Ripple marks in quartzite

Ripple-marked quartzite strata

at Tiradentes, Minas Gerais state, Brazil

Figs. 1-3: Wave-rippled quartzite slabs in the streets and (Fig. 3) a garden in the town of Tiradentes, Minas Gerais. Photos taken on 19,22 November 2025 in Tiradentes.

"Rock of the Month # 295, posted for January 2026" ---

Ripple marks in quartzite, a distinctive sedimentary structure

Ripples in sand, formed by the action of waves on beaches or in shallow water, are a common form of sedimentary structure (another, less often preserved, would be pits produced by rain drops falling onto soft, plastic sediment). Ripples can be preserved in both clastic and carbonate (sandy or limy) sediments, but seem especially often preserved in sandstones and quartzites (an especially tough variant of sandstone). Ripples are preserved in sediments of ancient (Precambrian) to modern settings. Sand, of course, occurs both in marine and terrestrial (deserts, rivers, lakes) settings (Welland, 2009). Moving sands driven by the wind result in familiar landforms: mounds, dunes of diverse forms, and sand ripples, as seen in the Gobi desert and elsewhere in Mongolia (Baasan, 2004). The physics of blown sand, with movement and entrainment of sand grains to form ripples, can be studied experimentally in a wind tunnel (McKenna Neuman, 2022).

Ripple marks are preserved in rocks of many ages. Structures such as ripples are illustrated in many textbooks (e.g., Holmes, 1965; Tucker, 2011; Plummer et al., 2016). While wave ripples are often preserved on cm- to dm- scales, larger "megaripples" may form under more extreme conditions (e.g., Allen and Hoffman, 2005). Here are a few examples of strata hosting ripple marks, oldest to youngest:

Precambrian: The Vindhyan Supergroup of northern India provides examples of rippled sediments (e.g., De, 2001; Raghuwanshi, 2007). In the Mesoproterozoic Sibley Group near Thunder Bay, Ontario, the upper Kama Hill Formation displays ripples and abundant dessication features, consistent with sheet flood deposition on an alluvial floodplain, during basinal extension and downdrop prior to initiation of the magmatism of the Midcontinent Rift (Cheadle, 1986). The Smithsonian Institution in Washington, D.C. has a spectacular slab from the Copper Country of northern Michigan: a big sheet of native copper from White Pine. The 147-kg sheet was mined at 2,066' / 630 m depth, the Proterozoic host shale (also related to the Midcontinent Rift) preserving water ripples (as noted at the museum in 2003).

Phanerozoic: The Silurian Grimsby sandstone of southwest Ontario and upstate New York is a hematitic quartz sandstone unit with structures indicative of deltaic subaerial to nearshore sedimentary facies (Lumsden and Pelletier, 1969). The Devonian Old Red Sandstone of Anglesey, north Wales, preserves ripple marks, cross stratification, flat bedding, sole marks and channels (Allen, 1965). Strata in the Carboniferous of southern Quebec display features such as ripple marks and dessication cracks (Jutras and Prichonnet, 2005). Rippled strata occur in the Cretaceous-Tertiary boundary section in southern Alberta (Eberth and O'Connell, 1995).

Quaternary, Recent: Ripples occur in interglacial sediments, the Don Beds, in Toronto (Eyles and Clark, 1988). Ripples also occur in the coastal sand sheets known as sandurs in southern Iceland (Bluck, 1974). Tidal flats with regular wave action, on a beach or in shallow water, are a typical environment for ripple development, as along the west coast of India (e.g., Saha et al., 2011). Ripples are documented also in Halifax harbour, Nova Scotia (Fader and Buckley, 1995).

Figs. 4-6: Wave-rippled quartzite. Photos taken on 23 November 2025 on the ridge (serra) above the town of Tiradentes. From L-R: loose slab on ascent of ridge; part of circa 5x3 metre outcrop of rippled quartzite atop cliff looking down to town (sorry: poor exposure, dark); and the view northwards along the ridge. The ridge appears to be largely quartzite, with some milky quartz veins, plus slate and orange, muddy outwash near base. No quarries were located on a six-hour walk, but clearly the abundant rippled slabs seen in the town are plausibly derived from the unit exposed on the ridge.

Quartzites in Brazil

Wave-rippled sandstone or quartzite is prominent in the rough-hewn paving stones in the picturesque town of Tiradentes, in the Brazilian state of Minas Gerais. Brazilian quartzites (with a variety of minor minerals, besides quartz) are a popular ornamental or building stone, and come from Minas Gerais, Bahia and Espirito Santo. Minor minerals may include phosphates and borosilicates, such as dumortierite (Morteani and Ackermand, 2004). Moore (1948) provided an early account of the mineral wealth of Minas Gerais, its history of mining, and the land: the town of Tiradentes is named for a national hero (ibid., p.490).

Quarried quartzites of Late Archean to Paleoproterozoic ages (2600 to 1750 Ma) were used extensively in Minas Gerais in the 18th and 19th centuries, as in Ouro Preto and Tiradentes (Costa, 2015). In Tiradentes, lower Proterozoic quartzites of the Tiradentes Formation were employed: these outcrop on the nearby Sao Jose Ridge (within walking distance of the city). These rocks are affected by low-medium grade regional metamorphism and may contain minerals such as micas, kyanite and Fe oxides. Weathering may form Fe hydroxides and thus tint the rock on exposure.

Quartzite as building stone is but one facet of the vast and diverse mining industry in the country: quartzites may also be the host rock to gold and uranium deposits (Darby, 1911; Dorr, 1969; Beurlen and Cassedanne, 1981).

REFERENCES

Allen,JRL (1965) The sedimentation and palaeogeography of the Old Red Sandstone of Anglesey, north Wales. Proc.Yorks.Geol.Soc. 35, part 2, no.8, 139-185.

Allen,PA and Hoffman,PF (2005) Extreme winds and waves in the aftermath of a Neoproterozoic glaciation. Nature 433, 123-127, 13 January.

Baasan,MT (2004) Aeolian Sands of Mongolia. Ulaanbaatar, 2nd edition, English translation, 506pp.

Beurlen,H and Cassedanne,JP (1981) The Brazilian mineral resources. Earth-Science Reviews 17, 177-206.

Bluck,BJ (1974) Structure and directional properties of some valley sandur deposits in southern Iceland. Sedimentology 21, 533-554.

Cheadle,BA (1986) Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay district, Ontario. Can.J.Earth Sci. 23, 527-542.

Costa,A (2015) Quartzite of historic buildings in Brazil: designations and degradation patterns. Geophysical Research Abstracts 17, EGU2015-7947, 1 p.

Darby,OA (1911) On the mineralization of the gold-bearing lode of Passagem, Minas Geraes, Brazil. Amer.J.Sci. 4th ser. 32, 185-190.

De,C (2001) Billion-year-old rain-imprinted tidal sea-coast in eastern Madhya Pradesh, India. Current Science 81 no.8, 1116-1124, 25 October.

Dorr,JVN (1969) Physiographic, stratigraphic and structural development of the Quadrilatero Ferrifero, Minas Gerais, Brazil. USGS Prof.Pap. 641-A, 110pp. plus map folder.

Eberth,DA and O'Connell,SC (1995) Notes on changing paleoenvironments across the Cretaceous-Tertiary boundary (Scollard Formation) in the Red Deer River valley of southern Alberta. Bull.Can.Petrol.Geol. 43, 44-53.

Eyles,N and Clark,BM (1988) Last interglacial sediments of the Don Valley Brickyard, Toronto, Canada, and their paleoenvironmental significance. Can.J.Earth Sci. 25, 1108-1122.

Fader,GBJ and Buckley,DE (1995) Environmental geology of Halifax harbour, Nova Scotia. Geoscience Canada 22, 152-171.

Holmes,A (1965) Principles of Physical Geology. Thomas Nelson and Sons Ltd, London, 2nd edition, 1288pp.

Jutras,P and Prichonnet,G (2005) Record of late Mississippian tectonics in the new Perce Group (Visean) of eastern Gaspesie, Quebec. Can.J.Earth Sci. 42, 815-832.

Lumsden,DN and Pelletier,BR (1969) Petrology of the Grimsby sandstone (lower Silurian) of Ontario and New York. J.Sed.Res. 39 no.2, 521-530.

McKenna Neuman,C (2022) How do they do it? Trent Environmental Wind Tunnel (TEWT) Edition II. Presentation 61 to Kawartha Geoscience Network (Kawartha and Region Earth Sciences, Engineering and Metallurgy Network, KREEM), Peterborough ON, delivered via Zoom, 01 March.

Moore,WR (1948) Brazil's land of minerals. National Geographic, 479-508.

Morteani,G and Ackermand,D (2004) Mineralogy and geochemistry of Al-phosphate and Al-borosilicate- bearing metaquartzites of the northern Serra do Espinhaco (state of Bahia, Brazil). Mineral.Petrol. 80, 59-81.

Plummer,CC, Carlson,DH and Hammersley,L (2016) Physical Geology. McGraw-Hill Higher Education, 15th edition, (xx+595+57=) 672pp.

Raghuwanshi,RS (2007) Petrographic and geochemical characteristics of the Kanar sandstone formation, NE of Barwah, Khargone district, Madhya Pradesh. J.Geol.Soc.India 69, 1298-1304.

Saha,S, Ghosh,A, Banerjee,S, Saraswati,PK and Burley,SD (2011) Characteristics of an open coast tidal flat: example from Daman, west coast of India. J.Geol.Soc.India 77, 409-418.

Tucker,ME (2011) Sedimentary Rocks in the Field: a Practical Guide. Wiley-Blackwell, 4th edition, 276pp.

Welland,M (2009) Sand, the Never-Ending Story. University of California Press, 344+16pp.

Graham Wilson, 28,29 December 2025

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