As the low-lying plains are altogether an alluvial deposit, the coarser sediments accumulate in the regions where the river first overflows its banks to spread out over the plains.
It is interesting to observe, as will be shown later, that during the Mesozoic era there was a land-mass in the north of Asia and another in the south, and between them lay the sea in which ordinary marine sediments were deposited.
The northern part can best be regarded as a low plateau (once marine sediments) sloping southward, traversed by the large diluvial valleys of the Mississippi, Red and Ouachita rivers, and recut by smaller tributaries into smaller plateaus and rather uniform flat-topped hills.
The coarser particles of the sediments are deposited near the shore as gravels, sand and muds, but the very fine particles remain in suspension in the colloidal form, and some of this may be acted upon by marine bacteria or (it is surmised) even utilized by diatoms as a source of silica.
They may safely be considered to be among the oldest auriferous sediments of the world.
Recent limestones are being produced in this way and also in some places by the precipitation of calcium carbonate by sodium or ammonium carbonate which has been carried into the sea or formed by organisms. The precipitated carbonate may agglomerate on mineral or organic grains which serve as nuclei, or it may form a sheet of hard deposit on the bottom as occurs in the Red Sea, off Florida, and round many coral islands in the Pacific. Only the sand and the finest-grained sediments of the shore zone are carried outwards over the continental shelf by the tides or by the reaction-currents along the bottom set up by on-shore winds.
Chemical and physical processes similar to those that have converted ordinary sediments into rock masses.
A very thin soil covers the Edwards Plateau, but on the Llano Estacado are brownish and reddish loams derived from the sediments of a Neocene lake.
The question is of the age of the sediments from which these were taken.
While igneous and metamorphic crystalline rocks form the bulk of the Adirondack area, it is surrounded by a ring of ancient Palaeozoic sediments in which these peripheral lowlands have been developed.
The Paleozoic sediments, ranging in age from Cambrian to Permian, occupy the Great Valley, the Valley Ridges and the plateaus still farther west.
Matthew, and are shown to contain fluviatile or channel beds with water and river-living forms, and neighbouring flood-plain sediments containing remains of plains-living forms. Thus we may complete the former physiographic picture of a vast flood plain east of the Rocky Mountains, traversed by slowly meandering streams.
,Among the known fossils are vermes, crustacea and probably brachiopods and pteropods The character of the sediments of the Proterozoic is such as to show that mature weathering affected the older rocks before their material was worked over into the Proterozoic formations.
Like the earlier Palaeozoic systems, the Devonian attains its greatest known thickness in the Appalachian Mountains, where sediments from the lands of pre-Cambrian rock to the east accumulated in quantity.
The site of these mountains had been, for the most part, an area of deposition throughout the Palaeozoic era, and the body of sediments which had gathered here at the western base of Appalachia, by the close of the Pennsylvanian period, was very great.
At this time these sediments, together with some of Appalachia itself, began to be folded up into the Appalachian Mountains.
Triassic SystemThis system has but limited representation in the eastern part of the United States, being known only east of the Appalachian Mountains in an area which was land throughout most of the Palaeozoic era, hut which was deformed when the eastern mountains were developed at the close of the Palaeozoic. In the troughs formed in its surface during this time of deformation, sediments of great thickness accumulated during the Triassic period.
These sediments are now mostly in the form of red sandstone and shale, with conglomerate, black shale and coal in some places.
This widespread submergence, followed by the deposition of marine sediments on the eroded surface of Comanchean and older rocks, is the physical reason for the separation of the system from the Comanchean.
They consist, in most parts of the country, of unconsolidated sediments, consisting of gravel, sand, clay, &c., together with large quantities of tuff, volcanic agglomerate, &c. Some of the sedimentary formations are of marine, some of brackish water, and some of terrestrial origin.
From these high lands sediments were borne down to lodge on the low lands adjacent.
The sites of deposition varied as the period progressed, for the warping and faulting of the surface, the igneous extrusions, and the deposition of sediments obliterated old basins and brought new ones into existence.
Thus the lower Eocene has some lignite in the eastern Gulf region, while in Teias lignite and saliferous and gypsiferous sediments are found, though most of the system is marine and of shallow water origin.
As in other comparable cases, this figure does not make allowance for the oblique attitude in which the sediments were deposited, and should not be construed to mean the vertical thickness of the system.
Under these conditions sediments from the high lands were washed out and distributed widely over the plains, giving rise to a thin but widespread formation of ill-assorted sediment, without marine fossils, and, for the most part, without fossils of any kind, and resting unconformably on Cretaceous, Eocene and Miocene formations.
The marine-part of the Lafayette is probably covered by sediments of later age.
The mountain structures originated in three great orogenic periods, the earliest in the Archean, the second at the end of the Palaeozoic and the third at the end of the Mesozoic. The Archean mountain chains, which enclosed the present region of Hudson Bay, were so ancient that they had already been worn down almost to a plain before the early Palaeozoic sediments were laid down.
Later the sediments lying to the south-east of this " protaxis," or nucleus of the continent, were pushed against its edge and raised into the Appalachian chain of mountains, which, however, extends only a short distance into Canada.
The Mesozoic sediments were almost entirely laid down to the west and south-west of the protaxis, upon the fiat-lying Palaeozoic rocks, and in the prairie region they are still almost horizontal; but in the Cordillera they have been thrust up into the series of mountain chains characterizing the Pacific coast region.
The ridges and intervening valleys, long parts of which have an approximately parallel trend from south-west to north-east, were formed by the erosion of folded sediments of varying hardness, the weak belts of rock being etched out to form valleys and the hard belts remaining as mountain ridges.
Clastic sediments are less abundant and there are fewer breaks in the succession.
The great variety of conditions under which the sediments and limestones were formed naturally produced corresponding inequalities in the thickness.
In western Europe the advent of the Carboniferous period was accompanied by the production of a series of synclines which permitted the formation of organic limestones, free from the sediments which generally characterized the concluding phases of the preceding Devonian deposition.
Sediments approaching to the culm type, with similar flora and fauna, were deposited in synclinal hollows in parts of France and Spain.
Looking eastward, towards central and northern Russia, we find a wider and much more open sea; but the continental type of deposit prevailed in the northern portion, and here, as in Scotland, we find coal-beds amongst the sediments (Moscow basin).
In North America, the crustal movements at the beginning of the period are less evident than in Europe, but a marked parallelism exists; for in the east, in the Appalachian tract, we find detrital sediments prevailing, while the open sea, with great deposits of limestone, lay out towards the west in the direction of that similar open sea which lay towards the east of Europe and extended through Asia.
Similar events were meanwhile happening in North America, for the seas were steadily filled with sediments which drove them from the northeast towards the south-west, and doubtless those movements which at the close of this period uplifted the Appalachian mountains were already operative in the same direction.
When the land reappeared a new series of valleys would at once begin to be eroded; and the subsequent degradation of these overlying sediments might reveal portions of the older topography, as in the case of the Great Glen, Lauderdale, and other ancient valleys.
The plains sediments contain important coal beds, which are worked in nearly every county in the state.
The Cambrian rocks in this region are nearly all soft sediments, some 600 ft.
The highland rocks no doubt once extended along the whole length of the state from north to south; but they are now crossed by a band of Upper Palaeozoic sediments, which extend up to the valley of the Hunter river and separate the Blue Mountains and the Southern Highlands of New South Wales from the New England tableland to the north.
The Devonian beds are well developed in the Blue Mountains, where the lower Devonian sediments at Mount Lambie are estimated to be Io,000 ft.
The terraces represent the out-cropping edges of hard sandstone layers included in the series of plateau sediments, and are named according to the colour of the rock exposed in the south-facing escarpments, the Pink Cliffs (highest), White Cliffs and Vermilion Cliffs.
Most interesting among these are the Henry Mountains, formed by the intrusion of molten igneous rock between the layers of sediments, causing the overlying layers to arch up into dome mountains.
The Jurassic and Cretaceous beds are ordinary marine sediments, but from the Cenomanian to the Oligocene the deposits are of the peculiar facies known in the Alps and Carpathians as Flysch.
They contain a great variety of sediments and igneous rocks.
- Wisconsin forms part of the inner margin of an ancient coastal plain and the oldland of crystalline rocks about which the plain sediments were deposited.