Slike strani
PDF
ePub

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 min. eral 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 Riverone in Warrensville, one near Northfield, and one in Boston, the last capping the Cuyahoga Valley wall and giving rise to the picturesque scenery of the "Boston Ledges." Eight of them lie west of the Cuyahoga River in the southwestern part of the Cleveland quadrangle, but only one is of much size, and four of the eight are very small. Two occur in the Berea quadrangle. Taken together these form all that remains within the quadrangles of this sandstone platform, the original surface of the plateau. The topographic prominence that rightly belongs to them has in many places been greatly subdued by heavy banking of glacial drift against their fronts and in the lower lands between them. Some of them stand out as prominent hills, 100 feet or more above the level of the surrounding territory, flat topped and steep fronted or surrounded by low ledges. Upon many the soil is thin and poor, and they remain largely forested. Between these hills the sloping surface is gently rolling from an altitude of 1,100 or 1,200 feet down to about 900 or 1,000 feet, whence the slopes descend more steeply to the valleys of the Cuyahoga and Rocky Rivers.

This region does not suggest a plateau, but southeast of these quadrangles the plateau surface is distinct and more nearly continuous. The margin of the plateau in the Cleveland region has been dissected by stream erosion until it has been reduced to long promontories projecting northward from the main area of the plateau, and outlying patches have been wholly separated from it.

Portage escarpment.--The Portage escarpment is the irregular slope 2 to 4 miles wide that descends from an altitude of 1,200 or 1,240 feet on the northwestern edge of the Appalachian Plateaus to 700 or 800 feet at the southeastern margin of the Erie Plain. irregular and discontinuous because of the large valleys cut into the plateau, and it bends away from its southwesterly trend into the walls of these valleys. Its upper, southeastern limit has the intricate outline of the dissected margin of the plateau surface; its foot, bordering the plain, is much more continuous and regular. Northeast of Cleveland, in the Euclid quadrangle, the Erie Plain is abruptly terminated immediately southeast of Euclid Avenue by the foot of the Portage escarpment, at an altitude of 680 to 740 feet. In the northern third of the Cleveland quadrangle the foot of the escarpment bends southward into the east wall of the Cuyahoga Valley. West of the Cuyahoga River the southern margin of the Erie Plain or the foot of the escarpment, is not so sharply defined. Where the west wall of the valley bends westward into the escarpment, across Independence Township, it lies along Rockside Road, at an altitude of about 800 feet. Across Parma Township the margin may be regarded as bending southwestward at an altitude of either 800 or 880 feet, depending on whether the low Berea scarp (see p. 13) is included or not.

In the south-central part of the Berea quadrangle the Portage escarpment assumes the southerly trend that carries it entirely across Ohio, Kentucky, and Tennessee. The northern and western threequarters of the quadrangle lie within the Erie Plain. The foot of the escarpment runs east of Berea, crosses the East Branch of the Rocky River near Strongsville, and runs east of the West Branch at the south line of the quadrangle. It is obscure in places but has an altitude between 800 and 900 feet.

The greater part of the Portage escarpment is made up of the long, gentle slopes of shale that descend from the Pottsville to the Berea terrace, or roughly that part of the Cleveland and Euclid quadrangles lying between 900 and 1,150 feet in altitude and of the Berea quadrangle between 800 and 1,150 feet. The general direction of the slope is northwest, but it is not sharply separated from the slopes of the valleys crossing the escarpment. There is some dissection by narrow, shallow stream valleys, but otherwise the slope is fairly uniform, descending from 40 to 80 feet to the mile and mantled everywhere with glacial drift. Rock outcrops are found principally in the stream valleys, and by no means in all, as many streams flow for miles in channels that nowhere cut through the drift.

The outcrop of the Berea sandstone makes a minor escarpment 30 to 50 feet high, which may be called the Berea scarp and which extends across the Euclid and Cleveland quadrangles in a southwesterly direction as far as the Cuyahoga Valley, where it swings to the south as a well-defined bench along the valley wall. As such it continues to the south margin of the Cleveland quadrangle, with a steady decrease in altitude, which brings it down to the level of the river about 3 miles farther south. There is a companion terrace on the west side of the valley. West of the Cuyahoga the Berea scarp continues westward and southwestward to the Rocky River at Berea, but the Berea sandstone declines in altitude, and the scarp is an obscure gentle slope of about 50 feet in a mile. At Berea the Berea sandstone descends below the level of the Central Lowland or Erie Plain and exerts no influence on the topography of the plain from Berea 13 miles westward to Elyria, in the Oberlin quadrangle, where it is again effective.

East of the Cuyahoga River the Euclid sandstone is present 50 feet below the Berea and caps part of a well-defined slope and terrace, the first rise of the Portage escarpment above the Erie Plain. West of the Cuyahoga Valley the Euclid sandstone and its terraces are absent, except for a small elevated area between 780 and 800 feet in altitude on the southern edge of the plain, in Brooklyn Township.

East of the Cuyahoga River the Portage escarpment forms three distinct steps, the first from 100 to 150 feet above the Erie Plain, largely under the influence of the Euclid sandstone; the second 50

а

a

[ocr errors]

feet higher and half to three-quarters of a mile south, the Berea sandstone; the third about 300 feet higher, 3 miles farther south and obscurely defined at the position of the Sharon conglomerate, at the edge of the Appalachian Plateaus.

West of the Cuyahoga River the Portage escarpment is not so high as on the east and consists of a single rise steeper than that east of the river, with the obscure Berea platform at its base.

Glacial deposits so thoroughly mantle the surface and clog the old valleys of the plateau and the escarpment slope that the present topography is much less varied than that which would be disclosed by the removal of these deposits. The plateau was deeply trenched by many old valleys, and its topography was much more rugged than it is now. It was a topography of mature stream dissection, in strong contrast to the present immaturity.

Two low morainic ridges, the Cleveland and Euclid moraines, slightly diversify the smooth slopes of the plateau and plain. The most westerly place at which the Cleveland moraine is recognizable as a distinct topographic feature is near Linndale, whence it runs to Brooklyn, following a course not far south of the valley of the main trunk of Big Creek. It is 20 to 30 feet high and about three-quarters of a mile broad. It is interrupted by the Cuyahoga Valley, but it reappears at Newburg, where it shows a number of knolls of gravelly drift, and continues east to Randall, where it joins the Defiance moraine and thence continues to the edge of the Cleveland quadrangle and beyond. In this part of its course, however, it is so low and so broad that its topographic expression is very indistinct. It is not mapped between Newburg and Randall.

The Euclid moraine shows only its west end in the Euclid quadrangle, just east of Euclid, as a thin mantle of till in a gentle ridge lying along the terrace at the top of the lowest or Euclid scarp. The terrace is not capped by the Euclid sandstone at this place. The Euclid moraine is noteworthy here as marking the west end of a series of moraines that follow the lower slopes of the Portage escarpment to the east end of Lake Erie, have much to do with the present configuration of the escarpment, and exert a profound influence upon the drainage of northeastern Ohio.

Erie Plain.-A considerable part of the Erie Plain is submerged beneath the waters of Lake Erie. Out in the lake is a channel of moderate depth that is probably best regarded as cut below the level of the plain, but with this exception the entire lake is shallow, and the bottom belongs with the Erie Plain. East of the Cuyahoga that part of the plain above the lake is only 2 to 3 miles wide and reaches the foot of the escarpment at an altitude of 700 feet at Euclid and 740 feet at Cleveland. The surface has a lakeward slope of 50 to 60 feet to the mile but is very smooth, except for the abandoned lake

« PrejšnjaNaprej »