88 TERTIARY SYSTEM. 



scratches and furrows. It was dry land, and much of it high and mountainous, 

 when the marine clays and sands were strewn over the territory adjacent to the 

 Gulf of St. Lawrence and the New England States, and dry land during the period 

 of the drift of the central part of the continent, and for geological ages antecedent 

 thereto. The precipitous ledges and profound valleys of denudation, the overhang- 

 ing rocks and castellated outliers, furnish incontestable evidence of the ordinary 

 eroding agencies through a period of time commencing anterior to the Tertiary 

 epoch. There are extensive driftless areas in Eastern and Southern Ohio free from 

 scratches and furrows on the surface rocks, and from drift, sand, gravel, and 

 bowlders, and they are characterized by outliers, monument rocks, sharp ridges, and 

 rugged scenery. The drift materials extend from the lakes to the sources of the 

 rivers that flow into the Ohio, and over more or less of the land intervening between 

 the head-waters ; but below this they occur only in the valleys of the larger rivers. 

 Wherever the valley was large enough to carry off the flow of water from the 

 north, the adjacent land was not overflowed, and the height of the water in the 

 valley is marked by river terraces. In Eastern Ohio, only those rivers having their 

 sources in the central and northern part of the State have river terraces, as the 

 Scioto, Hocking, and Muskingum, while the smaller tributaries, such as Raccoon, 

 Shade, and little Muskingum, have not a vestige of drift, or scratch, or furrow, 

 from their sources to the Ohio. The Ohio River Valley was large enough to carry <>fF 

 the water that flowed across Ohio and Indiana, and hence no drift crossed the valley 

 until it reached the western part of Kentucky. Throughout the drift area of Ohio, 

 Indiana, and Illinois, it is common in excavations below the drift to find an ancient 

 soil of vegetable mold resting upon stratified rocks in place. Beech, sycamore, 

 hickory, and cedar have been found where they grew prior to the drift; but beneath 

 the ancient soil no striated or furrowed rock has ever been discovered. 



§ 194. There is a driftless area in the south-western part of Wisconsin, covering 

 about 13,000 square miles, or nearly one-fourth of the State, and which extends into 

 Northern Illinois, North-eastern Iowa, and Eastern Minnesota. There is no drift, 

 sand, clay, or gravel, and, as in all cases where these do not occur, there are no 

 scratches or furrows on the surface of the rocks. This area was not overflowed by 

 the lake, and is a region of narrow, ramifying valleys, narrow, steep-sided, dividing 

 ridges, whose directions are toward every point of the compass, and whose perfectly 

 coinciding horizontal strata prove conclusively their subaerial erosion. The ravines 

 are all in direct proportion to the relative sizes of the streams in them. North and 

 east of this driftless area, from 25 to 75 miles, there is a scantiness of drift and 

 numerous outliers, attesting the ordinary effects of erosion. The " Stand Rock," in the 

 dells of the Wisconsin, the isolated ridges and peaks in the central part of the State, 

 rising from 100 to 300 feet abruptly from the low ground around them, and composed 

 of horizontally stratified sandstone, or of sandstones capped with limestone, prove 

 the regular erosion for ages, and are quite inconsistent with any single mechanical 

 eroding power that must have operated upon the whole country alike. In Dakota 

 County, Minnesota, there is an outlier of the St. Peter's sandstone known as " Lone 

 Rock," whose summit is 100 feet higher than the surrounding country, and 

 from which many other outliers are in view; and yet in the valleys the drift prevails 

 and bowlders abound. In Wabasha County, the " Twin Mounds," and in Olmsted 

 County the "Sugar Loaf Mound" and the "Lone Mound," attest in like manner 



