Some layers of sedimentary rock contain features called concretions. Concretions are masses of sediment that have been very tightly cemented together. Concretions are often very hard and resistant to weathering. They can occur in almost any shape, although rounded to disk-like shapes are very common. Many concretions form around organic material. Concretions thus often contain fossils.

If you were going for a hike, and were walking over sedimentary rocks, you might find a concretion. If these rocks were from Paleozoic or Mesozoic, you might be lucky enough to find a concretion that contains the beautiful fossil shell of an ammonite.

Ammonite shell inside of 
a concretion

A concretion in sedimentary rock