Gravestones may be simple upright slabs with semi-circular, rounded, gabled, pointed-arched, pedimental, square or other shaped tops. During the 18th century, they were often decorated with "memento mori" (symbolic reminders of death) such as skulls or winged skulls, winged cherub heads, heavenly crowns, or the picks and shovels of the gravedigger. Somewhat unusual were more elaborate allegorical figures, such as Old Father Time, or emblems of trade or status, or even some event from the life of the deceased (particularly how they died). Large tomb chests, false sarcophagi as the actual remains were in the earth below, or smaller coped chests were commonly used by the gentry as a means of commemorating a number of members of the same family. In the 19th century, headstone styles became very diverse, ranging from plain to highly decorated, and often using crosses on a base or other shapes differing from the traditional slab. By this time popular designs were shifting from symbols of death like Winged heads and Skulls to Urns and Willow trees. Marble also became overwhelmingly popular as a grave material during the 1800s in the United States. More elaborately carved markers, such as crosses or angels also became popular during this time. Simple curb surrounds, sometimes filled with glass chippings, were popular during the mid-20th century.