Sedimentary rocks, or rocks formed from sediment, dust, garden soil, particulates from the air, substances from lakes, rivers, creeks, and ponds that have become stationary, are of three basic divisions: siliciclastic – or clastic – chemical and biochemical. And these, for added simplicity in identification are further subdivided.
(Siliciclastic sedimentary rocks)
The identifying feature of clastic rocks is their inability to dissolve in water. A large part of their make up is silica. These are further divided into types such as conglomerate rock, or round coarse gravel; Breccia, or angular fragmented gravel; sandstone made up of sand; siltstone from mud; shale, which is a finer type of mud, possibly formed with more silica.
(Chemical sedimentary rocks)
This group contains two types of limestone, from crystal or fossil sources; chalk, chert, gypsum, rock salt and bituminous coal. They are often vastly different in appearance, and their one similarity is their primary origin being formed from an earth chemical.
Although limestone, both types and chalk have the same chemical calcite and in pretty much the same proportions, the differ in their construction. Chert is from quartz, gypsum while resembling chalk is of a different chemical makeup while still having some of the qualities of calcite but having other chemical properties as well. Bituminous coal is simply a composition of vegetative matter. Rock salt, known chemically as Halite, is sodium chloride, or NaCl.
(Biochemical sedimentary rocks)
This is simply sedimentary rocks that are mixed but are primarily formed by calcium carbonate, or calcite. The biochemical attached to the definition sets them apart from the chemical rocks in that parts biological or life giving properties, such as oxygen and the other chemicals necessary for life are also found in these rocks. Basically, they are carbonates.
From Thinkquest it is learned that “Seventy percent of all the rocks on earth are sedimentary rocks”. And of course that being the case, then why does that figure remain constant although rocks are constantly breaking up and reforming? Would that not mean that at some times there may be more or less of a that statistic?
Obviously not. Since the properties that bound the rocks together in the first place will be the same type of rocks that will attract the sand pebbles, the minute fragments of mud, or the other broken off particles that finally settle with another cluster of substances to form a rock. The location, the size, the exact amount of chemicals will be different, but in the finality, it will still be a sedimentary rock, or a particulate on its way to becoming a hardened mud clod, a bit of shale, a sandstone.
How exactly do these rocks form? They form from pressure or from some kind of glue like substance the grabs and hold these future rock fragments together. Forming from pressure mean they pressed on by the weight of the rock layers above them and over time they gradually become part of the surrounding mass of like and unlike materials.
In rocks all kinds of fascinating forms such as leaves, fossils of animals or whatever else was caught in the mud slide, the volcanic eruption, or whatever else aligned these future rock particles together. Rock collecting is a fascinating and a continuing hobby for children of all ages.