Thou sure and firm-set earth.
—Shakespeare
Glacial drift, as explained before, consists of clay, sand, gravel, and boulders. Under the glacial drift is solid rock. In some places the solid rock is only a few inches below the glacial drift. In other places it is several hundred feet below. This solid rock is called bed rock. Bed rock is found everywhere upon the land and even on the sea bottom. It has been calculated by scientists to be between 40 to 50 miles thick. All the loose material that we call clay, sand, gravel, and boulders is nothing but broken-up or decayed bed rock. In St. Paul bed rock can be seen along the banks of the Mississippi, the channel of Phalen Creek, some of the streets that run from Pleasant Avenue to St. Anthony Hill, on Oakland Street, and in other places. On the West Side (Riverview) there are many places where bed rock can be seen, especially on South Wabasha Street.
Wherever the bed rock comes to the surface, so that it can be seen, we speak of it as an outcrop. In the places just mentioned there are such outcrops.
In St. Paul there are only two kinds of bed rock that crop out. The topmost layer is Trenton limestone, and the one under it is St. Peter sandstone. The Trenton limestone has recently been divided into three distinct layers: Galena limestone, Decorah shale, and Platteville limestone.
From 10 to 12 feet of Galena limestone, consisting of limestone and shale, can be seen on both sides of the St. Paul Gorge, but particularly at the south side of the High Bridge near the brickyard. The Galena limestone is largely a conglomerate (cemented pebbles) of black-coated limestone pebbles. This structure goes to show that the original unbroken limestone was softened and more or less broken up by the weather. The rock pieces were then rolled along on the bottom of flowing water, and, being knocked both against one another and the rocky river bed, lost their corners and sharp edges and became rounded. The cement that fastened the pebbles together is dissolved limestone.
In the lower part of this formation are many fossils, among others corals, which are extinct. The fact that corals are found in the Galena limestone shows that at the time when the limestone was laid down the site of St. Paul was covered by a sea.
Fossils are nothing else but imprints, impressions or traces of ancient animals or plants in bed rock. When these animals or plants died, thousands of years ago, the rock in which they are now found was still soft. The dead animals and plants fell on the soft rock and made an imprint of themselves in it. In case the animals and plants were left uncovered by sand, mud, or clay, they decayed and left nothing but their bones. The bones also decayed if exposed to the weather for a long time. If, however, the animal or plant was covered up air-tight with clay or sand of some depth, the mold of the plant or animal was preserved and filled with mineral matter by water to form a perfect cast. The fact that these are marine plants and animals is conclusive evidence of the presence of a sea at the time of formation.
Decorah shale crops out on both sides of the St. Paid Gorge. The largest outcrop is near Pickerel Lake, not far from the south end of the High Bridge. The formation is some 50 to 60 feet thick (wherever it has not been removed) and consists largely of shale (compact layers of clay), though the shale is intermixed throughout by layers of limestone. The whole formation, especially the limestone, is rich in animal fossils, thus proving the presence of a former sea.
The Platteville Limestone is about 30 feet thick, and is practically all solid limestone, except its lowest layers, in which it gradually changes into shale. It is exposed on both sides of the St. Paul Gorge and is found in St. Paul proper from Wabasha Street along West Seventh Street to Fort Snelling. Marine life was abundant when this rock was formed in a somewhat shallow sea. Marine algae (a low order of plants) are very common. The animal life included many crawling, swimming, and burrowing wormlike creatures.
The St. Peter sandstone about 150 feet thick is immediately under the Platteville limestone. It is a very white sandstone, consisting of well-rounded sand grains that are barely cemented together. This rock is, therefore, rather porous and easily breakable. It is one of the purest, most unmixed rocks of this vicinity, being more than 99% pure silica, from which glass is made. The rock comes to the surface near Wabasha Street Bridge on both sides of the river,near Carver's Cave, Indian Mounds Park, and various other parts of the St. Paul Gorge. Numerous fossils of marine mollusks (shell animals) are found in this formation, among others the well-known orthoceras. The fossils found, however, are mere imprints, as their shells (fossil shells are sometimes preserved with very little change) have been dissolved by waters coming from the surface above in the same way as all other limestone matter which had been originally in the rock. Good places for finding the fossils are Dayton's Bluff, Highwood, and South St. Paul.
Below the St. Peter sandstone there are many layers of limestone and sandstone, indicating that wherever there is limestone the site of St. Paul was under a sea about 2,000 feet deep, and that wherever sandstone is found the site presented a shoreline. The fact that there are these different layers of bed rock indicates that wherever limestone is found the site was sinking and wherever sandstone is found it was rising. Thus this site has been repeatedly a moderately deep sea, a shallow sea, and land. These changes must have occupied millions of years.
The abundance of glacial drift in St. Paul furnishes an almost limitless supply of sand and gravel, so necessary for building houses and for the construction of highways. All
the sand and gravel necessary for the making of mortar, cement, and concrete is abundant, as shown by the many large and small gravel and sand pits in and about the city. The red glacial drift, brought here by the next to the last ice invasion, is best for building purposes, because it is practically free from limestone pebbles and broken-up shale.
The shale deposits of the Galena limestone and Decorah shale are used for the manufacture of brick and tile. Platteville limestone has been extensively quarried for building purposes along both banks of the St. Paul Gorge, especially in the West Seventh Street district. Before the use of cement for foundations, Platteville limestone was about the only material used for this purpose in St. Paul. In the Wahasha-West-Seventh-Street district, Platteville limestone is covered with little or no glacial drift and excavations for cellars must be blasted out of the formation.
As the sand grains of the St. Peter sandstone are round and polished, the rock is not very suitable for cement work. It is, however, extensively used for molding sand in iron foundries and as such furnishes a commercial article of some importance. It has also been found suitable for the manufacturing of sand-lime brick. As it is practically free from iron and lime, it can be used to great advantage for the making of fine glass and pottery. This sandstone is used by the Ford plant to manufacture glass for automobiles.
The lowest bed rock is granite, which must at one time have been molten. The different layers of limestone and sandstone on top of the granite have been formed in later geologic times from igneous rocks (granites and lavas) such as outcrop respectively at St. Cloud and Taylors Falls. The original rock was gradually broken up by such various forces of Nature as water, ice, and freezing.
Within more recent times the whole region has been subjected to several ice invasions. As shown before, the most striking events that took place during and after the ice age are, first, the deposition of glacial drift and glacial hills; second, the formation of a number of lakes; and, third, the work of River Warren in producing the River Warren Falls, the St. Paul Gorge, the lakes in St. Paul and vicinity, and the many rock terraces of the city.
QUESTION'S
Describe the principal kinds of bed rock.
In what order do they lie?
How could the lower rocks crop out?
What evidence is there that St. Paul was once covered by a sea?
Try to find places where the three kinds of rock may be seen.
No comments:
Post a Comment