FORT BRAGG (NORTH CAROLINA) PROJECT PHOTOMICROGRAPHS

This is a petrographic study of Early to Late Woodland era quartz and rock fragment tempered pottery recovered from the Fort Bragg Military Reservation in the eastern Piedmont of North Carolina. This investigation is part of a larger study by Dr. Joe Herbert and others. Fifty (50) sherds were examined in order to see if there were any textural or mineralogical characteristics that would assist in the form and type separation of this pottery.

The point count categories used in this study were paste (considered to be the plastic component of the clay body; mainly clay minerals in origin with very fine grained aplastic grains such as quartz and feldspar, and now, after firing, amorphous glass), quartz (separated by grain size), feldspar (noted as feldspar unless optical characteristics, primarily twining style, allowed separation into either plagioclase feldspar or potassium feldspar), mica (muscovite or biotite), opaques, other (includes epidote/clinozoisite and pyroxene), rock fragments (pyroxene ± plagioclase diabase fragments, quartz polygranular rock fragments (sometimes with fluid inclusions and rutile needles), quartz + feldspar rock fragments (both plagioclase and k-spar as microcline) and sedimentary and metamorphic rock fragments of the eastern Piedmont (generally in small abundance), grog (refired pottery fragments with or without aplastic mineral grains) and two types of fragments that could be classified as ACF (argillaceous clots or fragments of air-dried clay; see Whitbread, 1986).

Observations were also made concerning characteristics resulting from the firing of the pottery as well as the changes observed either as a result of use or of burial. These sherds display oxidation features (commonly a red to red-orange colour) on one or both of the inner and outer sherd surfaces, extending into the sherd for several millimeters. The region between these oxidized zones (often called the core) for some of the sherd is reduced and is either black to smoky gray in colour. Observations and measurements of the size of the oxidized zones and the degree of oxidation to reduction were noted in this examination. Lastly, some of the sherds show secondary alteration (observed as a colour change) in fractures and along broken edges. This mineralization may have resulted due to burial and interaction with ground water or as a result of usage.

All of the photomicrographs are taken using a BH-2 Olympus transmitted (and reflected) polarized light microscope. The thin-sections used are standard (30 micron thick) and epoxy-impregnated. Field of view, magnification, and type of observation (i.e., plane polarized light - PPL - vs. cross polarized light - XPL) will be indicated.

Schematic example of fiber orientation in the Fort Bragg sherds. Size of sherd displayed is 25 mm (long) by 5 mm (wide).

PHOTOMICROGRAPHS

JMH-006 (2.5X, plane polarized light; field of view is 5 mm lengthwise)

One group of these samples is dominated by igneous pyroxene + plagioclase rock fragments that are probably derived from the Jurassic age diabase dikes that cut through the eastern and central Piedmont of North Carolina. In the Fort Bragg area exposures of these can be found north and west of the military reservation and where stream downcutting has exposed these thin and narrow dikes. For comparison with these sherds, Jurassic diabase dike samples were acquired from Albemarle where there is good outcrop exposure. The rock fragments found in this group are identical with those from Albemarle.

In thin section these rock fragments range from medium to very coarse in size, allowing for them to be observed at the macroscale. These sherds have about ~ 30 modal % (or more) of these rock fragments. The other aplastic components in this sherd are pyroxene and plagioclase mineral grains whose size range and appearance suggests that they are derived (probably by sedimentary reworking and weathering) from the same source. Rock fragment in this sherd is ~ 3 mm x 3 mm. Green grains are the pyroxene, the intersecting lathe are plagioclase feldspar with Carlsbad twinning. The identity of the pyroxene is probably a clinopyroxene (augite) and the plagioclase is Ca-rich (probably laboradorite).

Note the dark colour of this sherd. It is possible that this is a reduced firing response, however because the majority of the aplastic material is Ca (and Mg-Fe rich), it is possible that this is what happens with an oxidizing firing of this type of material.

JMH-006 (2.5X, cross polarized light; same as previous image)

Under cross-polarized light (XPL) the clinopyroxene exhibits high second order inference colours and the plagioclase lathes show the characteristic twinning. The radiating habit of these intergrowths is well exhibited here. Note that some bright (orange-red and green) pyroxene mineral grains are seen below this rock fragment.

JMH-006 (2.5X, plane polarized light; field of view 5 mm)

Another example of the pyroxene + plagioclase rock fragments of this sherd.

JMH-006 (2.5X, cross polarized light; same as previous image)

Under cross-polarized light (XPL) note that the pyroxene has a range of interference colours.

JMH-006 (10X, plane polarized light; field of view 1.5 mm lengthwise)

In the lower left is a rock fragment of pyroxene + plagioclase. In the upper right is a pyroxene grain showing its characteristic high relief and fractured appearance (at higher magnification can be seen cleavage).

JMH-006 (10X, cross polarized light; same as previous image)

Under cross-polarized light (XPL) the identity of these fragments is distinct. Note also that above the pyroxene + plagioclase grain is a polygranular quartz rock fragment. Some monocrystalline quartz is found at the very fine to fine grain size and a few quartz rock fragments are found. Note the plagioclase (polysynthetically twinned) mineral grain to the right of the polygranular quartz rock fragment.

JMH-018 (2.5X, plane polarized light; field of view 5 mm)

Another group in these samples is dominated by quartz ± feldspar (often microcline, but some have both plagioclase and microcline) and quartz polygranular rock fragments. This group of sherds has a range of mica content, some with mica (generally muscovite, but some biotite) up to ~ 10 modal % and others with a trace to none visible. This sherd shows the blocky to subangular shape of the feldspar and the subrounded to subangular shapes of the quartz mineral and rock fragments. A small amount of mica lathes can be seen to the left of the image. Colour of these sherds is generally red-brown (brick) and sometimes a thin oxidation zone at the sherd edge is observed.

JMH-018 (2.5X, cross-polarized light; same as previous image)

Under cross-polarized light (XPL) the tartan plaid twinning of microcline (k-spar) is evident (top of sherd to the left of center). However, some of the feldspar is untwinned (see blocky grain to the right of center near top of sherd). In this case the feldspar is identified, but not separated into either plagioclase or k-spar (if this becomes important, staining techniques will have to be used).Most of the angular (to subangular) grains are monocrystalline quartz, but there are a few polygranular grains (to the left of center need the mica lathe).

JMH-018 (10X, plane polarized light; field of view 1.5 mm)

A subset of this group has a potential indicator mineral. Four (4) grains of tourmaline (glassy green mineral in center) as part of a quartz rock fragment were found in this sherd (and a couple of others). Tourmaline is often associated with late stage granitic (felsic) magmatism and pegmatites. It is a resistant mineral and often is found in heavy mineral suites in stream sediment samples.

JMH-018 (2.5X, plane polarized light; field of view 5 mm)

JMH-018 is also interesting as it appears to have an opaque black glaze on one surface of the sherd (exterior?). It appears to have been applied to the vessel before firing as some particles have been caught up in the opaque material (see grain surrounded by opaque material at top of sherd). Ann Cordell (2000), in the study of 20 preliminary sherds from the Ft. Bragg area, has a sherd (#21; site 31GT392) that has a comparable aplastic component distribution to this group. In addition, sherd #21 from that study also has remnants on one surface (exterior) of a black opaque glaze such as observed in JMH-018.

JMH-018 (10X, plane polarized light; field of view 1.5 mm)

Here is a close up of a portion of the (potential) opaque black glaze.

JMH-018 (10X, plane polarized light; field of view 1.5 mm)

Grog is found as a small component of this group of sherds. Grog, in the definition I have used for this investigation, is fragments of pottery that has been incorporated (either by accident or intent) into the vessel and refired. Grog, as a term, can be a slippery (and confusing) component. I have attempted to use a few simple aspects to constrain this usage. Grog, in this study, is applied to dark coloured (red-black, dark blood red, black-red) fragments with a range of shapes from subrounded to subangular. They appear to be refired and often have a separation void surrounding them (although it may not completely encircle them). They have a range of aplastic components (generally quartz ± feldspar ± mica ± mafic minerals or opaques) that are subangular to subrounded. Lastly, the arrangement of the grog fragment and the surrounding matrix is examined to determine if the orientation is parallel or non-parallel (especially with respect to the inclusions).

This example is subangular with subrounded to subangular quartz mineral and quartz polygranular rock fragment inclusions, one which has a fluid inclusion and rutile needles (to the right of center). The separation void is nearly complete. The orientation of the grog fragment to the surrounding matrix is non-parallel.

CLAY TEST TILE PHOTOMICROGRAPHS

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Last revision 22 July 2013