Bread made from a natural starter cultivated from wild yeast and kept alive from generation to generation by putting aside a portion of each sponge (see below) for the next bake. Some starters are said to have been in existence since the gold rush days. Because the process is longer and slower than that of regular yeast breads, the dough develops a unique sour character and a thick crust. It is said that there is particular yeast, found only in San Francisco, that makes San Francisco sourdough better than others. Desum and levain are also types of sourdough bread. We generally distinguish between sweet French bread, made with yeast and using the slow-rise method, from sourdough French, which uses a starter and has a different flavor and character. See, The Sound of Crust.