A "Chicago in Stone and Clay" Companion, Part 5: The Two Generations of Prudential Plaza
This series complements my award-winning guidebook, Chicago in Stone and Clay: A Guide to the Windy City's Architectural Geology. Henceforth I'll just call it CSC.
The CSC section and page reference for the building featured here: 5.7; pp. 47-50.
Looking north-northeastward from the western side of Michigan Avenue, a little north of Monroe Street.
Just to be clear, Prudential Plaza consists of two skyscrapers, predictably named One and Two Prudential Plaza, and the thin slices of open space below them.
"One Pru," known simply known as the Prudential Building before "Two Pru" was erected, is a standard expression of 1950s Modernism, while its younger and taller sibling with the rocket-ship shape is an equally standard expression of late-twentieth-century Postmodernism. We'll focus on the latter later in this series. For now we turn our attention to One Pru.
From this distance down on Michigan, the boxy high-rise with the huge antenna seems to have an exterior of two different materials: something metallic, and a buff-colored something-or-other that might be stone. And when we go to maximum magnification of the image, we see that the buff stuff is not just on the side elevations. It also forms vertical spacers between the windows and metal spandrels of the facade.
The metal, it turns out, is aluminum, the most abundant metallic element in the Earth's crust. Nowadays it's most frequently extracted from a strange-looking ore rock called bauxite. In posts to come we'll see how the aluminum here has been crafted to provide an ornamental effect visible at closer range.
And our guess that the buff-tinted material is some type of rock is completely correct. Specifically, it's the most widely employed rock in American architecture—the Salem Limestone. This selection, quarried in southern Indiana, has been super-popular with designers ever since the Gothic Revival and Richardsonian Romanesque styles held sway in the closing decades of the nineteenth century.
Renowned for its great workability and its capacity for taking and holding fine carved detail, the Salem is a carbonate classified by sedimentary petrologists as a grainstone and biocalcarenite. In essence, these terms indicate that it's composed of particles, mostly sand-sized, which are fossil fragments of marine-organism hard parts.
As I often point out in my Stone and Clay books, given the ubiquity of the Salem in the built landscape of Chicago and Milwaukee, this rock originated when what we now call the American Midwest was largely covered by an epeiric (continent-covering) saltwater sea. This was about 345 Ma ago, during the Mississippian subperiod of the Carboniferous.
In those days, what is now the lower portion of the Hoosier State was an interface between the land and that sea. And just offshore, in the biologically abundant shallows of shoals, tidal channels, and lagoons, the broken remains of ancient creatures tossed and tumbled by currents and surf came to rest on the bottom. There they were ultimately bound together and lithified with calcite cement.
And incidentally, if you've never heard of the Salem before and thought One Pru is clad in Bedford or Indiana Limestone instead, just take a stress pill and relax. Those two monikers are the Salem's most common trade names. "Salem" itself is a reference to the rock's geologic source, classified by stratigraphers as the Salem Limestone formation.
For much more on the site touched upon here, get and read Chicago in Stone and Clay, described at its Cornell University Press webpage.
The other photos and discussions in this series can be found in my "Chicago in Stone and Clay" Companion album. In addition, you'll find other relevant images and descriptions in my Architectural Geology: Chicago album.
A "Chicago in Stone and Clay" Companion, Part 5: The Two Generations of Prudential Plaza
This series complements my award-winning guidebook, Chicago in Stone and Clay: A Guide to the Windy City's Architectural Geology. Henceforth I'll just call it CSC.
The CSC section and page reference for the building featured here: 5.7; pp. 47-50.
Looking north-northeastward from the western side of Michigan Avenue, a little north of Monroe Street.
Just to be clear, Prudential Plaza consists of two skyscrapers, predictably named One and Two Prudential Plaza, and the thin slices of open space below them.
"One Pru," known simply known as the Prudential Building before "Two Pru" was erected, is a standard expression of 1950s Modernism, while its younger and taller sibling with the rocket-ship shape is an equally standard expression of late-twentieth-century Postmodernism. We'll focus on the latter later in this series. For now we turn our attention to One Pru.
From this distance down on Michigan, the boxy high-rise with the huge antenna seems to have an exterior of two different materials: something metallic, and a buff-colored something-or-other that might be stone. And when we go to maximum magnification of the image, we see that the buff stuff is not just on the side elevations. It also forms vertical spacers between the windows and metal spandrels of the facade.
The metal, it turns out, is aluminum, the most abundant metallic element in the Earth's crust. Nowadays it's most frequently extracted from a strange-looking ore rock called bauxite. In posts to come we'll see how the aluminum here has been crafted to provide an ornamental effect visible at closer range.
And our guess that the buff-tinted material is some type of rock is completely correct. Specifically, it's the most widely employed rock in American architecture—the Salem Limestone. This selection, quarried in southern Indiana, has been super-popular with designers ever since the Gothic Revival and Richardsonian Romanesque styles held sway in the closing decades of the nineteenth century.
Renowned for its great workability and its capacity for taking and holding fine carved detail, the Salem is a carbonate classified by sedimentary petrologists as a grainstone and biocalcarenite. In essence, these terms indicate that it's composed of particles, mostly sand-sized, which are fossil fragments of marine-organism hard parts.
As I often point out in my Stone and Clay books, given the ubiquity of the Salem in the built landscape of Chicago and Milwaukee, this rock originated when what we now call the American Midwest was largely covered by an epeiric (continent-covering) saltwater sea. This was about 345 Ma ago, during the Mississippian subperiod of the Carboniferous.
In those days, what is now the lower portion of the Hoosier State was an interface between the land and that sea. And just offshore, in the biologically abundant shallows of shoals, tidal channels, and lagoons, the broken remains of ancient creatures tossed and tumbled by currents and surf came to rest on the bottom. There they were ultimately bound together and lithified with calcite cement.
And incidentally, if you've never heard of the Salem before and thought One Pru is clad in Bedford or Indiana Limestone instead, just take a stress pill and relax. Those two monikers are the Salem's most common trade names. "Salem" itself is a reference to the rock's geologic source, classified by stratigraphers as the Salem Limestone formation.
For much more on the site touched upon here, get and read Chicago in Stone and Clay, described at its Cornell University Press webpage.
The other photos and discussions in this series can be found in my "Chicago in Stone and Clay" Companion album. In addition, you'll find other relevant images and descriptions in my Architectural Geology: Chicago album.