Today's cloud display in the skies of the Conejo Valley was a treasure to behold. We see these types of pillowy formations maybe a handful of times a year. I believe these are called stratocumulus clouds. According to the Wikipedia article, "A stratocumulus cloud belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and the whole being at a lower height, usually below 2,000 metres (6,600 ft)."