A Love of Bridges, Part 10: A Steely Study in Vermilion and Azure | Golden Gate Bridge, San Francisco to Marin County, California, USA (1937)

Taken from the eastern walkway, just south of the southern tower. And looking up at it!

For more on Golden Gate geology, see Part 8 of this set.

Walking across a beautifully engineered bridge is by no means the equivalent in danger to walking the plank of a pirate ship. Still, there's always a small voice in the back of one's head asking, "Will this thing really hold up until I'm across?"

Here are some splendid reasons it will hold up: the well-anchored tower, replete with Art Deco ornament, and its supporting stays and cables. All these are made of a geologically derived material that balances the immense stresses while yielding to the wind in proper proportion. This wonderful substance, of course, is steel, the very synonym of strength. It's the alloy of iron and carbon, and nowadays often of other elements as well.

As I mentioned in Part 9, steel was probably first discovered and utilized in Asia Minor, in the nineteenth century BC. However, its widespread use in architecture, bridge building, and other civil-engineering applications didn't occur, albeit in a nice numerical symmetry, until the the latter half of the nineteenth century AD.

Iron, the main constituent of steel, can be found in economically viable quantities in a number of different geologic settings. This should come as no surprise, because it's the second most plentiful metal in our planet's crust. Traditionally, it's been extracted from such oolitic sedimentary deposits as Wisconsin's Ordovician Neda Formation, and from wetland environments, where it was called bog iron.

In modern times, however, it mostly comes from the Earth's huge supply of Banded Iron Formations (BIFs), such as those found in the northeastern Minnesota and Michigan's Upper Peninsula. These strange and striking sedimentary beds, composed of alternating layers of red jasper and silvery hematite or magnetite, formed almost exclusively in Archean or Paleoproterozoic times, when our atmosphere had a considerably lower free-oxygen content.

While geologists still debate the details of how BIFs formed where and when they did, and why they are not forming now, they agree that there are two basic types. The Lake Superior variety was deposited over large areas—relatively undisturbed foreland or ocean basins. On the other hand, the Algoma BIFs occupied more restricted environments, where there was also a significant amount of submarine volcanic activity.

The other photos and descriptions of this series can be found in my Love of Bridges album.

Taken in: United States / California / Fort Winfield Scott (show map)

Taken on: July 1, 1971

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Camera: MAKER NAME 96660, ISO 50 (more info) (hide info)

Camera:	MAKER NAME 96660
ISO:	50
Flash:	Auto, Did not fire
Exposure Program:	Program AE
Metering Mode:	Average
White Balance:	Auto
Color Space:	Uncalibrated
Software:	Inpaint
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