Quick why do we have limestone? what do iron bands mean?

First question is - Why do we have limestone? Formed from this type of sediment are biological sedimentary rocks. Their biological organ is often revealed in the rock fossils. Some limestones can form by direct precipitation of calcium carbonate from marine or fresh water. (The second question is) - What do iron bands mean? The rocks are "banded" because the iron minerals deposited in alternating bands with silica and sometimes shale. The banding might have resulted from seasonal changes in organism activity steel mill. Most iron ore is used to make steel. Here a steel slab is being cut to length in a steel mill.

Answer a bell shaped curve;