Potsdam Formation
Washington County, New York

Sand grains made of quartz and a larger, multicolored quartzite pebble were rounded by abrasion in moving water, perhaps on an ancient beach. Later, they were cemented by dolomite to form sandstone. Look for the concentric shapes in the dolomite crystals. These are growth zones, formed as the crystals grew to fill the spaces between the sand grains. The rhombic pattern is typical of dolomite. The quartzite is an earlier generation of sandstone that was metamorphosed, deformed, eroded, and recycled as a grain in this younger rock.

Crossed polarizers with gypsum plate

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