My next post about the Geology of the National Parks Through Pictures is a park we visited in 2005 as I moved from Texas to New York.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
While driving up from Texas to New York we were able to visit the famous Gateway Arch that was in the former Jefferson National Expansion Memorial. Although there isn't much geology associated with the steel structured arch itself, there is always geology to talk about.
The Gateway Arch is clearly the centerpiece of the park, hence the name change. The Arch itself is 630 feet tall, 630 feet wide, and goes 60 feet into the ground. It is composed of steel and concrete. The outside skin is 1/4-inch stainless steel, followed by a layer of concrete, then 3/8-inch structural steel on the inside. The shape of the Arch is termed a catenary arch, which is the shape of a free-hanging chain when held at both ends.
As noted, the Arch descends 60 feet into the ground and is actually anchored within the St. Louis Limestone. The St. Louis Limestone is Mississippian in age (~335 million years old) and was formerly outcropped along the river's edge but extensive quarrying for use within the city has limited the availability. The St. Louis Limestone is a grey marine limestone that weathers to chalky white, sparsely fossiliferous, fine-grained, and occasionally dolomitic. Available fossils include bryozoan and echinoderm fragments. The entire city is underlain by the St. Louis Limestone, which can get up to 120 feet thick in places.
The Arch sits within the floodplain of the Mississippi River, where the floodplain is 6 to 10 miles wide. After the river front bluff of St. Louis Limestone was quarried away, construction began on levee in 1835. This enabled the waterfront to be used as a port where the goods on the larger ships from the lower Mississippi could be transferred to smaller ships for transport up the smaller upper Mississippi, Illinois, and Missouri Rivers. The original levee, made of mud, eventually was replaced with a levee made from the locally quarried St. Louis Limestone.
They have trams that can take you to the top of the Arch where you can hang out as long as you want, however the lack of bathrooms deems that eventually you will need to leave.
Looking west you can see the other two main buildings of the Gateway Arch National Park. Here is the Old Cathedral. From up close there are actually several building stones that are used in abundance in the building from the surrounding region. Started in 1831, the Cathedral took 3 years to complete, with new stones being added during later renovations. The sides and rear walls of the Cathedral are composed of the same St. Louis Limestone that the Arch is anchored in. These blocks were likely quarried from the nearby riverfront. The front wall and columns are of an unknown dolomite, however it is likely the Joliet limestone, a Silurian age (~430 million year old) massive, very fine-grained, tan colored stone. Windowsills throughout the cathedral are made from Mississippian age Salem Limestone (~335 million years old). These were stone products sold as "Indiana Stone" and were likely installed during renovations in the early 1960's. The steps leading up to the cathedral are crystalline pink Precambrian granite from the nearby St. Francois Mountains. These steps replaced the original sandstone steps back in the 1880's.