It typically takes millions of years for sedimentary rocks to form.
If you've ever visited the Grand Canyon or been by any river or cliff where you observed horizontal stripes in the rocks there, then you have seen sedimentary rock. Sedimentary rocks always form where they has been water, so they provide a geological record of sorts. They may indicate where a body of water, such as a lake or river once was, and also reveal the minerals and even the plant and animal life that was there at the time that layer of sediment was laid down.
The Basic Process
Sedimentary rock is formed when minerals are carried, usually by water but sometimes by wind or gravity, to a place where they are deposited in layers. As the layers grow, the weight on the lower levels increases until the sheer amount of pressure being exerted on them causes them to fuse and harden into rock. Each layer may be slightly different from the others, as weather and other forces conspire to drop different materials at different times.
Clastic
Clastic, also called silicaclastic, rocks are formed out of other rocks and bits of sand and clay. If you can see chunks of different minerals inside a rock, then it is a clastic rock. The minerals that make up clastic rocks are not dissolvable in water and have silica as one of their major components. They call be very small particles, such as the "silt" that makes up siltstone, moderately small, like the sand that makes up sandstone, or the size of pebbles more than two millimeters across, such as those to be found in a clastic rock descriptively named conglomerate.
Chemical
Chemical sedimentary rocks form out of minerals, such as salt and gypsum, that are able to dissolve into a solution. If the minerals become highly concentrated in the water, either because of evaporation or deposition, then they form rocks. Iron is a chemical sedimentary rock formed when iron and oxygen form in a solution together. A large portion of these rocks are composed of calcite, and they are called carbonates. Dolomite is one example of a chemical carbonate.
Organic
Organic sedimentary rocks are formed from the accumulation of plant and animal debris---that is to say, organic matter. They are also called biochemical rocks because, like chemical rocks, their minerals start out in a solution. The only way that those minerals are removed from the solution is by absorption into an animal or plant. When that animal or plant dies, then the minerals in its skeleton form rock. Classic examples of this are coal and some varieties of limestone.
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