From a text technological viewpoint, memorials can be divided (as can most text) into 'official' and 'unofficial' categories, with some blurring of these delineations. The official is most easily represented by the stone or metal, high profile and highly visible national monuments, like Mount Rushmore, the Vietnam Veterans' Memorial, or the new 9/11 Memorial. Stone and metal, both materials that are of the earth, are enduring, transchronological, and meant to be permanent. They connect the monument with its viewers and reconnect the viewer to the earth, which, unlike us, is eternal (in theory, at least). These monuments are public, and thus placed in areas where they can be visited, but there is a sharp contrast in the ways in which visitors can interact with the memorial. Mount Rushmore can only be viewed from hundreds of feet away; its vastness and stylisation means it can be seen and understood, but not touched. It is iconic and representative: we don't need names for the faces (well, Americans don't).
The Vietnam Veterans' Memorial and the 9/11 Memorial, on the other hand, are also vast in scale, but this is more to do with the tragedies they commemorate and the desire to name victims individually. While these are still official and public, then, they are deeply personal and intimate, and permit unofficial acts of memorialisation from visitors, who leave mementoes, notes, flowers, and other commemorative artefacts close to the names of their loved ones. Since most of those remembered by name do not otherwise have a site for reflection--few have graves--this is the place where those still present come to remember the absent. The name--carved into stone or stencil cut into copper--becomes a touchable text, a text that by its presence recalls the individual and his or her place in the world.