The rolling rock leaves its scratches on the mountain.
—Ralph Waldo Emerson
It is evident that, perhaps thirty or forty thousand years ago, the site of Saint Paid was covered by a great ice sheet. It may be asked how we dare to make so strong a statement. When we see rabbit tracks in the snow, we know that a rabbit has been there. We can just as easily discern the tracks—we call them traces—of the great ice sheet. It has left unmistakable evidence of its presence. It would be interesting to take a trip about the city some day in search for the footprints of this ice mountain that could boast of ability to change the whole face of nature. We should find glacial scratches, glacial hills and lakes, dry old river channels, waterfalls, and many other evidences of a mighty hand at work. Some of the effects of this glacier are so perfect that they seem to have been done by an intelligent artist. These will be explained, and you will see them, too.
This glacier has been called mountainous. It is thought to have been two thousand feet high. It was formed in the very far North and very gradually made its way down into the temperate zone where it must finally have melted away. Generally speaking, this immense ice sheet extended on the East to the Ohio River and on the West to the Missouri. It advanced and melted and advanced repeatedly. In all there were six advances and six retreats. This process is supposed to have continued for more than 500,000 years.
The first ice invasion, although it extended farther south in Minnesota and Wisconsin, does not seem to have left any effect in Saint Paul. The second invasion, which came from the western shore of Hudson Bay, made grooves in the limestone (top layer of the bed rock) from one to two feet deep. Such grooves can be found in quarries that have not been filled in. It deposited, also, pebbles and boulders that came from Manitoba. Gravel deposited by this ice sheet is to be found on the bluffs of the Mississippi River south of Pickerel Lake and east toward the High Bridge.
After a long interglacial period there was another advance, called the Illinois invasion. There is likewise little, if any, definite evidence of its effects around St. Paul. Following this invasion, came the Wisconsin ice sheet. This ice sheet, as it evidently melted in the vicinity of St. Paul, left a ridge-like dump of boulders, gravel, sand, silt, and clay which were dropped from a point a little south of White Bear Lake to Minnetonka. This deposit is called a terminal moraine. This glacier brought a large mass of red-boulder clay (ground boulders) which is still seen in many parts of the city.The later and last Wisconsin ice sheet reached only the extreme northwestern part of St. Paul (St. Anthony Park), brought a thin gray layer of glacial drift (boulders, gravel, sand, silt, and clay), and moved northeastward to the St. Croix River.
If you wonder how it is possible to know that there were many glaciers and that they produced the effects ascribed to them, a little reasoning will show you. Remember what we said about the rabbit tracks. In all parts of the city is a more or less thick layer of boulders, gravel, sand, silt, and clay that is different from the bed rock below it. Sometimes this layer is only a few inches thick, as in the vicinity of Wabasha and West Seventh Streets. In other places, it is one hundred feet or more in depth, as near the Central High School and along Snelling Avenue from about Como Avenue extending some distance beyond Randolph Street.
This material is unassorted except where flowing water may have separated it according to size. These deposits are called glacial drift. But how do we know that glaciers brought them?
So far as known, there are only two ways in which we can account for their presence. Either they must be of the original bed rock and formed here, or they must have originated elsewhere and have been brought here. Because they arc different from the bed rock, we conclude that they must have ben transported from some other source. Then, if they were transported, how were they transported? What were the natural agencies that could bear and bring so vast and heavy a load?
Could wind have been the means? Not at all. Fine clay and sand are often carried by wind. Results of this kind are found in some of our western states and northern Africa. It would be impossible, however, for wind to move coarse gravel and large boulders.
Could flowing water produce these results? The finer materials, including sand, coarse gravel, and even boulders, might have been pushed along by a strong current, but it is inconceivable to believe that the gigantic rocks that weigh many tons could have been carried for considerable distances even by a mighty river. Water also would have sorted out the different materials. We must, therefore, find some other solution than flowing water.
The only other natural transporting agency is ice. Will it satisfy the requirements? A broad and deep sheet of ice could carry boulders weighing many tons. Ice alone, too, because it would melt, and as it melted, would dump its load irregularly and without any attempt at assortment, just as we find it. If we accept this theory, it is easy, then, to conjecture how tremendous and powerful these glaciers must have been. From other facts, too, we can tell in what directions they moved and prove that the agency was ice.
An examination of boulders in the vicinity discloses the fact that they usually have one or more flat surfaces. Motion under a heavy glacial load made them flat-sided. If water had been the agency, it would have turned them over and over and tended to make them round. It can easily be seen that rocks would be likely to sink to the bottom of an ice sheet and to scrape along other hard surfaces.
Then the scratches on the bed rock, sometimes a foot or more in depth, so deep that we may call them furrows, are conclusive proof that some ponderous moving agency carrying on its under side rough substances, which must have been rocks, cut these furrows with irresistible force.
Except for a few sand hills (dunes) in St. Anthony Park, there are just two kinds of hills in and about St. Paul—river bluffs and glacial hills. The river bluffs skirt the Mississippi River and the lower part of Phalen Creek. All the other hills are of glacial origin; that is, they were dumped where they are by melting glaciers.
Broad streams carried some of the terminal moraines away and spread them at a distance in level deposits, called out-wash plains. Riverside, across the Mississippi, Dayton's Bluff, Arlington Hills, and the Merriam Park district are examples. In Hamline there is a red out-wash plain. To the north is a gray, glacial, gravel plain. Lake Phalen and Lake Como are of glacial origin. Their basins were formed by rings of glacial hills. They are, therefore, called rim-lakes.
It would be difficult to picture a bird’s-eye view of the land in this vicinity before the great glaciers transformed it, but it will be easier to gather some conception of the work they did and of the importance and value of their mission. In these we see more of that supreme wisdom that molded and furnished the earth for the dwelling and use of man.
QUESTIONS
How can we tell that glaciers were here?
How can we tell in what direction they moved?
Where in St. Paul can we find mixed gravel pita?
How could a glacier make a lull? A lake?
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