There is abundant and unassailable evidence that at one time, ages ago, a vast ice sheet covered the whole of the northern part of North America, extending south in Illinois to a point between latitudes 37 and 38. This is the most southerly point to which the ice sheet reached. From this point the line of extreme flow runs off in a northeasterly and northwesterly direction. The northeasterly line is through southeastern Ohio and Pennsylvania, striking the Atlantic Ocean about at New York, thence through Long Island and up the coast of Massachusetts. Northwesterly it follows the Mississippi River to its junction with the Missouri, which it crosses at a point some miles west of this junction, following the general course of this river a little south of it through the States of Missouri, Nebraska, Dakota, and Montana. The lines, especially the northeasterly one, are very irregular, shooting out into curves and then receding. This line of extreme ice flow is marked by glacial drift so prominently that no one who has studied glacial action can doubt for a moment what was the cause of these deposits. The line is called the "terminal moraine." By examining a map of North America and tracing the line of the moraine as we have described it, it will be seen that about two-thirds of North America was at one time covered with ice to a greater or less depth. How deep, is simply a matter of conjecture, but in the central portions of the great glacier, where was the bulk of snowfall, it must have reached a depth of several miles to account for the enormous pressure that would be required to carry the ice so far southward.