the United States of America

Most of Illinois, Indiana and western Ohio have a drift cover remarkable for its small relief and distinguished by a total absence of lakes. The flatness is not absolute, for the retreating ice front halted at various stages, building recessional moraines, a few of which have the distinctive topography of moraines, while others are barely visible, being made apparent only by their effect on drainage. The major streams have cut down moderately into this surface and tributaries are developing, but not much headway has been made except near the Ohio and the Mississippi. A margin near these streams belongs to an older drift sheet (Illinoian) while most of the section was again covered by ice in the last (Wis consin) glacial epoch. West of this section in Iowa and northern Missouri and parts of eastern Nebraska and Kansas, are the Dis sected Till Plains, distinguished from the Till Plains east of the Mississippi by greater erosion. This difference results from the fact that the glacial mantle west of the Mississippi was spread in the Kansan glacial epoch and is the oldest glacial drift widely ex posed in the country. Complete drainage is re-established; val leys now occupy fully f our-fifths of the area. In the type of agri culture that distinguishes the central lowland (grains and domestic animals) the Till Plains and Dissected Till Plains are pre-eminent.

South of the latitude of Kansas City (39°) the central low land (Osage section) was not glaciated. It extends almost to central Texas in a belt nearly 200 m. wide. The underlying rocks dip gently west beneath the Great Plains syncline and away from the Interior Highland. As the surface slopes in the opposite direc tion the formations outcrop in parallel strips trending nearly north and south. The stronger beds form cuestas with east-facing escarpments, some of them so eroded as to make hilly belts a few miles wide.

Interior Highland.

Surrounded by the lowlands of south central United States is the Interior Highland. Its northern and larger part consists of the Ozark plateaux on nearly horizontal though slightly domed rocks. The rocks of its southern third are folded along east-west axes. Altitudes in the Ozark plateaux are generally less than 1,600 ft., being highest near the centre, but in northern Arkansas on the southern slope of the dome the strong sandstone beds of the upper Carboniferous are preserved in a plateau that is locally more than 2,000 f t. high and ruggedly eroded, making the Boston mountains. In south-eastern Missouri in the midst of the plateau stand the St. Francois mountains, composed of granite, rising no higher than the surrounding hori zontal sediments which once covered all. These ancient granite mountains were buried by marine sediments and are now being exhumed. They are among the best examples of their class. The Ozark province is one of the leading sources of lead and zinc.

The belt of folded structure in Arkansas and Oklahoma, the Ouachita mountains (150 is essentially like the Appalachian Ridge-and-Valley province, the product of two cycles of erosion, with ridges on the stronger outcrops and intervening lowlands on the weaker rocks. The rocks of the Arkansas Valley are but mildly folded and although the stronger sandstones make either flat or sloping uplands the whole strip is less uplifted than its neighbours. This province has large supplies of coal. The

Ouachita mountains have the famous Hot Springs of Arkansas and produce the novaculite (natural honestone) of the United States. In and near it also are important bauxite deposits.

Great Plains Province.

The vast area known as Great Plains is a north-south belt averaging about 400 m. in width between the mountains of western United States and the low lands of its centre. Its height at the base of the mountains is 5,000 to 6,000 ft. and at its eastern edge ',so° to 2,000 feet. An eastward slope of 1 o ft. to the mile is common. A part of this great area is literally plain, most of the northern third is pene plain and considerable areas on the borders, both in the east and west, are dissected into hills. All of it is semi-arid and the only trees are in river bottoms or on certain isolated mountains like the Black Hills. Most of it has good soil without the necessary water to take full advantage of the fertility. Perhaps a tenth part may be irrigated.

The flat portions of the province are best illustrated in the Llano Estacado (Staked Plains) of Texas and eastern New Mex ico (13d on the accompanying map and table). Interrupted by the valleys of transverse streams these flats extend north to southern Nebraska and, in modified form, to the South Dakota boundary forming a belt known as the High Plains. These are remnants of a vast alluvial slope made by coalescing alluvial fans from the mountains. When the streams ceased their depos iting habit and began again to erode, the eastern edge of the fluviatile deposit was first to suffer. In Texas the edge is re treating westward as a well defined escarpment, there being but a few miles between the flat upland and the lowland on the east. In Kansas and adjacent States the wasting edge of the former upland plain is represented by a broad belt of hills (13e). In Nebraska, a thick mantle of loess obscures the transition from high plains to central lowland. Between the belt of preserved high plains and the mountains, erosion was also more favoured, partly by original slopes, partly by the nature of the vegetation. Here (13f fluviatile mantle, where formed, was de stroyed and the present landscape is one of varied relief. The South Platte, Arkansas and Pecos have broad terraced and well farmed valleys, but north-eastern New Mexico and the adjacent part of Colorado is a region of plateaux, trenched by angular valleys and surmounted by various features of volcanic origin. The Edwards plateau at the southern extremity of the province (I3i) is a limestone tableland almost as flat as the Llano Estacado but without the fluviatile mantle. It is cut off on the south and east sides by the great Balcones fault which for 30o m. forms the inner boundary of the coastal plain. Streams are dissecting the plateau at the edges and have in part removed the same limestone from a large area just north of the Edwards plateau. This hilly section in central Texas (13k) is in a stage of the erosion cycle intermediate between the youth of the well pre served plateaux and the old age of the lowlands on the north.