Geology of the Blue Heron Nature Preserve, Atlanta, Georgia

By naturalist L. Scott Ranger

There are two stories to tell about the geology of the Blue Heron Nature Preserve: the very small scale of rocks on the ground at the preserve; and, the very large scale of both time and space of the preserve and its place on the surface of planet earth.

On the ground at the preserve… The entire property—and much of this part of North Atlanta—is located on a geologic formation mapped as the button schist (POb). Unlike most geologic formation names, this really isn’t a name, but simply a description of the kind of rock that is found here. It has been called this since the early twentieth century but became more formalized in 1966 with the publishing of Michael W. Higgin’s The Geology of the Brevard Lineament Near Atlanta, Georgia as Bulletin 77 of the now defunct Georgia Geological Survey.

This map locates the Blue Heron Nature Preserve in its North Atlanta setting. National Park Service (NPS) Geologic Resources Inventory (GRI) program. 2012. Unpublished Digital Geologic Map of the Northern portion of Chattahoochee River National Recreation Area, Georgia (NPS, GRD, GRI, CHAT, CHTN digital map) adapted from U.S. Geological Survey Open-File Report Series digital map by Dicken et. al. (2005). National Park Service (NPS) Geologic Resources Inventory (GRI) program. Geospatial Dataset-2188723. https://irma.nps.gov/App/Reference/Profile/2188723

National Park Service (NPS) Geologic Resources Inventory (GRI) program. 2012. Unpublished Digital Geologic Map of the Southern portion of Chattahoochee River National Recreation Area, Georgia (NPS, GRD, GRI, CHAT, CHTS digital map) adapted from a U.S. Geological Survey Miscellaneous Investigations Series map by Higgins et. al. (2003). National Park Service (NPS) Geologic Resources Inventory (GRI) program. Geospatial Dataset-2188724. https://irma.nps.gov/App/Reference/Profile/2188724

Higgins, M.W., T.J. Crawford, R.L. Atkins & R.F. Crawford. 2003. Geologic map of the Atlanta 30’x60’ quadrangle, Georgia. Scientific Investigations Map 2602, U.S. Geologic Survey.

Note the general trend of the different colors, each representing a unique rock formation, as they all line up northeast-southwest. More on that later.

Here is the technical description of this geologic unit from Higgins et al:

Button schist (Permian? to Upper Ordovician?)—Gray to silvery, tan-weathering (±chlorite)-plagioclase- quartz-sericite button schist (Higgins, 1971) with C-S texture (Berthé and others, 1979); S-C mylonite of Lister and Snoke (1984) with fish-scale texture and locally displaying fish-flash (Simpson, 1986, p. 252); locally manganiferous; in many places including (±chlorite)-sericite-quartz-plagioclase phyllonite with mica-fish and fish-flash, and, locally, lenses and slivers of sheared chlorite-actinolite (±hornblende)- plagioclase and chlorite-actinolite-plagioclase hornblende amphibolites. Weathers to a red soil with buttons (mica porphyroclasts, mica-fish) scattered on the ground surface. Probably derived by shearing mostly of mixed unit (OZm).

So what does this mean to the casual observer, walking along the paths of the preserve? Button schist gets its name as these words graphically describe a prominent feature of the rock, spindle-shaped “buttons” of somewhat resistant mica crystals that sometimes protrude from the rock, but more often are seen as “lenses” in the texture of the rock itself. Some describe it as having a “fish-scale texture”. If you look carefully on the ground on a sunny day for the bright, shiny flat mica crystals either in the soil or in the rocks, you are seeing the “buttons” or “fish-flash” as the mica crystals reflect the rays of the sun to your eyes as a bright glint of light.

Schist is a low-grade metamorphic rock which means it has been changed by relatively low heat and low pressure into what we see today from something else. It has changed so much that determining what it originally was can be very difficult. Schist is characterized by having a wavy series of parallel lines clearly visible in at least some orientations of the rock. These lines are called foliation and have nothing to do with sedimentary layers and everything to do with metamorphic pressure. Within these lines are usually large amounts of mica.

Mica is a mineral with large amounts of silica that forms sheets and can often be easily broken or split along these foliations with a fingernail. This is called cleavage and is characteristic of micas. The two most common micas are the light-colored muscovite (common mica, isinglass, or potash mica) and darker biotite. Both minerals are found in the rocks here with the larger flashes coming from the muscovite and the tiny dark shiny spots in the rock being biotite. This rock underwent at least two episodes of metamorphism, and perhaps as many as seven! These have virtually obscured what the original rock was and Higgins et al simply guess that they are “Probably derived by shearing mostly of mixed unit (OZm)”. Here we need to learn about shearing and some other names that have been given to this rock formation. Bedell notes that “rocks within the Brevard Fault Zone (BFZ) have been described as button schists, phyllonites, phyllites, and mylonites. Names have been applied depending on localities studied and personal interpretations.”

Bedell, A.L. 2003. Polymetamorphism and deformation within the Brevard Fault Zone outside of Atlanta, Georgia. Master of Science Thesis, the University of Georgia.

Mylonite is a term applied to schistose rocks nearly always found in fault zones. Here the metamorphism not only includes heat and pressure but motion. As the protolith (original rock) undergoes heat, pressure and motion, the new crystals that form are usually far smaller than the original and produce a rock with a very fine grain. In the fault zone, the protolith is sheared, that is, stretched in opposite directions. In these diagrams, the shear directions are indicated by the half arrows. “S” (schistosité, schistose) planes are formed with the shearing and form an “S” shape and are recrystallized into micas or other platy minerals. “C” (cisaillement, elastic deformation) planes form perpendicular to the “S” plane in with less stress or shearing on the minerals.

Egger, A. undated. Geology 360 - Kinematic Indicators in Shear Zones. PowerPoint lecture, Central Washington University.

Within each of these shear planes a secondary shearing develops at an acute angle to the main shear that is very conducive to the development of flat mica crystals. If the shearing continues, some of the areas of “S” planes get ductiley stretched and thinned on the ends in the direction of the main shear. As the rock containing these metamorphic crystals weathers, the more resistant mica crystals rise to the top of the weathering surface and flake off into the developing soil as “mica fish”. Phyllonite is a mica- rich mylonite. Phyllite is a highly foliated mica-rich rock derived from slate.

The soil that develops from this mica schist is mapped as part of the “Urban land-Grover-Mountain Park complex” or UgE on the Fulton County Soil Survey with this typical profile: Surface layer: 0 to 4 inches—dark yellowish brown gravelly sandy loam Subsurface layer: 4 to 11 inches—yellowish brown gravelly sandy loam Subsoil: 11 to 14 inches—yellowish red and strong brown sandy loam 14 to 25 inches—red and strong brown sandy clay loam 25 to 31 inches—red sandy loam Substratum: 31 to 80 inches—yellowish red, red, strong brown, and dark grayish brown loamy sand Marshall, C.G. 2008. Soil Survey of Fulton County, Georgia. Natural Resources Conservation Service. At the large scale… To fully understand what we see on the ground at the preserve, we need to take a look at how this little spot of planet earth relates to the rest of the planet, or at least the southeastern United States.

Here, we’ve moved up higher above the preserve and see how the local geology fits into a more regional Atlanta area picture. Note the general trend of the lines and colors go northeast-southwest except for the northwestern corner of this map. With this larger scale, Lake Lanier and Allatoona Lake are obvious. For our purpose, Lake Lanier is the most important as it is formed by the Chattahoochee River. The narrow bands of color and lines roughly correspond to the channel of the Chattahoochee River and they define the Brevard Fault Zone (BFZ).

The area in shades of purple and pink on this map is the Piedmont Province characterized by thick layers of hard crystalline rock. The purples are metamorphic rock, mostly gneiss. The pinks are granite. On the bottom left is the Ben Hill Granite that shows an area of dramatic shearing where it meets the BFZ. Just to the right of center bottom are the Stone Mountain and Panola granites.

Gneiss is a high-grade metamorphic rock where the protolith was subjected to relatively high heat (>600 °C) and high pressure (<0.6 GPa) from a regional (large area) metamorphic event. It is a foliated rock, usually with alternating bands of dark and light crystals that form perpendicular to the direction of pressure. The parent material can be just about any kind of rock, but many form from other hard crystalline rock or granite making its chemistry complex. While gneiss contains some small amounts of mica, the crystals are usually small and most commonly of the dark biotite form which help differentiate them from schist.

Granite is an igneous (fire-formed) rock that originated deep enough in the earth to melt all the crystals and form a very hot (<1260c) liquid called magma. Because of its intense heat, it rises through the crust of the earth, melting its way with the roof material above it being incorporated into the mix. If it reaches the surface, the molten material erupts on the surface as is called lava. Granite forms when the rising plume of liquid rock stalls, cools, and crystallizes out as a granular rock. The faster it cools, the smaller the crystals, the slower it cools, the larger the crystals. Not being subjected to regional pressure, the crystals from in random patterns. So granite never has lines, swirls or any pattern at all. If you see a pattern in a hard crystalline rock, it is not granite but gneiss. Any surface exposure of granite is an indication that all the roof material has been eroded away. Stone Mountain and Panola Mountain are monadnocks, an isolated hill or mountain that rises above a relatively flat plain. The Ben Hill Granite forms a relatively flat surface with occasional large boulders such as at Boat Rock in west Atlanta.

This Google Earth image with the Geology of Georgia overlain gives us a more regional view of the geology and topography. The purples are the Piedmont. The Blues of northwest Georgia are the Valley and Ridge and Cumberland Plateau. The yellows and blue of south Georgia is the Coastal Plain.

Across the Piedmont is a strong linearity that appears just northeast of Montgomery, Alabama and continues to the border of North Carolina and Virginia. This is the Brevard Fault Zone (BFZ). Exactly what this structure is remains something of an enigma. Bedell (2003) includes a list of 42 different interpretations of the fault from 1905 with Keith calling it a syncline (a “U” shaped fold of strata). When Jonas called it a thrust fault (a low angle fault where one surface is thrust over another) in 1932 he was the first to recognize more of its true nature. Reed and Bryant in 1964 interpreted it as dextral strike slip fault (where the blocks separated by the fault move horizontally to their right). The far northeast corner of the Ben Hill Granite shows a clear “smearing” of this limb of the granite in a right-handed direction that gives evidence for this motion. Reed and others in 1970 interpreted it as a sinistral strike slip (where blocks separated by the fault move horizontally to their left) and root zone.

Even dating the activity on the fault is enigmatic as it has been considered to be a product of the Taconic (ended about 440 million years ago mya), Acadian (ended about 325 mya) and Allegehenian (ended about 260 mya) orogenies! All three are sometimes been grouped together as the Appalachain Orogeny. An orogeny is a mountain-building event caused by the collision of continents. The BFZ has often been considered to be the suture where the Alleghenian Orogeny thrust what is now Africa upon what is now North America. On-the-ground geology indicates this is not so as there are very similar rock units on both sides of the fault. The geology is complex enough that there may be truth in all of these interpretations as there is evidence of metamorphism in the zone from all these orogenies!

Regardless of this complexity, the rock of the Blue Heron Nature Preserve was created by movement and the stresses that resulted from this fault. It begs the question as to what it originally was. Higgins et al speculate it is part of the “informal mixed unit” OZm that does not have a formal name! It is called a “mixed unit” as it has not been mapped or correlated at small scale on the ground and contains rock of several related, but distinct forms. Here is its formal description for Higgins et al:

Informal mixed unit (Middle Ordovician? to Late Proterozoic?)—Lustrous, medium- to coarse-grained (garnet)-sillimanite-biotite-muscovite schist that is locally slightly graphitic and is commonly slightly manganiferous; medium- to coarse-grained, locally porphyroblastic biotite-quartz-plagioclase and biotitequartz- potassium feldspar-plagioclase gneisses; light-gray, medium-grained granite gneisses; and fine- to medium-grained, dark-green, ocher-weathering hornblende plagioclase and plagioclase- hornblende amphibolites. Locally contains garnet (chlorite)-biotite-muscovite-quartz-feldspar gneisses. Generally contains pods and lenses of chlorite, hornblende, and actinolite schists. Characteristic of the mixed unit is the presence of scattered 0.3- to-1-m-thick beds of fine-grained, blocky and sooty weathering, (magnetite)-spessartine quartzite (gondite, coticule rock) (OZmm), commonly interbedded with medium-grained, pink- to purple-weathering garnet-sillimanite-biotite-quartz-muscovite schist and fine- to medium-grained, dark-green to blackish-green, ocher-weathering hornblende-plagioclase amphibolite. Brown- to blackish-weathering manganiferous schists commonly are interbedded with the manganiferous quartzite, and manganiferous schists also occur without the quartzite.

What this material represents is the complex of “stuff” that was between what is now North America, a vast ocean that closed with the Alleghenian Orogeny what Higgins et al consider the “allocthonous oceanic assemblage”. An allonchton is a body of rock that formed somewhere apart from its present location. That this is oceanic is sure as the assemblage is full of rock that surely was originally erupted as basaltic lava in an ocean setting (for example, the Ropes Creek metabasalt).

Late Permian (260 mya) palinspastic reconstruction of Pangaea.

The word palinspastic is fun and comes from the ancient Greek πάλιν (pálin, "again") + σπαστικός (spastikós, "drawing") which means “drawing again” or an interpretation of what the world looked like long ago. Dr. Blakey has included ghost outlines of current boundaries to help interpret where this world of 260 mya was.

Blakey, R. Global Paleogeography. Northern Arizona University. http://jan.ucc.nau.edu/rcb7/globaltext2.html

With three continental collisions in three orogenies, some 260 million years of erosion with the development of the modern Atlantic Ocean, there has been plenty of time to shear, press, heat and whatever else to create the button schist of the Blue Heron Nature Preserve.