BH250-27
Title
Subject
Description
Minor Minerals: muscovite, sericite, zircons
Texture: non-foliated, sugary appearance, glassy luster, well rounded zirons along a line indicating paleo-layering, grain boundary migration, straight or loate grain boundaries, formation of subrains within larger parent grains.
Optics: large quartz crystals perfect for optic axis figures, off centered optic axis figure, flash figures and andulos extinction in quartz.
The Baraboo Quartzite is a metamorphosed quartz-rich sandstone prominently exposed in the Baraboo Ranges of southern Wisconsin. Stratigraphic and sedimentological analyses reveal abundant cross-bedding, ripple marks, and tidal bedding—especially in the lower portions—indicating deposition in a fluvial-to-shallow marine environment.
Petrology and Mineralogy:
Sample BH250-27 is representative of the Baraboo Quartzite, consisting of over 90 vol.% quartz. The quartz grains are well-rounded, typically ranging from 0.25 to 2.0 mm in diameter, and display sutured boundaries formed by metamorphic quartz overgrowths during solid-state recrystallization. Accessory minerals occur in trace to minor amounts and include white micas (muscovite–sericite), detrital zircon, and minor opaque oxides.
In several thin sections, alignment of rounded zircon grains is visible, likely marking a preserved sedimentary bedding plane from the original sandstone protolith now transformed into quartzite.
Cement and Color:
Grain surfaces are coated with thin films of iron oxide (hematite), which act as cement and impart the characteristic brick-red color of the rock. There is essentially no carbonate cement; any cementing material is limited to iron oxide and local silica overgrowths from metamorphism. The extreme purity of the quartz framework contributes to the rock’s high hardness and resistance to weathering.
Texture and Structure:
Primary sedimentary structures such as lamination and cross-bedding are preserved but overprinted by broad crenulations and weak slaty cleavage from low-grade metamorphism. Overall, the rock retains a dominantly sedimentary texture with only subtle metamorphic overprint.
Correlative Units:
Regionally correlative units of similar age and composition include the Sioux Quartzite of southwestern Minnesota and eastern South Dakota, and the Washington County Quartzite of Iowa.
One common and diagnostic feature of the Baraboo Quartzite in thin sections is the presence of sutured boundaries between quartz grains a key textural indicator of the rock’s metamorphic history and recrystallization processes.
Sutured (interlocking or serrated) grain boundaries are highly irregular, embayed contacts between adjacent quartz grains that resemble puzzle pieces. Unlike straight or smooth boundaries, these are characterized by lobes and embayments that penetrate neighboring grains, reflecting internal grain boundary migration.
These sutures form through solid-state recrystallization during low- to medium-grade regional metamorphism, involving several processes:
- Pressure Solution: Quartz dissolves at points of high stress along grain boundaries, especially where grains are in contact. The dissolved silica is transported by pore fluids and reprecipitated in areas of lower stress.
- Grain Boundary Migration: Quartz grains adjust their boundaries to reduce total interfacial energy, producing interlocking, sutured textures.
- Dynamic Recrystallization: Under conditions of differential stress (e.g., during folding), dislocation creep and recovery can drive additional grain boundary modification, enhancing suturing.
These sutured grain boundaries indicate moderate metamorphic conditions sufficient for quartz recrystallization but not intense enough to completely erase original textures or form equigranular, polygonal grains typical of higher-grade metamorphism.
Quartzite and Human Use Across Cultures and the Fossil Record:
Quartzite, a hard and durable metamorphic rock, has been widely used by human societies for grinding, milling, and tool-making due to its abrasiveness, wear resistance, and geological abundance.
Quartzite in Eritrea: Milling Grains:
In Eritrea and the Horn of Africa, quartzite was traditionally used to make grinding stones and mullers for processing staple grains such as sorghum, barley, millet, and teff. Grain was ground into flour for foods like injera using hand-held stones on flat quartzite slabs. Its hardness (~Mohs 7), rough texture, and local abundance in Proterozoic terranes made it ideal. Archaeological finds from sites like Adulis and Matara confirm quartzite’s long-standing role in food production.
Quartzite Use Among Native American Groups:
In the Baraboo region of Wisconsin, Native American groups extensively used Baraboo Quartzite, a dense and durable local material, for stone tools. Sample BH250-27d is an example of such quartzite tool use.
Quartzite Tools and Hominin Fossils:
Quartzite has been one of the most important raw materials for early hominins in making stone tools, which are often found alongside hominin fossils, helping us understand early human behavior, technology, and evolution.
Quartzite as a Toolstone: Quartzite’s hardness, durability, and availability made it a preferred material for producing sharp flakes, scrapers, handaxes, and other stone tools. Early human ancestors associated with the Oldowan (2.6 million years ago) and Acheulean (1.76 million years ago) industries frequently used quartzite at African archaeological sites. Quartzite tools are common at key paleoanthropological locations such as Olduvai Gorge (Tanzania) and the Turkana Basin in Koobi Fora and the Nachukwi area of Kenya. With the oldest stone tool dated today is the Lomekwi tools about 3.3 million years old. Older stone tools have also been found at Melka Kunture, Omo, Hadar, and the Middle Awash in Ethiopia.
Association with Hominin Fossils:
Quartzite tools are often found in direct association with hominin skeletal remains, providing critical context for behavior, tool use, and subsistence. Their presence alongside fossils of Homo habilis, Homo erectus, and other early humans helps establish timelines and cultural phases. These tools often show wear consistent with butchery, plant processing, and other survival activities.
Significance in Human Evolution:
Quartzite tools represent technological advances in early hominin cognition and manual dexterity. Their durability allows them to survive well in the archaeological record, often outlasting bones and organic materials. The selection and transport of quartzite by early hominins also reveal insights into landscape use, resource selection, and mobility.
Summary:
Quartzite has been a vital raw material in early human toolkits and is closely linked to hominin fossil sites throughout Africa. These durable quartzite artifacts provide essential evidence of early technological behavior and remain a cornerstone in interpreting the human evolutionary record.
Coverage
GPS Coordinates: 43.419219, -89.895472