is prominent and continuous from Cleveland eastward to Albany, N. Y. In the vicinity of Cleveland the escarpment that bounds the Appalachian Plateaus turns more to the south and passes between Columbus and Newark, through Chillicothe, and on southward into Kentucky. In the northern part of Ohio this escarpment is as high as the Portage escarpment, on the east, but its slope is more gentle and it is not so impressive. South of Newark it is bold and precipitous and is fully 300 feet high.

FIGURE 1.-Outline map of Ohio, showing physiographic provinces. The Cleveland district is shown by the shaded area. The numbers at the sides of the area indicate the three quadrangles: 1, Cleveland; 2, Euclid; 3, Berea

Northwest of the Appalachian Plateaus is the Central Lowland, which includes the lowland that surrounds the southern part of the Great Lakes and extends west as far as the border of the Great Plains. This region consists of rolling plains rather than hills, but its altitude in many places is equal to that of the Ohio part of the Appalachian Plateaus. In places the lowland is very hilly.

In the region of the lower Great Lakes the Central Lowland consists of a number of separate and distinct plains, which are known

by the names Erie Plain, Huron Plain, and Ontario Plain. The lowest of these plains is the Ontario; this is separated from the Huron by a distinct rise or escarpment, and the Huron is separated by a similar feature from the higher Erie Plain. The sequence of terraces and escarpments can be carried farther, as the Erie Plain is separated from the generally flat top of the Appalachian Plateaus by the Portage escarpment.

The Erie Plain lies at the base of the Portage escarpment across western New York and extends along the prolongation of the escarpment across northwestern Pennsylvania and northeastern Ohio. It forms a large portion of the Ontario Peninsula between Lakes Huron and Ontario, and Lake Erie occupies a shallow depression in its surface. In New York the plain becomes very narrow east of Buffalo, and it pinches out against the Portage escarpment just east of Auburn. From Buffalo to Cleveland the width of the plain between the shore of the lake and the escarpment is only a few miles, but west of Cleveland the escarpment swings away from the lake toward the southwest, and the plain broadens rapidly and increases in altitude. Within Ohio it ranges in altitude from 573 feet at the lake shore to nearly 1,000 feet. Its relief is slight, and it is on the whole a gently rolling, northward-sloping plain, interrupted only by morainic ridges and by the beaches and low cliffs of the glacial lakes.

Across western New York the Erie Plain is sharply separated from the Huron Plain by the low Onondaga escarpment. West of Buffalo, however, this escarpment becomes barely discernible, so that in Ontario the Huron and Erie Plains are practically merged into one. In Ontario in general the southwestern half of the triangular peninsula between Lakes Ontario and Huron belongs to the Erie Plain and the northeastern half to the Huron Plain. Westward across this peninsula the Huron Plain gradually rises and becomes higher than the Erie Plain, parts of it south of Georgian Bay being more than 1,500 feet above sea level.

The Niagara escarpment separates the Huron Plain from the Ontario Plain. East of Rochester these two plains merge into one by the disappearance of the escarpment, but at Rochester it is about 100 feet high, at Niagara about 200 feet, and it crosses the Ontarian Peninsula with steadily increasing altitude. It turns northwestward at Hamilton and continues to Georgian Bay, where, in the Blue Mountains, it has an altitude of more than 800 feet. Thence it crosses to northern Michigan, turns southward through eastern Wisconsin, and dies out in northern Illinois.

The Ontario Plain lies between the Niagara escarpment and the Laurentian Plateau. In New York its altitude ranges between 250 and 500 feet above sea level. Across Ontario the altitude increases and attains more than 700 feet south of Georgian Bay.

The Cleveland area includes the margins of the Appalachian Plateaus and the Central Lowland and the border zone between them. The larger topographic features are due to the bedrock surface and incised stream valleys, largely masked but not obliterated by the covering of drift left by the glaciers, and the minor features are due to glaciation and the action of glacial lakes. The northwestern twothirds of Ohio, including the Cleveland area, was covered by the great ice sheets of the Pleistocene epoch; the southern boundary of the glaciated area crosses Ohio from the middle of the east side to the southwest corner of the State, running obliquely across plateau and lowland.

DRAINAGE

The greater part of northern Ohio is drained to the St. Lawrence River and the remainder to the Ohio River. The Erie Plain is drained directly into Lake Erie, except that part lying east of Buffalo, which drains into Lake Ontario. The Ontario Plain is drained chiefly into Lake Ontario, but the western part drains into Lake Huron and the extreme eastern part into the Mohawk River.

The northern and northeastern margins of the Appalachian Plateaus drain into Lakes Erie and Ontario, chiefly through short streams that head not far back from the crest of the escarpment, through which they flow in deep, narrow valleys. The Grand River, in northeastern Ohio, is an exception, in that its valley across the escarpment is very broad. The Genesee River, in western New York, is a notable exception of another sort, as it rises in northern Pennsylvania far back from the escarpment and trenches it in a picturesque gorge. In northern Ohio the Grand, Chagrin, Cuyahoga, Rocky, Black, and Vermilion Rivers all rise not far within the plateau and flow north to Lake Erie. The greater part of the plateau in Ohio is drained southward into the Ohio River by means of streams that, in southeastern Ohio, occupy deep, narrow valleys; among these the Muskingum is chief. The Scioto and Miami, of central and southwestern Ohio, occupy much broader, shallower valleys. The divide between the drainage to Lake Erie and that to the Ohio runs rather close to the lake. It is inconspicuous and consists for the most part of low morainic ridges.

The preglacial drainage of the Appalachian Plateaus ran to the Erie Plain from all of eastern Ohio, northern West Virginia, and western Pennsylvania, a large part reaching it through the Grand River Basin. The drainage area of the Cuyahoga also extended farther south than at present. The change to present conditions came largely as a result of the glacial occupation. For a long time the glaciers wholly blocked the lower courses of many northwardflowing streams and forced them to seek outlets to the south. As the ice margin melted back from south to north the streams flowing southward filled up some of the northward-sloping drainage courses

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

STRATIGRAPHY AND STRUCTURE

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 escarpments 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.

GEOLOGIC HISTORY

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