n. a thin layer of geologic deposits, of varying thickness in different parts of the world, found between the geological strata identified as Cretaceous and the strata above, identified as Tertiary; also, the time point or period marking the boundary between the Cretaceous and Tertiary periods.
Syn. -- K/T boundary. [ PJC ]

☞According to a theory gaining acceptance (as of 1997), these deposits were formed as the debris of a large comet or meteorite impact on the earth, which threw up a large quantity of dust into the atmosphere, causing profound though temporary climatic change, and caused or hastened the extinction of numerous species, including the dinosaurs. This hypothesis was first postulated by Luis and Walter Alvarez on the basis of an excess of iridium found in the boundary layer, and was later supported by additional evidence of various types. The impact is believed to have occurred at the edge of the Yucatan peninsula in Mexico, forming what is termed the Chicxulub crater, which is partly under the Gulf of Mexico, is not evident from surface topography, and was detected primarily by gravity anomaly readings and subsurface geological characteristics. [ PJC ]

a. [ Pref. post- + tertiary. ] (Geol.) Following, or more recent than, the Tertiary; Quaternary. [ 1913 Webster ]

a. (Geol.) Earlier than Tertiary. [ 1913 Webster ]

a. [ L. tertiarius containing a third part, fr. tertius third: cf. F. tertiaire. See Tierce. ] 1. Being of the third formation, order, or rank; third; as, a tertiary use of a word. Trench. [ 1913 Webster ]

2. (Chem.) Possessing some quality in the third degree; having been subjected to the substitution of three atoms or radicals; as, a tertiary alcohol, amine, or salt. Cf. Primary, and Secondary. [ 1913 Webster ]

3. (Geol.) Later than, or subsequent to, the Secondary. [ 1913 Webster ]

4. (Zool.) Growing on the innermost joint of a bird's wing; tertial; -- said of quills. [ 1913 Webster ]

Tertiary age. (Geol.) See under Age, 8. --
Tertiary color, a color produced by the mixture of two secondaries. “The so-called tertiary colors are citrine, russet, and olive.” Fairholt. --
Tertiary period. (Geol.) (a) The first period of the age of mammals, or of the Cenozoic era. (b) The rock formation of that period; -- called also Tertiary formation. See the Chart of Geology. --
Tertiary syphilis (Med.), the third and last stage of syphilis, in which it invades the bones and internal organs. [ 1913 Webster ]

n.; pl. Tertiaries 1. (R. C. Ch.) A member of the Third Order in any monastic system; as, the Franciscan tertiaries; the Dominican tertiaries; the Carmelite tertiaries. See Third Order, under Third. Addis & Arnold. [ 1913 Webster ]

2. (Geol.) The Tertiary era, period, or formation. [ 1913 Webster ]

3. (Zool.) One of the quill feathers which are borne upon the basal joint of the wing of a bird. See Illust. of Bird. [ 1913 Webster ]