Mountains ares, 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 Eureka Springs, these most recent rock layers have long since been removed by erosion, but they persists as close as Pond Mountain and Hillspeak, only a few miles away.
The Science of Stratigraphy
There is a story written in every rock. Geology is the art and science of reading that story as it is written in the stone, and putting it into words on paper in a form that is useful and understandable to society. The words in the rocks are written in layers, from bottom to top, rather than from left to right across a page. They are punctuated with changes in shape, size, and chemical composition. Sometimes, putting words to the stony silence is easy… it’s just a matter of learning to read the language of the rocks and stones. Other times it is difficult, and we have to look for tiny whispered secrets. Microscopic fossil changes, the wrong atoms migrating to the wrong places, a simple gap, or a misplaced stone might tell us a great but subtle secret.
Structural geology is the story of the processes that drive geological changes at the largest scale. These are the major plots, chapter headings, and character twists that make the story interesting and that change its direction. In structural geology, big things happen. Tectonic plates collide and push against each other, causing mountains to rise and tilt the surrounding land. Neighboring regions are pushed below sea level, and the shapes of continents change.
These kinds of effects, while clearly visible in the large view (mountains for instance!), are some of the hardest processes to make sense of in the smaller scale rock record. The least subtle landscape changes are some of the most subtle changes in the rock. The rise of a mountain range might be expressed in the local rock as a subtle change in sand grain composition. The retreat of a sea shore for a million years can be expressed as nothing… literally a gap in the rock, but without any opening. It can be as if a person stacked playing cards on a table day after day, and then left for a year before returning and resuming the process. A tiny layer of pebbles is the equivalent of dust gathered on a card while they were away, and might be all we have to work with. These large scale changes often leave such subtle clues and happen so slowly, that they disappear behind the smaller scale changes more boldly expressed in the rock. By contrast, a single severe storm may leave a visible layer of sediment or even impressions of raindrops on a surface that was once a mudflat, above the ocean’s surface.
Much of what geologists are able to understand by interpreting the rock record comes from examining rocks that form parallels in modern environments. Coastal sands, deep water shales, and storm blown silts can be examined in modern environments to better understand their ancient corollaries.
We do our best, and we tell the story with some art as well as science.
Sedimentary rocks, like those found around Eureka Springs, have layers. These layers are called strata. And their study is called ‘stratigraphy.’
Stratigraphy is the branch of geological science that deals with the study of rock strata, or layers. Stratigraphic study of a region starts with looking at how the rock layers can be divided up into units defined by meaningful differences in character and composition. Named units are described in terms of gross composition, meaning what kind of rock they are made of (sandstone, limestone, shale, etc.), and in terms of location, shape, and size. Stratigraphic study may also look at indicators of age, changes in texture or composition within and between layers, the presence or absence of fossils, or how rock units relate to other adjacent layers, among many other subjects.
The sequence of stacked strata in a given location may be referred to, depending upon whim or context, as ‘the stratigraphic column’, ‘the vertical succession’, ‘the stratigraphic profile’, some combination of these terms, or simply ‘the rock layers.’ The phrases are interchangeable. No matter what the sequence is called, the geologist is referring to the real world layering that is immediately or subtly visible when gazing at an exposed section of sedimentary rocks such as a road cut or cliff. These layers have meaning. They reflect difference and change over time. The changes may be as profound as a volcanic ash layer laid down over a sandy beach, or they may reflect something as subtle as the change in direction of water flow as the tide moves in and out.
Reading the stratigraphic profile, like any sort of detailed investigation of clues, is both an art and a science. Like Sherlock Holmes visiting a crime scene, the geologist reconstructs a series of past events and surroundings based upon limited clues.
Named Rock Layers
To more easily identify and talk to each other about rock layers, geologists have created a system of naming. We divide rocks up into groups, formations, and members. ‘Formations’ are the fundamental unit. They are grouped together into ‘Groups,’ or subdivided into ‘Members.’