Chapter 4, The Boone Formation

Starting at the hilltops and working our way downward, we’ll look at the individual layers of rocks that make up the town. The rust-red stained chert that marks the upper hill slopes is called the Boone Chert. It dates from the Mississippian period of Earth History, about 340 million years ago.

Rocks and Minerals of the Boone Formation

Quartz

Quartz is the single most common mineral in the earth’s crust. It makes up 12% of the earth’s crust, and a substantially higher percentage of the exposed rocks in Eureka Springs.

Most people in this area are aware of quartz in the form of single quartz crystals, generally meaning specimens a few centimeters in size or larger. Such examples are pervasive on shelves and in pockets and drawers around here, since central Arkansas was once the world’s largest producer of this kind of crystal. Our familiarity with quartz crystals has led to a misnomer, however. We conflate the terms ‘quartz’ and ‘crystal.’ While all quartz is crystalline, there are thousands of other crystals that are not quartz, including virtually every rock-forming mineral that makes up the planet. All minerals are crystalline. It’s part of the definition of the word mineral. The word ‘crystalline’ means that the atoms making something up occur in an orderly and recurring pattern. And a ‘crystal’ is a continuous stack of such atoms, whether so small that we can’t see the edges of the tiny stacks without a microscope (making something micro-crystalline), or large enough to hold in the palm of our hand.

Large quartz crystal found near Mt. Ida, Arkansas.

Most of the quartz in Eureka Springs occurs as the micro-crystalline aggregates, flint and chert. The words flint and chert are more or less interchangeable, but custom uses ‘chert’ when the material is opaque and light in color, like our local material. Chert composes the majority of the upper hill slopes in town, dominating the bulk of the rock unit we call the Boone Chert Formation. It looks like limestone at a glance because of impurities trapped when the material formed from the gradual dissolution and replacement of limestone from layers of seafloor mud that contained a lot of silica. But when we cut and examine the hackly, fractured rocks of the upper hill slopes under a microscope, we find it to be made up almost entirely of quartz. The word describing the replacement of limestone with quartz is ‘silicification.’ The Boone Chert that comprises our upper hill slopes started out as mud on the bottom of a shallow inland sea. The mud was composed mostly of carbonates, but was also rich in silica, probably from wind-blown volcanic ash. Over time, the mud solidified to form limestone, but then the silica slowly replaced and excluded the limestone, preserving both the textures of the mud itself and the fossils it contained. You can generally recognize the Boone Chert in town by the reddish color of the soil and clay that occurs richly in any pocket when the rock is disturbed. The red clay weathers to a grey soil where it has been exposed over a long period.

Lower down our hillsides, quartz occurs as sparse nodules in the members of the St. Joe Formation (meaning the bluffs), and in the Clifty Sandstone layer. The Clifty Sandstone is a tough, white layer of sandstone that can be seen halfway up the stairs to the bluffs behind Basin Park. It is exposed many other places around town as well, and occurs as a <1 meter to 2 meter thick layer with a rough, sandy texture and distinct appearance. In most places, it is about 2 feet thick. The Clifty Sandstone is made up almost entirely (99%+) of sand grain sized individual quartz crystals rounded by wear from waves near an ancient coastline. The layer is tough, forming the underpinnings of the flat shelf that most of Spring Street and several other neighborhoods rest upon.

In the lower valleys, Quartz also occurs as banded nodules and as occasional druzy surfaces within voids in the the rocks. Druzy refers to a paving of tiny, well formed crystals, just large enough to see with the unaided eye, that cover a rock surface, often quite attractively.

A pocket of druzy quartz in Eureka Springs, Arkansas.

Fossils of the Boone Formation

Crinoids

Crinoids are a group of ancient sea creatures related to starfish, sand dollars, and sea urchins. Numerous related species still live in the world’s oceans. They are echinoderms, or members of the phylum echinodermata. Crinoids are found in the fossil record from the Ordovician (nearly 490 million years ago) to the present, but the ones in and around Eureka Springs all date from the Mississippian period, about 323-359 million years ago. They were so numerous in the Mississippian seas in this region that one occasionally encounters rocks known as crinoidal coquina. Coquinas are fossil containing rocks composed of more fossil material than of surrounding sediments such as sand or mud. Crinoidal coquinas are sometimes made up almost entirely of crinoid fragments, suggesting these creatures were extraordinarily numerous, and that their fragmented bodies must have at times covered the floor of the inland sea that rested where Eureka Springs now sits.

The crinoid fossils around Eureka Springs are almost always, if not always, fragments. Small disks and columns represent stems, smaller disks made up the feathery tops of the organisms, and one occasionally finds pieces of the rounded bases of the organisms as well. They occur in the Boone Chert Formation and Saint Joe Formation. These rock layers are both from the Mississippian period, and make up the hill slopes from the bottom of the bluffs in and around town, visible above Basin Park, to the hilltops. To the north, in southern Missouri, the same rocks that we call the Boone Chert are named the Burlington Keokuk Formation. The same fossils can be found there. Starting several miles south of town, some crinoids can also be found in younger rocks from the Pennsylvanian period.

To learn more about crinoids, you can read an excellent Wikipedia article at https://en.wikipedia.org/wiki/Crinoid

Brachiopods

One of the more common fossil groups found around Eureka Springs are the spiriferida. They are an extinct group of brachiopods that existed from the Late Ordovician to the Middle Jurassic. The ones found in and around Eureka Springs, Arkansas, are from the earlier part of the Mississippian period, about 340 to 360 million years ago, and occur in the Boone Chert Formation and in the members of the St. Joe Formation, meaning from the bluffs to the hilltops..

Fossil brachiopod (a seashell) of the Order Spiriferida, found in crinoidal limestone of the Boone Chert Formation at Eureka Springs, Arkansas, dating from the Osagean Stage of the Mississippian period, or about 340 to 348 million years ago.

Fossil brachiopod of the Order Spiriferida in the crinoidal limestone of a member of the St. Joe Formation (the bluffs) at Eureka Springs, Arkansas.

Bryozoans

Bryozoans are as common among marine organisms today as they are in the fossil record. There are something like 4,000 known species today. Bryozoans are colony forming aquatic invertebrate organisms. Though their calcareous colonies sometimes contribute to reef formations, they differ from corals in that the individual organisms that make up the colonies have diverse and specialized roles. Some gather food, some eliminate waste, some handle reproduction. The details vary from species to species among this incredibly diverse group. The most common bryozoans in the fossil record in and around Eureka Springs are the fenestrate bryozoans, with a distinctive fossil that resembles screen wire mesh. Fenestrate bryozoan fossils are found in the Boone Chert Formation and the members of the St. Joe Formation, meaning in the higher elevation parts of town, from the bluffs to the hilltops.

Fossil fenestrate bryozoan found in silicified limestone of the Boone Chert Formation at Eureka Springs, Arkansas, that lived in a shallow inland sea during the Osagean Stage of the Mississippian period, or about 340 to 348 million years ago.