374 Story of the Mississippi-Missouri .— Clay pole. 
independent delta but these are now buried and lost in the 
greater one formed by their union. 
Hundreds of thousands, perhaps millions of years have 
passed away since the Mississippi-Missouri river was finally de¬ 
veloped and complete. All the later Tertiary ages saw it in per¬ 
fect running order, mainly as it exists in our own time, except 
near its mouth where the annexation of the great southwestern 
streams has been in process. The Pliocene age in North Amer¬ 
ica seems to have passed quietly without any of those special 
changes which were witnessed by the ages that immediately 
preceded it. But our river-system did not pursue the even 
tenor of its way without disturbance and interruption even 
after its development was complete. Once at least, probably 
twice, and possibly several times has its domain been invaded 
by the icy hosts of the frost-king. Creeping slowly down from 
the high lands of Canada they have overrun the northern 
states carrying ruin and destruction before them. Slowly 
they marched forward sweeping the very soil clean beneath 
them and burying all under one white deluge of ice and snow. 
Dispute as we may about the causes of the ice-age there is no 
ground for doubting its reality. The invasion from the north 
spread farther and farther southward until all New England 
was covered. It gained allies from the then snow-clad Adir- 
ondacks and the two crept on, crossed the great lakes, landed 
on their nearer shores and slowly overwhelmed the adjacent 
country. Ohio, Indiana, Illinois, Michigan, Wisconsin, Min¬ 
nesota, Dakota, Nebraska, Iowa and Missouri disappeared 
under the ice-sheet. The slopes of the western mountains 
sent down their reinforcements and the states along their east¬ 
ern flank were buried beneath the glacier. More than half the 
continent disappeared and the entire basin of the Mississippi- 
Missouri was blotted out, except possibly its mouth. The 
Ohio then for a time resumed its rank as the great river of the 
continent, but even this was probably subject to more or less 
interruption. 
Such was the condition of North America during the ice-age. 
In all probability this inroad occurred twice for there is very 
strong ground for believing that the glacial-era consisted of 
two extreme periods separated by an interval when the cold 
was less severe. It is also probable that the extent of the 
.glacier was less in the second onset than in the first. The 
