Many sedimentary rocks show a distinct history, leaving a geologic record written in salt casts, ripple marks, and fossils. Sometimes, the colors of the rocks tell their own history. Although both green and red mudstones owe their rich colors to iron, the red sediments oxidized in open air, and the green sediments formed under water, being oxidized when plants freed the oxygen that had been held captive in carbon-dioxide.

Northwest Montana is a rich mine of sediments, some of which, when polished, display a greater creative artistry than any man could accomplish. It is also rich in meta-sediments like marble, slate and talc.

CLASTIC SEDIMENTS

     Clastic sediments contain mroken materials that have once been part of other rocks or organisms. "Clastic" means "broken", and mudstone, sandstone and cochina fall into this category.

Mudstone is formed from very small silt particles. It varies in color from browns and yellows associated with shale to rich reds, greens and blacks, tinted by impurities like iron and magnesium. It varies in texture from loose shale to indurated (compacted by overlying layers which squeeze the air and water out) mudstones. Some of these are hard enough to be easily polished, which gives them a high, glossy shine and even more intense colors.

Claystone is similar to mudstone. Most geologists lump the two together into "siltstone". Claystone has a much higher percentage of clay, which is a product of the breakdown of granitic or volcanic rocks. You can easily tell if a eock is claystone by smelling it when it's wet. It has that musty, earthy smell that all young schoolchildren are familiar with. Polished claystone shows more subdued colors than mudstone and has a softer, satiny finish tha resembles enamel.

Dolomite is formed from shale that is rich in magnesium. Its color ranges from brown to bright gold, and it varies from loose and crumbly to quite hard. The dolomite of Glacier Park is noted for its black magnesium "dentrite" inclusions, so named because they resemble small plants.

Limestone is not, strictly speaking a clastic sediment, although it can contain broken pieces of other rocks or sediments. It is formed either from calcite, which is precipitated out of water by plants or volcanic activity, or the small calcitic exoskeletons of tiny marine organisms. It can range in hardness on the Moh's scale from 3 to 5 depending on the amount and type of impurities it contains. The hardest we've found is dolomitic limestone, because of its magnesium content.

1. Chalk is very loosely compacted, and easily recognizeable in formations, such as the white cliffs of Dover, seen in the photograph at the top of this page. It is formed from the remains of tiny, one-celled sea creatures.

2. Cochina is formed by the compaction of millions of various broken sea shells. It's very common on the coasts of Florida, and its presence in Montana speaks to us of ancient inland seas.

3. Stromatolites are fossils of blue-green algae. They are the earliest known fossils and have been dated back over a billion years, and yet their descendants survive to this day, relatively unchanged.



Stromatolites were formed when vast mats of algae spread out upon fresh or sea waters. They used the carbon-dioxide that was abundant in the water, causing calcite to precipitate and form a crust over them. The algae would break through the crust and the process would be repeated. Stromatolites show this growth record in beautiful swirls or concentric circles. When stromatolites used so much carbon-dioxide, it was converted into oxygen, which made the atmosphere breathable for animal life. The new oxygen-rich atmosphere turned many exposed sediments into vivid colors of yellow, orange-yellow and red. Where there was a lot of magnesium, the dolomitic limestone formed by the stromatolites turned from deep blue to black, which provided a sharp contrast with the warm colored dolomite sediment layers which interbedded it.

   Pink stromatolite found in Australia.

This contrast of colors formed beautiful gemstones, which vary from warm, earthy colors to pinks and greens. The most common stromatolites of Glacier National Park and its surrounding areas are deep blue-black contrasted by yellow to orange-yellow. We have found pink and green ones here that have metamorphosed into marble, but they're very rare. They may have been carried down here from Canada by glaciers.

Stromatolites are very hard for limestone and have a high glossy shine when polished.

4. Marl is a mixture of limestone with mudstone and/or clay. These, as well as other mineral impurities, give marl a variety of beautiful colors and patterns. Most of the decorative and gemstone marbles around the world began as marl. There are also various mineral inclusions which add to marl's beauty such as biotite mica, hematite and iron pyrites, which also add beauty to lapus lazuli, which is a form of marble. We even have a polished marl with gold sediments between the bedding layers.

5. Conglomerate- There is a rock called "puddingstone" that is fairly common. It usually forms when calcite cements several small stones together into larger ones, which are called conglomerate. Hematite, silica, and marl can form conglomerates also, but the limestone ones are more commonly found. We also have a few talc conglomerates which metamorphosed from limestone. These are some of our most treasured polished sediments, because the high degree of difficulty polishing them entails, as well as many hours of labor.

6. Breccia is very similar to conglomerate, but the pieces are broken instead of smooth. The most beautiful examples of brecciated limestons are found in the "ruin" marbles of Italy. Breccia can also be formed from pyroclastic rock and ash from volcan eruptions.

7. Concretions form when a fossil or small particle of sand is encircled by layers of calcite, hematite, or cryptocrystalline (crystals so small they can't be seen with the naked eye) quartz. Fossil hunters crack open limestone concretions searching for plant casts. The most beautiful concretions are septarian nodules, which are found in many collections.

Cryptocrystalline Quartz forms from siliceous ooze secreted by sea organisms, which, despite its ignoble beginnings, becomes agate, jasper, flint and chert. Many fossils of sea life have been preserved because of this ooze. There are also quartz-mudstone/claystone hybrids which are much harder and more beautifully colored than the other earthy sediments. They are also extremely difficult to polish due to brittleness.

    Opal is a hydrated, cryptocrystalline quartz. It's beautiful

Evaporites are minerals that precipitated out of great bodies of water that evaporated. The most famous formations being found around the Dead Sea in Jordan and Death Valley in California. They can also be seen in the great salt flats around Salt Lake in Utah. Many evaporites like salt and potash are mined.

Ash is less known as a sedimentary rock, but bedding layers can easily be seen in these deposits, the most famous of which are in Pompei and around Mt. Saint Helens in Washington. Some ash beds also contain conglomerates formed from pyroclastic (fire-broken) rocks from more violent volcanic explosions.

META-SEDIMENTS

 :    Meta-sediments form through either regional metamorphism, caused by the increased heat and pressure from hundreds of feet of overlying rock, or contact metamorphism, caused by geothermic gasses and/or liquids being intruded into the sediments. In both cases, the high temperatures involved melt the sediments, taking away some minerals and adding others. All metamorphic rocks can be recognized by their crystalline structure.

Meta-quartzite is formed when temperatures are so high that they melt the sand grains. Meta-quartzite can be distinguished from quartzite because it will bread through the sand grains instead of around them. When rock-dealers sell polished "quartzite", it's really meta-quartzite.

Slate is formed by regional metamorphosis of shale. It can be easily recognized by it cleavage (it breaks along former bedding layers instead of through them as shale does) and by the tinking sound it makes when it strikes other rocks. It is commonly used as building material for roofs and patios. Some slates can be polished and have a satiny finish similar to claystone, but have richer colors.

Phyllite is a result of slate that goes through further metamorphosis. It commonly contains crystals of mica.

Talc forms from limestone that contains iron-pyrites. It doesn't require heat as other metamorphic rocks do. When the pyrites in limestones are exposed to a lot of water, they bread down into limonite (rust) and a mild sulphuric acid. This acid changes the composition of its parent limestone into talc, a much softer mineral familiar to all of us. Talc is also found in marble formations, such as those in southwestern Montana, which is full of talc mines.

Talc ranges in color from white to green, red or even black, depending on the impurities of the original limestone. We have several polished limestone and mudstone specimens that contain layers of talc. Polished talc has a dull, satiny shine, and the few specimens that we polished required over twenty hours of work.

Marble is formed by either contact or regional metamorphosis. Pure marble, like pure limestone is white in color. The most famous example is the Carrera marble of Italy which is the most sought after material for statues. Any colors found in marble are due to mineral impurities.

Lapis Lazuli is an expensive gemstone found primarily in the middle east. Its beautiful blue and gold colors come from lazurite and iron pyrite inclusions.

Fire Marble was popular in the 1950's as a gemstone, but has been forgotten. It has beautiful orange, red and green inclusions that have a fire-opalescent look. It can be found in Northwestern Montana, but it's parent marl is more common.

Fossiliferous Marble is fairly common in this area. It ranges in color from almost white to medium grey to black. There is a myth that fossils can't survive the metamorphic process, but we have several polished specimens as well as slate fossils. Most of the specimens we have found are crinoidal. Crinoids are sea lilies which look more like wheels in their fossilized form. Stromatolitic Marble is also a fossil, but makes gorgeous gemstones and is much rarer. It's colors range from yellow and blue-black to pinks, greens and whites. Unfortunately, the latter colors are not in good enough shape to be polished. There are Australian specimens that have been polished, however.


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