Theories as to time relations of orogeny and epeirogeny are only as strong as the available evidence; this evidence leaves much to be desired. In many regions total effects of orogeny and epeirogeny are apparent but not the stages by which they were built up; in others, superimposed structures demonstrate the episodic nature of the process without indicating a correlation of the episodes with the standard time scale. Chief evidence for orogenic and epeirogenic episodes therefore remains the sedimentary record—the occurrence of unconformities and of various types of clastic sediments. In many places this evidence is available only in the marginal zones of mobile belts (as in the Southern Appalachians) and may not reflect details of orogenic history in the interior of the belt. Where evidence is available in the interior of the belt (as in the Maritime Provinces) it is less complete for the main orogeny than for the minor succeeding episodes. A further difficulty is lack of precision in correlation of the sedimentary record between different regions. Block faulting in the Rio Grande area of New Mexico, epeirogenic uplift of the Rocky Mountains in Colorado, and compressional deformation of the Coast Ranges of California all seem to have occurred near the Pliocene-Pleistocene boundary, but whether they are simultaneous or closely spaced separate episodes cannot be determined. Much of the evidence regarding time relations of orogeny and epeirogeny is negative and can be interpreted in various ways according to the theories or predilections of the geologist. The writer’s predilections are that orogeny and epeirogeny were episodic rather than continuous; that episodes affected fractions of continents rather than whole continents or all continents; and that episodes may be expressed in one place by compression of the crust, in another by tension, at one place by orogeny, at another by epeirogeny.

It is well known that the Boulder batholith region experienced intensive plutonism, volcanism, and tectonism that all began in Late Cretaceous time, after at least 700 m.y. of structural and igneous inactivity except for sporadic epeirogeny. Recent stratigraphic, structural, paleontologic, arid, especially, radiometric evidence makes it possible to date these dynamic events rather closely. The time relations that are revealed do not form a simple sequence of volcanism-folding-thrusting-batholith emplacement, as has often been supposed, but involve an intertwined complex. Significant volcanism began ~ 85 m.y. ago in late Coniacian or early San-tonian time, with deposition of the thick, local tuffaceous Slim Sam Formation. Volcanism climaxed from 77 to 79 m.y. ago, in early Campanian time, when the region was buried under at least 10,000 feet of calc-alkalic volcanic and volcaniclastic rocks, which included many sheets of welded tuff — the Elkhorn Mountains Volcanics —, and a vast amount of contemporaneous ash was airborne beyond the region. Major volcanism ceased ~ 73 m.y. ago, late in the Campanian, not to recur until early Eocene time, ~ 50 m.y. ago. The bulk of the batholith was emplaced beneath and within the volcanic edifice in early and middle Campanian time, during a 6 m.y. span from 78 to 72 m.y. ago, and some leucocratic masses were intruded during the next few million years, so that the whole batholith was emplaced within about 10 m.y. Folding at and near the site of the batholith began in late Coniacian or Santonian time and culminated before middle Campanian time; the main folding north and east of the batholith was post-Campanian, probably Maestrichtian. Thrusting began before middle Santonian time, and recurred intermittently well into the Maestrichtian, or even a little later. Thus volcanism, plutonism, folding, and thrusting began and ended within a few million years of each other, during the last 20 m.y. of the Cretaceous. Major folding, thrusting, and volcanism started about the same time, though not always at the same places, and a little earlier than plutonism. In any given locality, volcanism ended before major folding; the climax of plutonism followed the climax of volcanism; thrusting preceded and accompanied plu-tonism near the batholith, but followed plutonism farther away; thrusting ended a little later than folding. These dynamic processes so closely related in time and space must also be genetically related in the Boulder batholith region. Gilluly’s (1965) conclusion that the orogeny which produced the great Cretaceous thrusts of Montana was “essentially without plutonic associations” is not tenable.