Indirect fossil preservation includes traces or clues of an organism’s life. The most common indirect fossils are molds and casts. After the remains of an organism have been buried in sediments, water moving through the sediment dissolves the fossil. A cavity in the rock remains and is called a mold. A cast forms when the mold fills up with another material. In some cases minerals such as calcite or quartz precipitate in the mold. Sediment may also fill the void. The formation of a cast is similar to putting Jell-O in a mold. When you remove the mold, you are actually eating a cast. 

Mold and cast

Routes of preservation