Visit http://www.makemegenius.com ,watch all Science & other educational videos for kids for free. When mountains are old, lots of rock gets worn away due to wind or weather erosion. Most of the broken parts of the rocks end up in the streams & rivers that flow down from these mountains. These little pieces of rock & sand are called sediments. When the water slows down enough, these sediments settle to the bottom of the lake or the sea they get into.Over a period of time the layers of sand and mud at the bottom of lakes & seas turns into rocks. These are called sedimentary rocks.Clay ,Chalk, Coal & Sandstone are all examples of Sedimentary rocks.