The great Appalachian system, running parallel to the Atlantic coast, and ending in northern Alabama, forms the geological axis of the southern states. Bordering the mountains proper is a broad belt of hills known as the Piedmont or Metamorphic region, marked by granite and other crystalline rocks, and having an elevation decreasing from 1,000 to 500 feet. The soil varies according to the underlying rocks, but is thin and washes badly, if carelessly tilled. The oaks, hickories and other hardwoods, form the forests. In Virginia this section meets the lower and flatter country known as Tide-Water Virginia. In the southern part of this state we come to the Pine Hills, which follow the Piedmont and stretch, interrupted only by the alluvial lands of the Mississippi, to central Texas. The Pine Hills seldom touch the Piedmont directly, but are separated by a narrow belt of Sand Hills, which run from North Carolina to Alabama, then swing northward around the coal measures and spread out in Tennessee and Kentucky. This region, in general of poor soils, marks the falls of the rivers and the head of navigation. How important this is may easily be seen by noticing the location of the cities in Georgia, for instance, and remembering that the country was settled before the day of railroads. In Alabama the Black Prairie is interposed between the Pine Hills and the Sand Hills, and this prairie swings northward into Mississippi. The Pine Hills give way to the more level Pine Flats, which slope with a gradient of a few feet a mile to the ocean or the gulf, which usually has a narrow alluvial border. Going west from Alabama we cross the oak and hickory lands of Central Mississippi, which are separated from the alluvial district by the cane hills and yellow loam table lands. Beyond the bottom lands of the Mississippi (and Red river) we come to the oak lands of Missouri, Arkansas and Texas which stretch to the black prairies of Texas, which, bordering the red lands of Arkansas, run southwest finally, merging in the coast prairies near Austin. In the northern part of Arkansas we come to the foothills of the Ozarks. These different regions are shown by the dotted lines on the population maps.