Five new states between 1815 and 1821 carried the area of settlement down the Ohio to the Mississippi, and even up the Missouri to its junction with the Kansas. The whole eastern side was filled with states, well populated along the rivers, but sparsely settled to north and south. The frontier wedge, noticeable by 1776, was even more apparent, now that the apex had crossed the Mississippi and ascended the Missouri to its bend, while the wings dragged back, just including New Orleans at the south, and hardly touching Detroit at the north. The river valleys controlled the distribution of population, and as yet it was easier and simpler to follow the valleys farther west than to strike out across country for lands nearer home but lacking the convenience of the natural route. For the pioneer advancing westward the route lay direct from the summit of the Alleghanies to the bend of the Missouri. The course of the Ohio facilitated his advance, while the Missouri River, for two hundred and fifty miles above its mouth, runs so nearly east and west as to afford a natural continuation of the route. But at the mouth of the Kansas the Missouri bends. Its course changes to north and south and it ceases to be a highway for the western traveller. Beyond the bend an overland journey must commence. The Platte and Kansas and Arkansas all continue the general direction, but none is easily navigable. The emigrant must leave the boat near the bend of the Missouri and proceed by foot or wagon if he desire to continue westward. With the admission of Missouri in 1821 the apex of the frontier had touched the great bend of the river, beyond which it could not advance with continued ease. Population followed still the line of easiest access, but now it was simpler to condense the settlements farther east, or to broaden out to north or south, than to go farther west. The flanks of the wedge began to move. The southwest cotton states received their influx of population. The country around the northern lakes began to fill up. The opening of the Erie Canal in 1825 made easier the advancing of the northern frontier line, with Michigan, Wisconsin, and even Iowa and Minnesota to be colonized. And while these flanks were filling out, the apex remained at the bend of the Missouri, whither it had arrived in 1821. There was more to hold the frontier line at the bend of the Missouri than the ending of the water route. In those very months when pioneers were clearing plots near the mouth of the Kaw, or Kansas, a major of the United States army was collecting data upon which to build a tradition of a great American desert; while the Indian difficulty, steadily increasing as the line of contact between the races grew longer, acted as a vigorous deterrent.