Slike strani

and dissected the surface sufficiently to divert much of the drainage into its present courses.


[ocr errors]
[ocr errors]

The consolidated rocks of that part of the Central Lowland east of the Mississippi River and of the Appalachian Plateaus are, except a few small igneous dikes, wholly sedimentary and of Paleozoic age, ranging from Ordovician to Carboniferous. The Laurentian upland, north of Lake Ontario, is occupied by a complex of both igneous and sedimentary rocks of pre-Cambrian age, most of which are greatly metamorphosed. Similar ancient rocks extend everywhere beneath the region and form the floor upon which the Paleozoic rocks rest.

The stratified Paleozoic rocks have a general slight southward dip away from the area of ancient rocks. The oldest of them crop out around the border of the Laurentian upland, and on the south successively younger formations adjoin them in rudely concentric belts. The general southward dip is disturbed in places by broad, gentle folds. In western Ohio occurs the broad, low dome known as the Cincinnati arch, the axis of which trends nearly north. The rocks dip gently away from this axis, westward into Indiana and eastward or southeastward into central and eastern Ohio. In western New York and Pennsylvania and northeastern Ohio the rocks dip southward, and their belts of outcrop have a general eastward trend. In central Ohio the trend of the outcrop belts has swerved to a northerly direction, with eastward dip, along the east flank of the Cincinnati arch. Just west of Cleveland the strata swing from the eastward trend of northeastern Ohio to the northerly direction of central Ohio. About 20 feet to the mile is a common rate of dip in these regions. Other slight swells of the strata exist, of less magnitude than the Cincinnati arch-too slight to require material modification of the statement that the general structure of eastern Ohio is monoclinal.

There is close agreement between the relief of the region and the surface distribution of the formations, the more resistant beds capping escerpments and steep slopes and giving rise to platforms, whose surfaces, together with the lower slopes of the escarpments, are occupied by less resistant beds. The Ontario Plain is occupied by Ordovician limestone and shale and early Silurian shale and sandstone. The Niagara escarpment is capped by a massive limestone of middle Silurian age, and the Huron Plain is occupied by late Silurian limestone and shale. The Onondaga escarpment is formed of the Onondaga limestone, of Middle Devonian age. The Erie Plain is formed on shales of Upper Devonian age, including also shales of probably early Carboniferous (Mississippian) age at and west of Cleveland. There the southward swerve of the formations causes discordance between the relief and the surface distribution of formations, so that at and

west of Sandusky much of the surface of the Erie Plain is formed on Silurian and Devonian limestones.

The rocks of the Portage escarpment in New York are sandstones and shales of Upper Devonian age. In passing into Ohio successively younger and younger rocks are found at the crest of the escarpment, and in the Cleveland district the Sharon conglomerate, of early Pennsylvanian age, forms its capstone. Within Ohio the Appalachian Plateaus are formed of shales and sandstones of Mississippian, Pennsylvanian, and lower Permian age.


Little is known of the pre-Cambrian history of the region, but at the beginning of the Cambrian it was a land area, and existed as such for a long time, so that erosion reduced it to a nearly even surface. During late Cambrian time it became submerged beneath the waters of a shallow, epicontinental sea, the first of a series of such seas that invaded and covered parts of the interior of North America in Paleozoic time. Frequent changes in the relative levels of land and sea shifted the position of these seas, changed their size and shape, and shifted also their connections with the oceanic basins. Climates altered from humid to arid and from warm to cool and the reverse, and a tremendously long and complicated history was recorded, many of the details of which have not yet been worked out.

During much of the Paleozoic era the region was covered by marine waters, and successive sheets of mud, calcareous mud, sand, and gravel were spread over the sea bottom. At times nonmarine deposits were formed, such as the delta sands and coals of the Pennsylvanian of eastern Ohio, or the gypsum and rock-salt deposits laid down in the salt lakes of the Salina epoch and now buried 1,800 to 2,000 feet beneath Cleveland. At other times the waters were largely or completely withdrawn and the surface was partly worn away, instead of receiving deposits. Many of the seas contained abundant marine life, which in the earlier seas consisted entirely of invertebrate animals. In some beds of the Ordovician, Silurian, and Devonian periods the calcareous shells of these creatures are so plentiful as to constitute the major part of the rock. Later in the era fishes appeared, as shown by their remains in the Cleveland shale, and plants flourished, which have been preserved in the coal seams of the “Coal Measures."

No locality shows a complete section of the known Paleozoic formations, because the seas alternated with land, leaving many areas without record of long passages of time. Everywhere deposition was frequently interrupted, and at times the surface deposits underwent erosion. When the sea returned and deposition was renewed the new beds were laid down on the worn surface thus produced, which remains as an evidence of the time interval between the two

sets of deposits. “Unconformities” of this kind are found at several horizons. The one beneath the Sharon conglomerate, at the base of the “Coal Measures," is the most significant one exposed in Ohio. A much less conspicuous one occurs beneath the Berea sandstone.

The first of the Paleozoic seas overspread the region in the later part of the Cambrian period. In the Ordovician and Silurian periods the region was many times submerged. Early Devonian seas barely reached it, but the Middle Devonian sea was widespread over it. A late Devonian sea also existed in the region but did not lay down thick deposits. Early Mississippian seas succeeded that of the late Devonian. During the later half of the Mississippian epoch the region was above sea level, and its surface was planed off by erosion. The land was low, however, and the thickness of material worn away was moderate. On the irregular surface thus formed, early in Pennsylvanian time, a thick deposit of sand and gravel with thin beds of mud and of plant débris, indicating strong currents, perhaps rivers, was formed over the region; this material has consolidated into the Sharon conglomerate. It was followed by the formation of a great thickness of Pennsylvanian shales and sandstones, with beds of coal, over all of southeastern Ohio. The sea then withdrew altogether from the region, and there is no evidence that it has since returned to any part of it, except for a recent occupation of the basin of Lake Ontario.

The Cincinnati arch was developed at the end of the Ordovician period and existed as a low dome over a large part of western Ohio throughout the remainder of Paleozoic time, sometimes as a land area, sometimes as a shoal. When not submerged it separated the marine waters covering Ohio from those on the west.

In the final stages of the Carboniferous period the whole region was brought above sea level and much of it was greatly elevated. The uplift was accompanied by profound deformation of the rocks along the east side of the Appalachian Plateaus and the region still farther east, the strata being closely folded and more or less faulted. It was by this disturbance that the Appalachian Mountains were formed. The Appalachian Plateaus, lying immediately west of this belt of intense folding, were broadly uplifted with only gentle folding, which decreased in amount westward, so that as a rule the folds in Ohio are perceptible only on careful determination of the altitude of the beds in different places. The uplift and deformation that produced the great mountain system in the eastern half of the Appalachian province probably gave the surface of much of the remainder, especially of its northern part, a northwestward slope, with considerable altitude at the base of the mountains, diminishing toward the country now occupied by the lakes.

The Mesozoic era thus began with the region above sea level and undergoing erosion, and it has probably never since been submerged. The surface sloped to the northwest or north, and the original streams must have flowed down this slope. Erosion was several times accentuated by renewed uplift of the land, accompanied by some warping or tilting of the surface, which influenced the courses of the streams. During the early part of the era the region remained undisturbed so long that the surface of the entire region, even of the mountains, was worn down to a comparatively featureless plain of low altitude, or "peneplain." The uplift next following, probably at the end of the Jurassic period, was accompanied by marked eastward tilting of the region now forming the Atlantic Coastal Plain, so that the sea has since repeatedly invaded it as far as the present "fall line,” and, as part of the peneplain was also involved in the tilting, the streams flowed down these new slopes toward the southeast. These streams had a short route to the sea compared with those flowing northwest, and this enabled them to extend their headwaters back into the plateau at the expense of the other streams.

This uplift of the plateau was great enough to revive the streams, which at once began to deepen their valleys across the old peneplain and to reduce the general surface. This erosion interval was not, however, as long as the preceding one, so that only the less resistant rocks of the district were worn down to a new peneplain. All the region north of the Portage escarpment and west of the Adirondacks was so worn down, and the Erie Plain is the remnant of the peneplain then formed. A subsequent uplift again lowered the grade of the streams, and the Ontario Plain was worn down below the level of the Erie Plain. During the Tertiary period the streams of the northern part of the Appalachian Plateaus in all likelihood flowed northward and northwestward into trunk streams that occupied the Erie and Ontario Valleys.

During the Pleistocene epoch great ice sheets, four at least, successively invaded the northern part of the United States from the Canadian plains. One or more reached as far south as Long Island, southern Ohio, Indiana, and Illinois, central Missouri, and northeastern Kansas. Evidence of four such invasions has been obtained in the Mississippi Valley. In Ohio there is certain evidence for at least two of these invasions, but by far the greater part of the glacial deposits are those of the last, or Wisconsin glacial stage. The ice of this stage spread out from the Labrador Peninsula and in western Ohio reached well toward the Ohio River. In eastern Ohio the high land of the Appalachian Plateaus so blocked its course that it extended only 60 to 80 miles south of the Lake Erie shore. The general direction of the ice movement over the Cleveland district was southward. As the ice advanced across the region it removed much of the loose surface material and somewhat eroded the underlying rock, especially where the rock was soft shale, so that the basal ice became heavily


[ocr errors]

clogged with rock fragments. When the ice melted away it spread a sheet of such material over the surface, most thickly over the depressions and valleys, so that the new surface was much smoother than the old.

A very prominent effect of the ice invasion upon the region was the modification of the drainage systems. Many valleys were filled with glacial deposits, some were deepened and widened by ice erosion, and northward-flowing streams were dammed by the ice or by glacial deposits and forced to seek new outlets southward. Divides were shifted, in places for long distances, and the directions of many streams were permanently reversed. Some of the new stream courses occupied old valleys but in reversed direction, such as the lower 50 miles of the Scioto; others followed wholly new routes, such as the Scioto above Marion. Many of the streams that follow new routes have since cut gorges with waterfalls where they enter the preglacial , portion-for example, the Cuyahoga. Most of the cataracts and gorges of northeastern Ohio and western Pennsylvania and New York, Niagara among the number, have thus originated.

When the invading ice melted away its southern margin moved slowly backward. In doing so it uncovered, in the present Great Lakes region, the western and southwestern ends of several basins whose natural outlets are on the east. After the ice front had withdrawn within these basins, it acted as a dam to those streams that flowed into them from the south and caused their waters to form lakes. For a long period it prevented these lakes from using their present outlets and raised them to levels of much higher outlets. As the ice withdrew farther or readvanced it uncovered lower outlets or blocked them again, and the lake levels fell and rose in consequence. When the present outlets were uncovered by the melting away of the ice the lakes finally fell to the level of these outlets. Traces of the successive changes in level are preserved in a series of old beaches extending entirely around the Great Lakes, some of them several hundred feet higher than the present surface of the lakes.

As a result of the tremendous weight of the ice over a very great length of time the northern part of the continent was, at the time of maximum ice occupation, depressed several hundred feet below its normal position. As the ice melted away and the weight was lessened, the continent slowly arose-a greater amount in the north, where it had been weighted most. As a result the shore lines, which were once horizontal, are now higher at the northeast than at the southwest.

The ice withdrew from the lower St. Lawrence Valley before the rebound of the continent was completed and while the outlet from Lake Ontario was yet below sea level, thus permitting the ocean


« PrejšnjaNaprej »