Breaking rocks up into smaller pieces. There are lots of processes that cause rocks to break apart into smaller pieces. No matter what causes the rock to break, we call the smaller pieces "sediment." A rock that forms from these smaller pieces of sediment is called a "sedimentary rock!" Keep in mind that even though sediments are 'smaller' pieces of rock, they might be still be the size of a house if they broke off a the side of a massive mountain. Regardless of whether they the size of a pea or the size of a car, we call all pieces of rock that break off from other rocks "sediment."
Moving those pieces.... Water, wind, and gravity are the main things that move pieces of rock from place to place. Sediment may get transported thousands of miles by one of the world's major rivers, or it may just go from the top of the hill to the bottom during a landslide. During this journey, a lot can happen to the sediment. For example, it can continue to erode into smaller pieces during transport. This happens because a piece of rock may bump into other rocks during transport and break into smaller pieces (erosion!). So transport can also include some erosion, and the longer it takes to transport a rock, the more chance it has to erode even more.
...until you stop moving the pieces. Once the sediment stops being transported, you have a bunch of pieces of sediment in one place. A pile of sand is an example, and you might find such a pile at a sandbar along the edge of a river. You can also find sediment that has been deposited at the bottom of steep hills, at the beach, in sand dunes, and many other places on earth. Bigger size pieces are harder to transport, so they tend to stop moving (deposition) before smaller pieces. For example, a huge boulder cannot move in a tiny trickle of water, but may move during a raging flood. Similarly, a muddy river flowing into the ocean may dump most of the larger pieces of sediment near the coast in what we call a "delta," but the really tiny pieces stay suspended in the water and can travel far out into the open ocean before finally settling to the bottom.
Turn the individual small pieces back into a solid rock by cementing them together. So far, we've broken a rock into tiny pieces and moved the pieces from one place to another so that we have a pile of loose sediments -- not a hard rock. The evolution of sediment into rock typically takes thousands of years or longer in nature. Most sedimentary rocks are held together by the minerals calcite and quartz that act like cement to hold the individual pieces of sediment together. The combination of higher temperatures and pressures speeds the process of cementation. If sediment continues to be deposited in the same place, newer layers of sediment will bury older sediment. The added weight of the newer sediment increases the pressure on the older sediment and squeezes the bottom layers. The layer of newer sediment also acts like a blanket insulating the lower layers such that the temperature also increases. So as sediments get buried by other sediments, they can eventually become "as hard as a rock!"The fact that burial is so important in the last stage of making sedimentary rocks also helps explain why sedimentary rocks tend to form in layers. Layers most often reflect individual pulses of deposition -- like individual floods, wet seasons, or even climatic periods lasting millions of years. If a river, lake, or ocean stays around for many years so that it experiences lots of deposition events, there will be layer on top of layer on top of layer in the same spot. Because the newest layers always form on the top (burying older layers), geologists can read these layers like the pages of a history book.
