The surface of the Moon has been subject to billions of years of collisions with both small and large asteroidal and cometary materials. Over time, these impact processes have pulverized and "gardened" the surface materials, forming a fine grained layer termed "regolith". The thickness of the regolith varies between 2 meters beneath the younger maria, to up to 20 meters beneath the oldest surfaces of the lunar highlands. The regolith is predominantly composed of materials found in the region, but also contains traces of materials ejected by distant impact craters. The term "mega-regolith" is often used to describe the heavily fractured bedrock directly beneath the near-surface regolith layer. The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the impacts. In most of the lunar regolith, half of the particles are made of mineral fragments fused by the glassy particles; these objects are called agglutinates. The chemical composition of the regolith varies according to its location; the regolith in the highlands is rich in aluminium and silica, just as the rocks in those regions. The regolith in the maria is rich in iron and magnesium and is silica-poor, as the basaltic rocks from which it is formed. The lunar regolith is very important because it also stores information about the history of the Sun. The atoms that compose the solar wind – mostly helium, neon, carbon and nitrogen – hit the lunar surface and insert themselves into the mineral grains. Upon analyzing the composition of the regolith, particularly its isotopic composition, it is possible to determine if the activity of the Sun has changed with time. The gases of the solar wind could be useful for future lunar ases, since oxygen, hydrogen (water), carbon and nitrogen are not only essential to sustain life, but are also potentially very useful in the production of fuel. The composition of the lunar regolith can also be used to infer its source origin. In contemporary linguistics, agglutination usually refers to the kind of morphological derivation in which there is a one-to-one correspondence between affixes and syntactical categories. Languages that use agglutination widely are called agglutinative languages. Agglutinative languages are often contrasted both with languages in which syntactic structure is expressed solely by means of word order and auxiliary words (isolating languages) and with languages in which a single affix typically expresses several syntactic categories and a single category may be expressed by several different affixes (as is the case in inflectional (fusional) languages). However, both fusional and isolating languages may use agglutination in the most-often-used constructs, and use agglutination heavily in certain contexts, such as word derivation. This is the case in English, which has an agglutinated plural marker -(e)s and derived words such as shame·less·ness. Agglutinative suffixes are often inserted irrespective of syllabic boundaries, for example, by adding a consonant to the syllable coda as in English tie – ties. Agglutinative languages also have large inventories of enclitics, too, which can be and are separated from the word root by native speakers in daily usage. Note that the term agglutination is sometimes used more generally to refer to the morphological process of adding suffixes or other morphemes to the base of a word. This is treated in more detail in the section on other uses of the term.