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

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

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waters to come into Lake Ontario for a short time, but they were drained out as the further upward movement of the land mass brought the present St. Lawrence River above sea level. This was the last occupation of the Great Lakes region by ocean water.

RELATION OF CULTURE TO GEOLOGY

On the comparatively level surface of the Central Lowland transportation routes are free to follow any desired direction at a minimum of expense. Railroads are generally laid out in straight lines between points, and the vehicle roads follow the section lines, usually 1 mile apart. These facilities for transportation together with the general fertility of the glacially deposited soils, have permitted a rather uniform distribution of a fairly dense population, such as characterizes the western half of the State of Ohio.

In the unglaciated Appalachian Plateaus in eastern Ohio, the surface 'is rugged, and the tops of ridges are from 200 to 400 feet higher than the floors of the valleys. Much of the surface consists of steep slopes, many with thin soils, and much of it remains forested. Railway routes must follow the valleys and, in passing from one drainage system to another, must overcome steep grades. They are more expensive to lay out and to maintain than those in the glaciated plains. Roads commonly follow either the valleys or the crests of ridges and are correspondingly crooked. Population is less dense and settlement more uneven than on the plains. On the other hand, the mineral wealth of the plateau is an incentive to settlement.

The northern part of the Appalachian Plateaus was covered by the Pleistocene ice sheets, and its valleys are largely filled with glacial deposits. Its surface is therefore less rugged than that of the unglaciated part of the plateaus, its soils are better and more uniform, and transportation routes are much less hampered by steep slopes, are usually independent of stream valleys, and are laid out in nearly straight lines between places.

The Appalachian Plateaus form a high rugged land barrier to east-west trade routes across Ohio, except those that traverse the low strip of the Erie Plain. The Erie Plain connects on the east with the Mohawk Valley lowland, separating the Adirondack highland from the Appalachian Plateaus, and these two lowlands form the only natural low-grade route north of the Gulf Coastal Plain between the Atlantic seaboard and the interior of the United States. The Lake Erie ports are consequently the natural points for transshipment of the coal of the northern part of the Appalachian Plateaus and the iron ore of the Lake Superior region. Largely because of the industries that have developed in this meeting ground of coal and iron, the Erie Plain is the most densely populated part of Ohio.

TOPOGRAPHY OF THE CLEVELAND DISTRICT

By H. P. CUSHING

SURFACE FEATURES

General character. The altitude of the surface of the Cleveland district ranges from 573 feet above sea level along the lake shore to 1,300 feet on the south margin of the Cleveland quadrangle, in Richfield Township. The Portage escarpment, which separates the Erie Plain from the Appalachian Plateaus, crosses the Eculid, Cleveland, and Berea quadrangles from northeast to southwest. The boundary between plateau and plain is not very sharp but forms a zone, in places several miles wide. As a rough approximation two-thirds of the district may be said to belong to the hilly plateau and escarpment and one-third to the plain. All three parts are notched by the Cuyahoga River, Rocky River and its branches, Euclid and Big Creeks, and Doan Brook, most deeply where these streams have cut into the plateau and escarpment.

The slope of the Portage escarpment from plateau to plain is not simple. In the Berea quadrangle it is a single slope descending from the plateau surface to the plain, but in the Cleveland and Euclid quadrangles this slope is interrupted by one or, in places, two minor platforms or terraces.

Three resistant sandstone formations in the rock section at Cleveland, separated by a varying thickness of weak shale, mark the surface of the Appalachian Plateaus and the two lesser platforms on the slope of the Portage escarpment. The uppermost and thickest of these sandstones is the Sharon conglomerate, of Pottsville (lower Pennsylvanian age. It is the youngest Paleozoic rock exposed in the quadrangles and is the capstone formation of the plateau across northeastern Ohio. A thickness of 150 to 350 feet of beds, chiefly shale, separates it from the Berea sandstone beneath, and 50 feet of shale lies between the Berea sandstone and the Euclid sandstone lentil at the base of the Bedford shale. Because of their resistance erosion has developed a terrace at the summit of each of these sandstones, and each terrace terminates in a steep front or low cliff. From the foot of these scarps gentle slopes run down to the surface of the terraces below.

Appalachian Plateaus.-The rolling uplands of the southeastern two-thirds of the region form the northwestern margin of the Appalachian Plateaus. The highest hills, whose altitude ranges from 1,150 to 1,300 feet, are more or less flat topped and are composed of the Sharon conglomerate. There is, however, only about 15 square miles of such territory, contained in a dozen separate areas, so that much of the rolling plateau surface is on the underlying shale. There are three such areas east of the Cuyahoga River-one in Warrensville,