until the start of the Viking Age in about 800 AD. Dating The question of the dates of the objects and events discovered through archaeology is the prime concern of any system of thought that seeks to summarize history through the formulation of ages or epochs. An age is defined through comparison of contemporaneous events. Increasingly, the terminology of archaeology is parallel to that of historical method. An event is "undocumented" until it turns up in the archaeological record. Fossils and artifacts are "documents" of the epochs hypothesized. The correction of dating errors is therefore a major concern. In the case where parallel epochs defined in history were available, elaborate efforts were made to align European and Near Eastern sequences with the datable chronology of Ancient Egypt and other known civilizations. The resulting grand sequence was also spot checked by evidence of calculateable solar or other astronomical events. These methods are only available for the relatively short term of recorded history. Most prehistory does not fall into that category. Physical science provides at least two general groups of dating methods, stated below. Data collected by these methods is intended to provide an absolute chronology to the framework of periods defined by relative chronology. Grand systems of layering The initial comparisons of artifacts defined periods that were local to a site, group of sites or region. Advances made in the fields of seriation, typology, stratification and the associative dating of artifacts and features permitted even greater refinement of the system. The ultimate development is the reconstruction of a global catalogue of layers (or as close to it as possible) with different sections attested in different regions. Ideally once the layer of the artifact or event is known a quick lookup of the layer in the grand system will provide a ready date. This is considered the most reliable method. It is used for calibration of the less reliable chemical methods. Measurement of chemical change Any material sample contains elements and compounds that are subject to decay into other elements and compounds. In cases where the rate of decay is predictable and the proportions of initial and end products can be known exactly, consistent dates of the artifact can be calculated. Due to the problem of sample contamination and variability of the natural proportions of the materials in the media, sample analysis in the case where verification can be checked by grand