Sandstone is a rock made of sand-size particles more or less well
cemented. It is recognized by the grains of sand which are dislodged or
scratched loose when the rock is broken, or when scraped with a piece of
steel or another hard rock. The old-fashioned grindstone is a sandstone
nicely cemented by nature.
[Illustration: Sandstone bluff near mine entrance, Crystal City.
(Photo courtesy of Pittsburgh Plate Glass Company.)]
Sandstone occurs in thin layers to thick massive beds and deposits which
may exceed fifty feet in thickness. In addition to possessing horizontal
bedding and parallel bedding planes, some sandstone displays beautiful,
intricate cross-bedding or cross-lamination.
The grains of sand composing the stone may be either angular or rounded.
They may sparkle in the light from reflections from their crystal faces,
or they may have dull, frosted surfaces. Sandstones are ordinarily
nearly white in color except where the grains are covered with coatings
of yellow or red iron oxide (rust).
[Illustration: Cross-bedding in sandstone north of Fredericktown.]
[Illustration: Sandstone in hand specimen. Magnification 4x.]
[Illustration: 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.
Asphaltic sandstone is a sandstone impregnated with a bituminous residue
from the evaporation of petroleum which once occupied the pores of the
rock. It has been reported from more than a dozen counties in western
Missouri, but the most extensive deposits are probably in Barton,
Vernon, and Lafayette counties.
Attention has been directed to the origin of sandstones from ocean
deposits of sand and from sand dunes, but it should be recognized also
that river channels and stream valleys which contain deposits of sand
(such as those on floodplains, river bottoms, and sand bars) may be
covered, and the sand consolidated to sandstone. Many years ago, even
long ago geologically, a large river, almost comparable in size to the
Missouri river, occupied a channel which is now represented by a long
narrow sandstone deposit extending from a little north of Clinton
through Warrensburg to Lexington and then east through Moberly almost to
Paris. Smaller channel sandstones are abundant in other areas in
Missouri.
The sandstones of the so-called Roubidoux formation, which occurs in
south central Missouri, commonly show well-preserved ripple marks on the
rock slabs. These marks were formed exactly as their name suggests—in
sand which was thrown into ripples by the shallow water in which it
accumulated and was covered and cemented so as to retain the ripple
forms.
[Illustration: Ripple marks in limy sandstone.]
Sandstone is used for building stone, walks, grindstones, furnace
linings, and rock gardens. Large quantities are mined each year near
Pacific, Festus and Crystal City, Klondike, and Hermann, for the
manufacture of glass and other uses. Common glass is a cooled melt of
relatively pure silica sand, soda ash, and lime. Asphaltic sandstone is
used in road building. Sand-lime brick are made of sand. Sand is used as
a molding material for metal castings, a parting substance between brick
in kilns, and in large quantities in concrete and mortar mixtures.