Here are several sources and interpretations for some frequently used symbols on gravestones. There is more than one interpretation for some symbols so interpretations must be used as possibilities, not certainties.
General Gravestone Symbolism
See Clasped Hands: Symbolism in New Orleans Cemeteries, by Leonard V. Huber, published 1982 by the Center for Louisiana Studies, University of Southwestern Louisiana. It is fully illustrated and deals only with gravestones and tombs.
See "Fraternal Organizations" by Alvin J. Schmidt from The Greenwood Encyclopedia of American Institutions, published by Greenwood Press, 1980, or see The International Encyclopedia of Secret Societies & Fraternal Orders by Alan Axelrod, published by Facts on File, 1997 or see Markers XI, "Ritual Regalia, and Remembrance: Fraternal Symbolism" by Laurel K. Gabel.
Woodmen of the World
Woodmen of the World derived from the Modern Woodmen of America, a fraternal group which was founded in 1883. Fraternal scholar William Whalen describes it as an insurance society with some fraternal lodge features. Woodmen advertised themselves as an organization for the "Jew and Gentile, Catholic and Protestant, the agnostic and atheist." The Woodmen of the World emblem is a sawed-off tree stump, often with a mallet or beetle, an ax, and a wedge: the motto "Dum, Tacet Clamat" (Though Silent He speaks") usually appears somewhere on the border. These Woodmen emblems are found throughout the United States, but the largest concentration is in the South and Midwest.
In the 19th century some larger urban areas had pallbearer or mourners' coaches. These would be the equivalent of a limousine today, but they certainly had features that separated them from an everyday coach. You may want to look up www.hearse.com on the Internet. They have photographs of funeral coaches of every size and may have a pallbearer's coach. There is also a national mourning collectors group.
Generally most mourning cards are black with gold lettering. Some have generic images, like a dove, flowers, etc. They were used as family mementos and may have been incorporated into a large piece. Some companies produced large lithographs with various mourning iconography, angels, doves, flowers, biblical verses, and there was a spot to place the mourning card within the lithograph. The whole thing was then hung on the wall. I have seen mourning cards in photographs taken of the flower arrangements from the funeral. The photographer propped the cards up near the flowers. I have seen these cards used in shadowbox frames with other artifacts from the person and their wake. The whole thing was then hung on the wall in tribute to them.
Clam shells, scallop shells, and other types of shells are a symbol of a person's Christian pilgrimage or journey through life and of baptism in the church. In the middle ages, Christians wore the scallop shell to indicate that they had made a pilgrimage to the shrine of St. James of Compostella in Spain. Placing a shell on a gravestone when visiting the site is an ancient custom and may in fact have several different meanings depending on the cultural background of the people placing the shells. The idea of crossing over a body of water to the promised land or crossing the River of Styx to the afterlife, the final journey to the "other side" is also part of the symbolism of the shell.
Hands are found on many gravestones. It may be the hand of God pointing downward signifying mortality or sudden death. The hand of God pointing upward signifies the reward of the righteous, confirmation of life after death. Praying hands signify devotion. Handshakes may be farewells to earthly existence or may be clasped hands of a couple to be reunited in death as they were in life, their devotion to each other not destroyed by death.
Some initials found on gravestones:
FLT stands for Friendship, Love, Truth, three degrees associated with the Independent Order of Odd Fellows. The Odd Fellows, first organized in the US in 1819, is a popular fraternal/benefit organization. The emblem of the Odd Fellows is usually shown as three links of a chain. A number on the stone is the local lodge number.
FCL stands for Fraternity, Charity, and Loyalty. These same letters were also used by the Daughters of Union Veterans of the Civil War and a similar hereditary group called the Ladies of the Grand Army of the Republic.
The obelisk is, to quote McDowell and Meyer in The Revival Styles in American Memorial Art, one of the "most pervasive of all the revival forms" of cemetery art. There is hardly a cemetery founded in the 1840s and 50s without some form of Egyptian influence in the public buildings, gates, tomb art, etc. Napoleon's 1798-99 Egyptian campaigns, the discoveries at the tombs of the Pharaohs, and our new Republic's need to borrow the best of the ancient cultures (Greek revival, classic revival, the prominence of classical studies and dress, etc.) led to a resurgence of interest in the ancient Egyptian culture. Obelisks were considered to be tasteful, with pure uplifting lines, associated with ancient greatness, patriotic, able to be used in relatively small spaces, and, perhaps most importantly, obelisks were less costly than large and elaborate sculpted monuments. There were many cultural reasons for the revival styles of the nineteenth century. Freemasonry, while part of the overall cultural influence, was not responsible for the prevalence of obelisks. If you would like to read more about some of these styles, see The Egyptian Revival: Its Sources, Monuments and Meaning, 1808-1859, by Richard Carrott.
What is the origin of the practice of all headstones facing east?
In many, but by no means all, early New England burying grounds the graves are positioned east/west. This east/west orientation is the most common orientation in other parts of the country and world as well. The earliest settlers had their feet pointing toward the east and the head of the coffin toward the west, ready to rise up and face the "new day" (the sun) when "the trumpet shall sound and the dead shall be raised" or when Christ would appear and they would be reborn. If the body was positioned between the headstone and the footstone, with the inscriptions facing outward, the footstone might actually be facing east and the decorated face of the headstone facing west. If the headstone inscription faces east, the body would most commonly be buried to the east of it. Much depends on the layout of the graveyard -- if there was a church or other building in the center of the burial site, where the high ground was located, the location of access roads, etc. Early graves were seldom in the neat rows that we are used to seeing. Burials were more haphazard, more medieval in their irregularity; families didn't own plots and burial spaces were often reused. The north side of the cemetery was considered less desirable and is often the last part of the burying ground to be used, or you may find the north side set aside for slaves, servants, suicides, "unknowns," etc. In many burial grounds graves face all four points on the compass. Sometimes a hilly site will have stones facing all four directions. With the coming of the Rural Cemetery Movement in the 1830s and 40s, an entirely new style of burial became popular. The ideal of winding roads and irregular terrain dictated the orientation of the monuments to a large degree.