27F: Trace Fossils
on the Coastal Plain
By Thomas Thurman
Began 15/Oct/ 2021
Posted 03/Jan/2026
After a talk in Perry, Georgia, a 9 or 10 year old girl & her father brought the fossils they’d found. The girl handed me a partial ladyfinger 4 or 5 inches long. This was in September 2021, I’d spoken about Houston County’s Natural History at the Houston County Library in Perry, Georgia.
Ladyfinger is a colloquial, amateur, term for a stone that is finger shaped and sized, usualy the brick-red color of oxidized iron. They occur commonly in the sediments of some Coastal Plain areas. I explained to the girl that this was a trace fossil, a fossilized burrow where some small animal, probably a shrimp, had burrowed through the seabed feeding as it went.
“The burrow is backfilled with iron-rich sediments which oxidize, or rust, giving them their rust color, and the sediments slowly harden into stone.” I said.
Ladyfingers aren’t common in Houston County, so I wasn’t expecting to see one. So, this page is dedicated to that little girl, whose name I failed to get, and the answer I fumbled through. Trace fossils are common in many places of along the Coastal Plain.
Ladyfingers aren’t common in Houston County, so I wasn’t expecting to see one. So, this page is dedicated to that little girl, whose name I failed to get, and the answer I fumbled through. Trace fossils are common in many places of along the Coastal Plain.
Ichnology
In 1982 Charles Schroder published a trace fossil guide for Georgia’s Wilkinson and Washington Counties. His Bulletin 88, Trace Fossils of the Oconee Group and Basal Barnwell Group of East Central Georgia ,will form the foundation of this page but other works will be involved as well.
To begin with let’s do just a few definitions; Schroder was studying ichnology. For our purposes the Dictionary.com definition of Ichnology applies.
“The branch of paleontology concerned with the study of fossilized tracks, trails, burrows, borings, or other trace fossils as evidence of the occurrence or behavior of the organisms that produced them.” Wikipedia also ranks coprolites (fossilized feces) as trace fossils since they give evidence of an animal’s behavior. But this page will primarily look at burrows and borings.
In 1982 Charles Schroder published a trace fossil guide for Georgia’s Wilkinson and Washington Counties. His Bulletin 88, Trace Fossils of the Oconee Group and Basal Barnwell Group of East Central Georgia ,will form the foundation of this page but other works will be involved as well.
To begin with let’s do just a few definitions; Schroder was studying ichnology. For our purposes the Dictionary.com definition of Ichnology applies.
“The branch of paleontology concerned with the study of fossilized tracks, trails, burrows, borings, or other trace fossils as evidence of the occurrence or behavior of the organisms that produced them.” Wikipedia also ranks coprolites (fossilized feces) as trace fossils since they give evidence of an animal’s behavior. But this page will primarily look at burrows and borings.
In ichnology species are regularly named for the traces they leave behind, instead of the animal which created them. The animal is frequently unknown or uncertain. The genus Thalassinoides, for example, is actually a burrow. Wikipedia says a T-shaped burrow. Schroder, in 1982, described them as Y-shaped burrows. “Thalassinoides composed of regularly or irregularly Y-branched, sinuous to slightly sinuous, inclined burrows 0.7cm to 10cm in diameter.” Intersections are inflated. Walls are slightly irregular and unlined or thinly lined with rather smooth exteriors.
Such burrows could have been made by worms or any number of crustaceans; but they are generally attributed to ghost shrimp living in muddy conditions. Since the animal is uncertain, the burrow itself is defined and named so that it can be compared to others, wherever they may be found.
Such burrows could have been made by worms or any number of crustaceans; but they are generally attributed to ghost shrimp living in muddy conditions. Since the animal is uncertain, the burrow itself is defined and named so that it can be compared to others, wherever they may be found.
Schroder was specifically looking at Oconee Group (Pre-Upper Eocene) and Barnwell Group (Primarily sand deposits of Upper Eocene Age). I understand that this isn’t very clear to many who are interested in the subject, but most of Georgia’s Coastal Plain trace fossils occur in the Oconee Group or Barnwell Group. So it’ll be useful to begin with brief descriptions of these two basic groupings of deposits or formations.
Terrestrially Sourced Sediments
Oconee & Barnwell Group deposits overlap from 34 to 36.5 million years ago. Both are generally terrestrially sourced sediments, clay & sand, carried by our ancient rivers and deposited into a sea whose coastline rested on or near the Fall Line.
The Groups are described towards thew base of this page.
Oconee & Barnwell Group deposits overlap from 34 to 36.5 million years ago. Both are generally terrestrially sourced sediments, clay & sand, carried by our ancient rivers and deposited into a sea whose coastline rested on or near the Fall Line.
The Groups are described towards thew base of this page.
Definitions for the Below
- Amastomose; a connection or opening between two thinks like leaf veins.
- Appressed; close to something else.
- Spreiten; a banded pattern of uncertain origin found in the infill of the burrows of certain fossil invertebrates.
- Boudinage; is a geological term for structures formed by extension, where a rigid tabular body is stretched and deformed amidst less resistant surroundings.
- Podiform; Pod-shaped.
- Ferrugninous; related to or containing iron.
- Vermiform; Resembling or having the form of a word.
- Supratidal Zone; just above the normal high tide line but frequently splashed by waves, not submerged except during the highest tide levels of the year.
Ichnogenus Trace Fossils (As described by Schroder)
Amphorichnus (Probable anemone trace)
- Description: short, vertical, conical to subconical structures 1.5 (.59”) to 4.0 cm (1.57”) in diameter having, blunt, convex downward apices (plural of apex) and smooth, unlined walls; enlarged crowns. Maximum length observed; 10 cm (3.93”).
- Occurrence and Distribution; Occurs in parallel-bedded, poorly sorted sands and sandy muds, within a few meters of the upper surface of the Oconee Group.
- Interpretation: Dwelling or resting structure for anemone or anemone-like organism.
Fraena (unknown grazer)
- Description; Unbranched, simple sinuous trails, consisting of a central furrow, flanked by a pair of low ridges. Total width, 3mm, maximum length observed about 5cm.
- Occurrence and Distribution; Occurs in thinly parallel bedded fine undifferential Branwell Group sands rich in heavy minerals, 3 meters above the upper surface of the Oconee.
- Interpretation; Crawling or grazing trace of an animal moving upon the substrate surface.
Ophiomorpha (Probable ghost shrimp)
- Description; Complex, ramified, dominantly horizontal burrow systems, thinly lined, exhibiting vertically stacked retrusive spreiten structures and sparsely nodose exteriors; irregularly ”Y” or less common “T” branched. Individual burrow segments cylindrical, smoothly curved or straight, and 1.50cm (.59”) to 2.0cm (.78”) in diameter; spreiten up to 30cm (11.8”) in height; maximum length of burrow segment observed at about 30cm (11.8”).
- Occurrence & Distribution; Occurs in coarse sand fill of shallow channels cut into upper surface of Oconee Group.
- Interpretation; dwelling structures for a species of thalassinidean shrimp, possibly a callianassid.
Phycodes (Worm-like creature)
- Description
- Straight to smoothly curved, somewhat flattened, tubular structures, which may anastomose into closely appressed bundles suggestive of spreiten structures;
- segments commonly grade into boudinage-like segments or elongate, discontinuous, podiform structures; cemented to, or otherwise associated with parallel ferrugninous tunnels;
- typically originating on ventral side of parallel ferrugninous tunnels; with closely appressed individual tubed curving around and obliquely upward, toward dorsal side;
- true branching not observed; rare, short, tongue-like protrusions may extend obliquely outward from walls of structure; terminations rounded; distinctly lined by 1mm thick sheath, apertures or interconnections with parallel ferrugninous tunnels not observed;
- diameters range from 0.5 to 1.5 cm and are rarely constant, eve within individual tubes; maximum length observed, about 15 cm.
- Occurrence and Distribution; Occurs in close association with parallel ferruginous tunnels in weathered crossbedded, poorly sorted, muddy, medium sand of channel fillings.
- Interpretation; these are feeding structures produced by vermiform (worm), sediment ingesting animal, close association with parallel ferrugninous tunnels suggestive of commensalism; discreet, thin, hydrous iron oxide lining suggestive of original organic lining; structures formed later than parallel ferrugninous tunnels with which they are associated.
Planolites A
- Description; Closely spaced, unbranched (?), sinuouos, randomly oriented burrows 1 to 4 mm in diameter, frequently truncation or interpenetrating one another; unlined walls leathery in appearance due to poorly defined, discontinuous, circumferential ridges; maximum length observed about 5cm (1.96”).
- Occurrence and Distribution; Occurs in massive muds and slightly sandy muds, widespread but confined stratigraphically to upper few meters of Oconee Group.
Planolites B
- Description; Sparse, unbranched, smoothly curved, cylindrical or subcylindrical slightly sinuous shafts, tunnels, and inclined burrows 1mm to 3mm in diameter. Diameters within individual burrows constant, walls smooth, thinly lined. Burrows commonly exhibit preferred orientation with preference vertical, horizontal, or inclined in order of decreasing frequency. Maximum length observed about 1 meter.
- Occurrence and Distribution; Occurs in some massive muds of the Oconee Group.
- Interpretation; Dwelling structures or feeding-dwelling structures of infaunal vermiform (worm-like) animal, possibly a species of polychaete, probably originated as open, lined burrows that were later passively filled.
Skolithos
- Description; Unbranched, discontinuous, straight to slightly sinuous shafts and deeply inclined burrows, roughly circular in transverse section. 1cm to 2cm in diameter, maximum observed length appo. 1 meter.
- Occurrence and Distribution; Occurs in massive to vaguely horizontal laminated muds.
- Interpretation: Feeding structures or feeding-dwelling structures of a vermiform, deposit feeding animal. Burrows were probably constructed in soft, unconsolidated mud substrates greater whiteness of burrow fill relative to adjacent sediments is apparently due to relative absence of iron compounds within burrow fills, which is probably, in turn, related to passage of burrow fills through the gut of the animal. (It consumed the iron?)
Thalassinoides A
- Description; Composed of systems of irregularly “Y” or “T” branched, slightly sinuous tunnels 1cm to 5cm in diameter, typical orientations of systems, dominantly vertico-horizontal, with some systems becoming more random in lower portions. Intersections dichotomous and inflated, or multiple and chambered with 5 or 6 intersections per chamber. Chambers may exceed 10cmin diameter, walls are unlined, moderately to slightly irregular, commonly exhibiting small, bluntly conical pits, maximum depth of penetration observed was 5 meters.
- Occurrence & Distribution; Occurs in massive muds and slightly sandy muds.
- Interpretation: Probably excavated by thalassinidean shrimp, possible a species of Callianassa in an intertidal, or shallow subtidal marine or estuarine environment of high current energy.
Thalassinoides B
- Description; Composed of regularly or irregularly Y-shaped, sinuous to slightly sinuous, inclined burrows .7 to 10 cm in diameter, typically anastomosing downward. (An anastomosis is a connection or opening between two things, typically cavities or passages) Intersections are inflated. Branching is along an axis. Walls slightly irregular & unlined or thinly lined with non-mamillated exteriors. Overall configuration is two or three dimensional with a maximum depth of penetration of 2 meters.
- Occurrence and Distribution; Occurs in massive muds, and rarely in bioturbated sand.
- Interpretation; Probably excavated by burrowing shrimp, possibly species of Upogebia and/or Alpheus in an intertidal or shallow subtidal marine or estuarine environment of moderate energy.
Trypanites
- Description: Short, cylindrical straight to slightly curved, steeply inclined borings, having constricted upper necks and enlarged. Bulbous lower terminations. (Shaped like a light bulb.) Diameters range from 3mm to 4mm across upper opening and 10mm to 12mm across widest lower bulbous point. Maximum depth (length) of about 40mm. Walls are unlined and commonly ornamented with circumferential striations.
- Occurrence and Distribution; Occurs in massive, slightly sandy muds at the upper surface of the Oconee Group. Distribution is patchy.
- Interpretation; Dwelling structure of a marine borer, probably a species of pholadid bivalve (piddock or anglewing). Boring was probably excavated into a firm mud substrate under intertidal to shallow subtidal, intermediate to high current energy conditions.
Unnamed Trace Fossils
Ferruginous Cones
Ferruginous Cones
- Description; Short, sub-cylindrical, straight, vertical to steeply inclined, elongate, pouch-like structures. They have pointed upper terminations and blunt lower terminations (shaped like an elongated teardrop).
- Occurrence and Distribution; Occurs in large-scale, crossbedded, slightly muddy medium undifferentiated Barnwell Group sands suprajacent to the upper surface of the Oconee Group.
- Interpretation; Dwelling structures, possibly of polychaetes (bristle worm) similar to Pectinaria. Longitudinal ridges may represent differential collapse of wall structure. Thin, distinct hydrous iron oxide lining strongly suggestive of original organic lining, hydrious iron oxide cement in structure fill suggestive of original presence of organic material.
Large Sinuous Shafts
- Description; Unbranched, slightly to markedly sinuous, cylindrical to sud-cylindrical burrows, 3cm to 5cm in diameter, consisting of straight to slightly sinuous shafts and steeply inclined burrow segments 30cm to 60cm in length. Uppermost segments steeply inclined, commonly exhibiting preferred orientation, lower terminations bluntly conical; unlined, slightly irregular walls show numerous claw sculptings and occasional small pits; maximum length observed about 2 meters.
- Occurrence and Distribution; Occurs in massive to vaguely laminated muds and crossbedded muddy sands at the upper surface of the Oconee Group.
- Interpretation; Interpreted as crab dwelling structures constructed from coherent mud and muddy sand. These are very similar to some modern species of supratidal crabs, crab which live just above the high tide line where their burrows are frequently splashed by waves but very rarely submerged. Orientation for top of burrow may show preference for prevailing winds.
Parallel Ferruginous Tunnels (Colloquially; Ladyfingers)
- Description; Infrequently “Y” branched, straight to sinuous, discontinuous, cylindrical to sub-cylindrical tunnels, commonly close-spaced vertical or lateral series. Rarely connected by short vertical shafts; lateral terminations blunt to pointedly conical, surface apertures not observed, diameters range from 0.5cm to 10cm but are typically 3cm to 4cm. Maximum observed length 1.54 meters.
- Occurrence and Distribution; Occurs in large-scale, tangentially crossbedded, gravelly, muddy, sands and sands within the Oconee Group. In horizontally bedded muddy undifferentiated Barnwell Group sands overlying Oconee material.
- Interpretation; Dwelling or feeding/dwelling structures possibly of a decapod crustacean (shrimp-like crusteacean). Vertical stacking may represent a response to slowly aggrading or degrading substrate surfaces. Parallel alignment suggests response to current direction. Hydrous iron oxide linings suggest diagenetic replacement of original organic linings.
The Oconee Group
41 to 90 million years old, spanning multiple periods.
(As defined by Huddlestun & Hetrick in 1991)
The Oconee Group is defined as pre-Upper Eocene, river deposited kaolin and kaolin-rich sands in the Fort Valley Plateau and Fall Lines Hills district of Georgia’s Coastal Plain. The age of the Oconee Group ranges from Late Cretaceous (66 to 90 million years ago) to the Middle Eocene (41 to 50 million years ago). While these deposits were being laid down the line marking the high-tide fluctuated along the Piedmont Province.
Yes, at this point the coastline rested on the southern Piedmont Province, north of the Fall Line.
The sea level changes were driven by the advance and retreat of glaciers. As glaciers retreated, heavy, monsoon rains flooded rivers and increased erosion into the rivers created deposits where the rivers drained into the sea on or near the Fall Line coastline.
This is how the Fort Valley Plateau and Fall Line hills were carved and how their sand and clays were left behind. Hilltops in Crawford County often have golf-ball sized quartz pebbles that are fractured and rounded. Some of which are so weathered they can be crushed by hand pressure. Pebbles which originally formed as crystals deep inside lost Georgia mountains which have long weathered away. The quartz rock was exposed by weathering, then transported by water and rounded into pebbles by abrasion. they were then left behind on a floodplain that slowly dried up. Later, flowing waters carved the plain into rugged, incised terrain leaving rounded pebbles undisturbed on many hilltops while grinding others to quartz sand and carrying them downrange via creeks and rivers to the sea.
This occurred during the Late Cretaceous (66 to 90 million years ago) and the Early Tertiary (34 to 66 million years ago.)
Oconee Group Deposits
41 to 90 million years old, spanning multiple periods.
(As defined by Huddlestun & Hetrick in 1991)
The Oconee Group is defined as pre-Upper Eocene, river deposited kaolin and kaolin-rich sands in the Fort Valley Plateau and Fall Lines Hills district of Georgia’s Coastal Plain. The age of the Oconee Group ranges from Late Cretaceous (66 to 90 million years ago) to the Middle Eocene (41 to 50 million years ago). While these deposits were being laid down the line marking the high-tide fluctuated along the Piedmont Province.
Yes, at this point the coastline rested on the southern Piedmont Province, north of the Fall Line.
The sea level changes were driven by the advance and retreat of glaciers. As glaciers retreated, heavy, monsoon rains flooded rivers and increased erosion into the rivers created deposits where the rivers drained into the sea on or near the Fall Line coastline.
This is how the Fort Valley Plateau and Fall Line hills were carved and how their sand and clays were left behind. Hilltops in Crawford County often have golf-ball sized quartz pebbles that are fractured and rounded. Some of which are so weathered they can be crushed by hand pressure. Pebbles which originally formed as crystals deep inside lost Georgia mountains which have long weathered away. The quartz rock was exposed by weathering, then transported by water and rounded into pebbles by abrasion. they were then left behind on a floodplain that slowly dried up. Later, flowing waters carved the plain into rugged, incised terrain leaving rounded pebbles undisturbed on many hilltops while grinding others to quartz sand and carrying them downrange via creeks and rivers to the sea.
This occurred during the Late Cretaceous (66 to 90 million years ago) and the Early Tertiary (34 to 66 million years ago.)
Oconee Group Deposits
- Pio Nono Formation
- Gaillard Formation
- Buffalo Creek Kaolin Member
- Huber Formation
- Marion Member
- Jeffersonville Member
- Deposits associated with & related to the Oconee Group
- Butler-Allon channel deposits
- Upper Eocene channel deposits
The Barnwell Group
34 to 36.5 to million years old
(As defined by Huddlestun & Hetrick)
The Barnwell Group is Late Eocene deposits (34 to 36.5 to million years old) dominated by quartz sand, all other mineral components are subordinate. Kaolin is absent. Quartz sand is a terrestrially sourced material typically transported via our rivers to a marine environment. It can also mean a beach deposit worked by tides. That said, the Twiggs Clay is also part of the Barnwell Group and it is nearly devoid of quartz sand, though there are sporadic sand lenses in the Twiggs Clay. The Twiggs Clay is devoid of kaolin. Essentially it is a montmorillonite clay (fullers earth) but Limestone can also be present.
Barnwell Group Members
34 to 36.5 to million years old
(As defined by Huddlestun & Hetrick)
The Barnwell Group is Late Eocene deposits (34 to 36.5 to million years old) dominated by quartz sand, all other mineral components are subordinate. Kaolin is absent. Quartz sand is a terrestrially sourced material typically transported via our rivers to a marine environment. It can also mean a beach deposit worked by tides. That said, the Twiggs Clay is also part of the Barnwell Group and it is nearly devoid of quartz sand, though there are sporadic sand lenses in the Twiggs Clay. The Twiggs Clay is devoid of kaolin. Essentially it is a montmorillonite clay (fullers earth) but Limestone can also be present.
Barnwell Group Members
- Clinchfield Formation
- Riggins Mill Member
- Treadwell Member
- Albion Member
- Utley Limestone Member
- Dry Branch Formation
- Twiggs Clay
- Irwinton Sand
- Griffins Landing
- Tobacco Road Sand
- Sandersville Limestone Member
Kaolin Isn’t Chalk
Kaolin is often referred to as chalk. For the record, kaolin is not chalk but a clay mineral, a very fine-grained silicate mineral. Its traditional use is the clay for making porcelain. Kaolin certainly looks like chalk, but chalk is a form of limestone made up entirely of compressed marine plankton fossils. While they look similar, they are very different. Kaolin is a terrestrially (land) sourced mineral. Chalk is formed in a marine environment by microscopic plankton settling on the seabed. Under a microscope you can see kaolin’s crystals & chalk’s fossils.
Kaolin is often referred to as chalk. For the record, kaolin is not chalk but a clay mineral, a very fine-grained silicate mineral. Its traditional use is the clay for making porcelain. Kaolin certainly looks like chalk, but chalk is a form of limestone made up entirely of compressed marine plankton fossils. While they look similar, they are very different. Kaolin is a terrestrially (land) sourced mineral. Chalk is formed in a marine environment by microscopic plankton settling on the seabed. Under a microscope you can see kaolin’s crystals & chalk’s fossils.
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Schroder's Bulletin 88 is available to download at right
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References
- Schroder, Charles H.; Trace Fossils of the Oconnee Group and Basal Barnwell Group of East-Central Georgia; Bulletin 88, Georgia Geologic Survey, 1982
- Huddlestun, Paul F. & Hetrick, John H.; The Stratigraphic Framework of the Fort Valley Plateau and the Central Georgia Kaolin District, Georgia Geological Society 26th Annual Field Trip. Vol. 11, Number 1, October, 1991
- Huddlestun, Paul F., & Hetrick, John H.; Upper Eocene Stratigraphy of Central and Eastern Georgia, Georgia Department of Natural Resources, Environmental Protection Division, Bulletin 95. 1986.
- Go Fish Education Center; | Department Of Natural Resources Division