Furthermore, if igneous source rocks furnished sediments, say, for Cambrian strata, whereupon those Cambrian sediments were deposited upon the source rocks, covering and sealing the source rocks from weathering and eroding agencies, how could Silurian sediments be subsequently extracted from the buried source rocks? How could the Silurian sediments emerge up through the Cambrian cover to be deposited upon the latter? An identical enigma arises as to all subsequent deposits up through the whole geologic column: How could younger strata be extracted from buried source rocks and be deposited upon older strata, which covered and concealed the source rocks? Attempts to overcome this enigma are responsible for the theory of alternate upheavals and erosions of first one region and then another, whereby the source rocks were exposed again and again to erosive agencies, enabling them to supply sediments for successive periods.
Sediments a Supercrust
Derivative, sedimentary rocks, except where “overthrusts” or volcanic ejectamenta have covered them, overlay igneous rocks beneath. Thus they form what may be called a supercrust. Obviously they were accumulated later than were the igneous rocks. There are of course restricted areas where ablation has removed a former cover of sedimentary rock, leaving exposed “shields” of “basement-complex” igneous rocks.
It is held that sedimentary rocks are either a mechanically accumulated detritus, such as sandstone, shale or conglomerate, or a chemical precipitate, such as rock salt or gypsum. It is the concensus of geologists that both classes of rocks came from weathering, chemical reactions upon, or leaching of, previously existing, indurated, igneous rocks. The original igneous rocks are assumed to have been situated at high, subaerial locales, from which the weathered material was eroded, transported downgrade by, and deposited as sediment in, water. The whole process of weathering, erosion, transportation and deposition can be clearly seen in operation today. Hence it is perfectly natural to conclude that the original and only source of sedimentary rocks was previously existent igneous rock masses.
It is held that as a result of peneplaining, in connection with successive crustal upheavals and subsidences, sedimentary materials have been worked and reworked time after time. Because the process of deposition in water can be seen at work today and because all sedimentary strata obviously were originally laid down horizontally and in parallel layers, it is assumed that the existence at any location of a veneer of stratified rock proves that the area at the time of accumulation was submerged either in a lake or in an arm of the sea which had temporarily transgressed the lowered land.
Surprisingly, the theory is not questioned, even though it develops contradictions. In many instances land fossils are found alternated with marine fossils in successive strata. To illustrate, way down below the rim of the Grand Canyon of the Colorado River, in Permian strata, again in the nearby Painted Desert, in Triassic strata, thousands of feet below top-most sedimentary strata of the Arizona-Utah Plateau, are footprints of short-legged animals and dinosaurs clearly impressed in solid rock. How come these lumbering land animals were able to walk on an ocean floor? Or had the floor, just at those moments, bobbed up from beneath the sea so that the old fellows could leave their “footprints in the sands of time” before the sands hardened into rock? If the latter is the true explanation, the surface must have sunk immediately after the awkward saurians passed, in order that subsequent layers of sediment could be deposited.
To overcome complications like the foregoing, proponents of the theory are obliged to assume that land areas time after time have been alternately uplifted and depressed above and below sea level. Yet such assumption conflicts with the general opinion that within geological time, aside from the great major orogenic revolutions, continental platforms and ocean basins have uninterruptedly maintained substantially their present relative positions. As Dr. Rachel L. Carson says in The Sea Around Us (p. 11):
In spite of theories to the contrary, the weight of geologic evidence seems to be that the locations of the major ocean basins and the major continental land masses are today much the same as they have been since a very early period of the Earth’s history.
So-called “derivative,” sedimentary rocks underlie top soil throughout major portions of nonmountainous areas of Europe, Asia, Africa, Australia, North and South America, and very probably also Greenland and Antarctica. Such rocks also dominate or occur plentifully in both inland and coastal mountains everywhere, excepting only volcanic mountains and those of purely igneous, basement complex. They also occur in islands everywhere. From evidence thus far obtained from borings, dredgings and photography, it appears that they also underlie the silt on all ocean floors. (Those who contend that sedimentary rocks accumulated from deposition of sediments in the seas can scarcely afford to question the last statement!)
Sedimentary rocks are so prevalent throughout the world as to indicate that at one time virtually the entire surface of Earth’s igneous, indurated crust must have been covered with a stratified, sedimentary supercrust; and that only subsequent weathering and transportation by wind, water and ice, have left some comparatively small and isolated areas devoid of such previous coverings.
The theory that Earth’s sedimentary supercrust was derived from older igneous rock, as was explained before, is predicated upon the assumption that first one and then another mountainous or highland region supplied eroded sediments to cover other areas which at the time were submerged beneath the sea. It is assumed that as time went on crustal uplifts and subsidences of regions to heights above and depths below sea level resulted in alternate areal accumulation and distribution of crustal sediments as we now find them. But the theory develops paradoxes. To illustrate, suppose areas A and B, let us say, during the Cretaceous period, were mountainous regions supplying eroded sediments to cover adjacent area C, which at the time lay submerged on an ocean floor.
According to the theory, areas A and B, after being “pene-plained,” must have sunk beneath the sea, for they are now also covered with sediments. And when we examine the sediments, we find that they too are Cretaceous! But whence did they get their cover? Area C could not have furnished it, for area C is still covered with Cretaceous strata. What perplexes exponents of the theory is the difficulty of determining the whereabouts of assumed source rocks for sedimentary strata spread all over the globe. As we shall see later, some very questionable assumptions have been made in attempts to overcome this difficulty.
- Other mysteries solved