In a literal sense "historical artifacts" and "historical sites" are all artifacts and sites dating after the introduction of written history in any region. For example, in New Mexico, these would be sites dating after AD 1540, the year of the first Spanish entrada into what would later become the State of New Mexico. In many instances, historical sites can also include those sites created by American Indians who possessed at least some Euro-American objects, and/or whose methods of construction were influenced to some degree by Euro-Americans. The National Historic Preservation Act defines antiquities as over 50 years old; therefore, even late 20th century historical sites may be considered eligible to the National Register of Historic Places. (It is important to note that only cultural resource specialists can make a determination regarding the eligibility of a cultural resource to the National Register of Historic Places; see chapter 1.) Given this time depth and regional/ethnic diversity there exists a wide variety of historic architectural designs made of materials such as adobe, sod, logs, planks, firebrick, formed concrete and, quite often, combinations thereof. Artifacts present at even the most humble of historical sites can number into the thousands; virtually anything listed in a nineteenth century mail-order catalog could be found on a frontier ranch.