The main work of youthful streams, like the Middle Fork of the American River, is to erode the landscape and carry away the sediments. As a result of that work rocks of all sizes are broken, tumbled and abraded by the force of the water and other rocks in the stream. Sediments are carried as part of the suspended load or bedload whenever the discharge and stream velocity are great enough to carry, bounce, or roll them along. Eventually, downstream, the stream valley will become mature then old age and the load will be deposited in a sedimentary environment like the bottom of the river, a natural levee, the floodplain, delta or the ocean. As more sediments are continually deposited on top of them, compaction occurs. Groundwater carrying dissolved solids, like iron, calcium carbonate, or silica, moves through pore spaces. Minerals may form from the dissolved solids and begin to cement the sediments together. Rocks created in this way are called clastic sedimentary rocks. When the particle size is about the size of mud or clay, the rock is called shale. Rocks with sediments the size of sand grains are called sandstone. If the predominant particle size is that of a pebble or cobble greater than 2mm diameter and rounded the rock is called conglomerate. When sedimentary rocks are buried even deeper underground the intense heat and extreme pressure begins to "metamorphose" or change the shape and mineral content of the rock. The clasts of sediment become recrystallized and the cobbles begin to flatten out. So, a conglomerate that has undergone metamorphism is called a "metaconglomerate" or a "stretched pebble conglomerate".