Cross-bedding in sandstone north of Fredericktown.
Sandstone in hand specimen. Magnification 4x.
Quartz sand grains. Magnification 21x.
The grains themselves are predominantly particles of the mineral quartz, although any rock or mineral of sand size may be present in sandstone. The quartz (see discussion of quartz on [page 41]) may have been derived from pre-existing sandstones or more directly from granite, porphyry, or other igneous rocks in which quartz crystallized when the hot liquid rock solidified. Today quartz grains which are weathering out of igneous rocks and sandstones are being carried by the Missouri river and tributaries to the Mississippi river and thence to the ocean, where extensive deposits of sand are accumulating, probably destined to become widespread beds of sandstone.
The grains of sand may be broken and become angular during their long trip to the ocean, or they may become rounded by rubbing against each other. If they exist in sand dunes, blown about by the wind before being cemented into rock, the grains usually become somewhat rounded. Even after sandstones are buried beneath other rocks, silica, which is carried in solution by ground waters percolating through the sandstone, may crystallize out on the sand grains and restore some brilliant, angular crystal faces to the otherwise rounded grains.
Cementation of loose sand to more or less firm sandstone is due to the presence of clay, iron oxides, or calcite (mineral of limestone) which may be deposited with the sand. All of these cements are softer and weaker than quartz, thereby being broken first and freeing the harder quartz when the rock is scratched or crushed.
[A] [variety] of very hard sandstone called quartzite is one that is so strongly cemented that it breaks through the sand grains instead of around them as is the case with ordinary sandstone. This condition is brought about by their being cemented with silica (chemically the same as quartz), which makes for essentially uniform hardness throughout the rock.
Quartzites are, as previously noted, extremely hard, and resist abrasion and chemical weathering. Reddish quartzite boulders occur rather abundantly north of the Missouri River in the glacial clay, sand, and gravel which overlie the sedimentary rocks that form the bed rock or country rock there. Locally, the hard, red quartzite boulders may be called “red niggerheads”, although the term “niggerhead” is more often applied to black or dark greenish black boulders of basalt (see [page 48]) also present in the glacial drift. It is to be recalled that the distinguishing hardness of quartzite is due to the hardness of the quartz grains plus the equal hardness of the silica cement.