12 GUIDEBOOK OF THE WESTERN UNITED STATES. 
due to the invasion of this country at various epochs in the past by 
great glaciers which filled existing valleys with the mud, sand, and 
gravel that they brought in from the north. (See footnote on pp. 
26-30.) When the ice vanished the streams found different courses, 
developing lakes and falls, features characteristic of a newly estab- 
lished drainage system. 
Thus the original drainage about St. Paul consisted of a stream 
flowing southeastward through the city and following the present 
course of Mississippi River to a point 10 or 12 miles below St. Paul, 
where it was joined by a large stream from the west, which crossed 
the present river valley 4 or 5 miles above Fort Snelling. At that 
time there was no stream flowing from Fort Snelling to St. Paul. 
This area was then a rocky upland. 
When the western ice sheet (see route map, sheet 1, p. 20) melted 
back from this region, all these old stream channels were filled with 
glacial materials, and the principal streams were developed along 
their present courses. After a time the glacier, which came from the 
north or northwest, melted back into the Red River valley, and 
Lake Agassiz (ag’a-see) came into existence. (See footnote on p. 32.) 
Warren River, the outlet of this large body of water, occupied the 
present valley of Minnesota River and received Mississippi River as 
a tributary from the north. This great river flowed across the rocky 
upland between Fort Snelling and St. Paul and cut a broad valley, 
upon the floor of which St. Paul has been built. Where it joined 
the old valley, at the site of St. Paul, there was a waterfall many 
times the size of St. Anthony. This fall gradually receded upstream 
until it reached the other old valley above Fort Snelling, and thus 
the inner rock gorge shown in the diagram was cut. 
The gorge of the Mississippi from Fort Snelling to the Falls of St. 
Anthony, which are a short distance below the station in Minneapolis, 
shows from its extreme narrowness that it is new and that it has 
only recently been carved out of the solid rock by the swiftly flowing 
stream. In fact, the process of carving the gorge of the Mississippi 
is still in operation, or was before it was arrested by the hand of man. 
1 Many take it for granted that the sur- 
face features of the earth have always been 
the same as they are to-day, that the val- 
leys have always been valleys and the 
hills and mountains always eminences 
overlooking them. The incorrectness of 
* such an assumption, however, can 
realized by watching: any small stream or 
rill after or during arain. It will be seen 
that the stream is busily engaged in cut- 
ting the sand or clay over which it flows 
and carrying it down the valley to be 
dropped where the current slackens in 
some pond or lake. Every > as 
it is loosened from the bed of the 
is carried or rolled along by . water a 
few 
be | because ek 
farther, as a tired man lays down his bur- 
denamomenttorest. By dropping many 
grains the power of the water is restored 
and the grain of sand is again picked up, 
