The buffaloes migrated annually from the north to the south, and throughout their habitat in the United States, their great trails were north and south trails. The rivers flowing mainly east or west into the Mississippi are crossed usually at right angles by the more important trails of the buffalo. The annual movement was caused not so much by the change of temperature (though buffaloes which remain sometimes in cold climates seek the warmer, secluded spots) as by the frozen condition of the ground and the depths of snow which buried the grasses upon which they fed. When, from various causes, the annual north and south migrations of the buffalo herds of the Far West were discontinued, an east and west migration took place--the herds moving westward to more protected portions of the country. As late as 1872, hunting parties made their headquarters during the summer at Hay's City, and in winter moved their quarters a hundred or a hundred and fifty miles westward of their fall camps; near Hay's City the grass was buried under ice and encrusted snow, while near Ellis the ground was bare. Thus unnaturally the migrations were turned east and west rather than north and south, and the trails which marked the former lines of migration were cut by deep-worn trails crossing them at right angles.[123] During the reign of the buffaloes in the Ohio basin their greater thoroughfares were undoubtedly made by their annual migrations, even though the extent of this movement did not exceed a few hundred miles. In this day the winters are appreciably milder along the Ohio river than even in the northern portions of the states of which it forms the southern boundary. And here, as in the Far West, the routes of the buffalo are north and south with here and there a great cross trail. These greater trails lay largely on the watersheds which the buffalo found with great certainty. He was an agile climber despite his great size and weight. Writes Mr. Allen, "They will often leap down vertical banks where it would be impossible to urge a horse, and will even descend precipitous rocky bluffs by paths where a man could only climb down with difficulty, and where it would seem almost impossible for a beast of their size and structure to pass except at the cost of broken limbs or a broken neck. On the bluffs of the Musselshell river I found places where they had leaped down bare ledges three or four feet in height with nothing but ledges of rocks for a landing-place; sometimes, too, through passages between high rocks but little wider than the thickness of their own bodies, with also a continuous precipitous descent for many feet below. Nothing in their history ever surprised me more than this revelation of their expertness and fearlessness in climbing."[124]