Geology: The Science of the Earth's Crust

by William J. Miller

Fig. 32.—Map showing the surface distribution of Archeozoic and Proterozoic rocks in North America. (Redrawn by the author after U. S. Geological Survey.)

All known evidence leads us to the remarkable conclusion that the climate of much, or possibly all, of Archeozoic time was not fundamentally different from that of to-day. There must have been weathering of rocks, rainfall, and streams much as at present as proved by the character and composition of the stratified rocks which formed in that remote era. The presence of graphite (“black lead”) in crystalline flakes scattered through many of the strata shows that the climate must have been favorable to some form of life, because graphite thus occurring quite certainly represents the remains of organisms, this matter being more fully discussed in a succeeding chapter. In passing it may be stated that climatic zones were then probably scarcely if at all marked off, as they quite certainly were not even during Paleozoic time. One of the great contributions of geology to human knowledge is that during the tens of millions of years from Archeozoic times to the present the earth’s climate has undergone no fundamental change or evolution. In the earlier ages there was greater uniformity of climate over the earth, and, during known geologic time there have been rather localized relatively minor fluctuations giving rise to glaciers, deserts, etc., but there has been no real evolution of climate at all comparable to the marvelous evolution of organisms—both animals and plants.

We shall now turn our attention briefly to a consideration of the second great subdivision of geologic time—the Proterozoic era. Rocks of Proterozoic Age comprise all of those which were formed after the Archeozoic rocks and before the deposition of the earliest Paleozoic (Cambrian) strata, these latter being rather definitely recognizable because they contain fossils characteristic of the time. Cambrian strata are, in fact, the oldest rocks which contain anything like an abundance of fossils, so that the separation of rocks of either Archeozoic or Proterozoic Age from the earliest Paleozoic is seldom difficult. But how may we separate the Proterozoic rocks from the Archeozoic? Fossils afford us no aid whatever, because no determinable fossils have been found in rocks as old even as the earlier Proterozoic. The two great groups of very ancient rocks do, however, show a number of differences which must be considered together. Thus, igneous rocks distinctly predominate in the Archeozoic, while stratified rocks predominate in the Proterozoic. All Archeozoic strata are thoroughly metamorphosed (changed from their original condition), while large masses of the Proterozoic strata are only moderately metamorphosed, or even unaltered, and therefore look much like ordinary strata of later ages. Archeozoic rocks have almost invariably been notably deformed by more or less folding, tilting, etc., while the Proterozoic rocks show relatively much less deformation. Another important criterion is the fact that the Proterozoic rocks, wherever they have been studied in relation to the Archeozoic rocks, always rest upon a profoundly eroded surface of the latter, that is, an unconformity separates the two great sets of rocks. This erosion surface is of still further interest because it is the very oldest one known, none having been recognized within the Archeozoic group itself. Even where the Proterozoic strata have been considerably metamorphosed and deformed, this old erosion surface may be recognized, and if the rocks below that surface possess the characteristics of the Archeozoic rocks as described above, the two great very ancient rock groups may be distinguished. One of the triumphs of geology during the last 25 to 30 years has been the recognition of the great rock group (Proterozoic) between the Archeozoic and Paleozoic, thus bringing to light the records of an era which lasted many millions of years.

The length of time represented by the Proterozoic era is by many believed to have been fully as long as all succeeding eras—Paleozoic, Mesozoic, and Cenozoic—combined. Twenty million years would be a very conservative estimate for the duration of the era. What is the nature of the evidence as recorded in the rocks which lead us to conclude that the Proterozoic era lasted such a vast length of time? The great thickness of Proterozoic strata (over 30,000 feet in the Lake Superior region), in the light of what we have already learned regarding the present rate of wear (erosion) of lands and deposition of the eroded materials under ordinary conditions, clearly implies millions of years of time for their accumulation. But the Proterozoic strata as we now see them are in most places not a continuous pile, that is they were not accumulated layer upon layer without notable interruption. Thus, the thick Proterozoic group of the Lake Superior region has been divided into four distinct, mainly sedimentary series separated from each other by erosion surfaces (unconformities). Each erosion surface represents a long time when the area was elevated and underwent profound wear before the next series of strata accumulated on the worn surface. That such times of erosion were geologically long is proved not only by the profound alteration (metamorphism) of one set of strata before another accumulated, but also by the fact that granite, which, as we have learned, is never exposed except where much overlying material has been eroded, actually formed parts of surfaces of earlier Proterozoic rocks upon which later ones were deposited. In the Lake Superior region there are not only three great erosion surfaces (unconformities) within the Proterozoic group, but also one at the base separating it as a whole from the Archeozoic group, and another at the top separating it from the Paleozoic group. It is, therefore, fair to conclude that the amount of time (millions of years) represented by these great erosion intervals was fully as great as the time needed for deposition of the existing Proterozoic strata.

In the Lake Superior region the older Proterozoic strata are nearly all more or less folded and altered (metamorphosed), and they have been intruded by considerable bodies of molten rock, mostly granite. The later Proterozoic strata have been much less deformed and in many cases they are practically unaltered. In this region a very remarkable event took place in late Proterozoic time. This was volcanic activity on a grand scale. We may gain some idea of the stupendous and long-continued volcanic outpourings from the fact that, based upon actual measurements of thickness, lava sheets, averaging about 100 feet thick, poured out one upon another until a pile about six miles high had accumulated.

In parts of the Grand Canyon of the Colorado tilted Proterozoic strata may be seen resting upon the profoundly eroded surface of the Archeozoic rocks of the inner gorge. The Proterozoic strata, 12,000 feet thick, consist of practically unaltered sandstones, shales, and limestones, associated with some layers of basaltic lava. An erosion surface (unconformity) separates the whole group into two distinct series, and the group is separated from the overlying nearly horizontal Paleozoic (Cambrian) strata in the walls of the Canyon by another erosion surface.

More recently the Proterozoic strata so finely displayed in the Rocky Mountains of Montana and southern Canada have been studied. These strata, at least two or three miles thick, are mostly unaltered sandstones, shales, and limestones, associated with some metamorphic and igneous rocks. As usual, these strata rest upon the eroded Archeozoic. They were more or less upturned and folded before deposition of the succeeding Paleozoic strata. Satisfactory subdivisions have not yet been worked out.

In North America most of the areas shown on the accompanying map contain more or less Proterozoic rocks. Rocks of this age are known to some extent in all continents where their general relationships seem to be much like those of North America. They have perhaps been most carefully studied in Scandinavia and the Highlands of Scotland, where the strata portions are about two miles thick.