n.; pl. Boundaries /plu> [ From Bound a limit; cf. LL. bonnarium piece of land with fixed limits. ] That which indicates or fixes a limit or extent, or marks a bound, as of a territory; a bounding or separating line; a real or imaginary limit. [ 1913 Webster ]

But still his native country lies
Beyond the boundaries of the skies. N. Cotton. [ 1913 Webster ]

That bright and tranquil stream, the boundary of Louth and Meath. Macaulay. [ 1913 Webster ]

Sensation and reflection are the boundaries of our thoughts. Locke. [ 1913 Webster ]

Syn. -- Limit; bound; border; term; termination; barrier; verge; confines; precinct. Bound, Boundary. Boundary, in its original and strictest sense, is a visible object or mark indicating a limit. Bound is the limit itself. But in ordinary usage the two words are made interchangeable. [ 1913 Webster ]

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 ]