Jason M. Barr July 29, 2019
Note: This two-part series is adapted from chapters seven and eight of my book, Building the Skyline: The Birth and Growth of Manhattan’s Skyscrapers (OUP, 2016), which is based on research (here and here) performed with Troy Tassier of Fordham University.
The Bedrock Myth
There’s a popular historical anecdote about Manhattan that’s frequently bandied about at cocktail parties and on hop-on-hop-off bus tours. It has to do with why skyscrapers are “missing” between Downtown and Midtown. In particular, the story says that Manhattan’s skyline is the way it is because of geology.
Since skyscrapers are so tall and heavy, they require proper foundations to prevent them from leaning, sinking, or cracking. One possibility is to anchor them directly to the bedrock. Manhattan Island—really just one big 13 mile (21 km) long boulder—would be ideally suited to support millions of humans and their structures.
But the bedrock is not flat. Starting at the southern tip of Manhattan, it is relatively close to the surface. Going north, say along Broadway, it begins to dip, and by the time you get to City Hall, a mile (1.6 km) to the north, the rock floor gives way to a valley, far below the streets. Then at about 14th Street the bedrock approaches the surface again, where it remains relatively close for the rest of the island. In parts of Central Park, the bedrock crops out above the ground, making for fun summertime climbing.
Correlation not Causation
The skyline tale says that developers naturally gravitated to those parts of the island—Downtown and Midtown—where bedrock was accessible. And, they avoided places in between where the rock was, presumably, too far down. Let me be clear: this notion is false–a case of confusing correlation with causation. It is true that skyscrapers more frequently appear where the bedrock is relatively close to the surface, but it is incorrect to conclude that access to bedrock was the reason. This is a myth. It perpetuates itself because it is constantly repeated by seemingly authoritative people—geologists and tour guides, in particular. It is appealing because, like all founding myths, it helps connect the dots between the great powers of heaven and earth and our humble lives.
But how did the myth begin? It seems it was first written down by a well-known New York geologist, Christopher Schuberth, who, in his 1968 book, The Geology of New York and Its Environs, took some liberties by steering away from his expertise to write about urban land use:
South of 30th Street…the mica schist, however, is still accessible in deep foundations or excavations. But when one reaches the north side of Washington Square in Greenwich Village, the situation changes rather abruptly. Here, near New York University, the surface of the bedrock drops appreciably—up to several hundred feet below the street level. Farther south, near Chambers Street, it comes again to within about one hundred feet of the surface. Dividing Manhattan into the “downtown” and “midtown” districts, the low-lying area—the area where the top of the bedrock lies hundreds of feet below the street level—has been filled in and built up almost entirely with glacial deposits and, more recently, artificial land fill. That is to say, the skyscrapers of New York City are clustered together into the midtown group, where the bedrock is within several feet of the surface, and the downtown group, where the bedrock again reappears to within forty feet of the surface near Wall Street…..In any event, it is readily seen how clearly the accessibility of the bedrock has, to some degree, controlled the architectural planning of the city.
From there, the great aha! went viral as the truth (to him) was “readily seen.” But to see two things together does not mean there is a connection between them. When one takes the time to review the evidence, the “facts,” like touching gossamer, dissipate in one’s hands.
Economics: The Real Story
The real story is not geology, but economics. As I have documented in another post, the key to understanding why skyscrapers are built begins with a comparison of the costs and benefits that arise from their construction. The benefit is the value created. Corporations, for example, pay generous rents for the ability to shelter their employees. Today, office rents in Manhattan average $77 per square foot ($829 per square meter) per month. For a one million square foot (92,903 square meter) skyscraper, well, that’s a lot of moolah. But to get it, the developer must first pay for the land and the cost of construction. Thus, the location and height of a skyscraper are determined, fundamentally, by the income relative to the cost of producing it, or, in real estate parlance, the return on investment.
With this in mind, let’s go back in time, to about 1885, when the notion of how to build skyscrapers was just dawning on the real estate community. Lower Manhattan was, arguably, the densest business district in the world. This clustering was bidding up land values, which was signalling to landowners that the demand for those locations was rising fast. This, in turn, incentivized developers to find innovative solutions to “make the land pay,” and get access to the revenues.
But, of course, they had to contend with the geology. The myth states that since the bedrock is just below the surface in lower Manhattan, skyscrapers were simply thrust down upon it like an empty beer can on a table. But this is not true. In fact, Downtown, from Wall Street to City Hall, has some of the worst geological conditions on the island. Builders in lower Manhattan built skyscrapers not because of the geological conditions, but despite them!
Watch Out for the Quicksand!
Just below the ground level, atop the bedrock, sit uneven layers of sand, gravel, and clay, with large boulders scattered throughout. Beneath the waterline, the sand morphs into a wet, viscous goo, known as quicksand. Anyone who’s watched the 1960’s television comedy, Gilligan’s Island knows that quicksand is a trap, instantly enveloping whatever falls into it. This was a major problem for developers in lower Manhattan. They couldn’t just dig their way to bedrock because once they hit the quicksand, it would flow back into the construction site (also dooming the foundations of surrounding buildings.)
The solution was the caisson—a large cube with no bottom. Workers inside the cube (“sandhogs”) would dig up the soil and pass it up through shoots to the surface. The box would slowly sink. Once it hit the quicksand, compressed air was pumped in to keep the soil out. Eventually the box would reach the bedrock. It was then filled with cement, and piers were built atop it up to the basement level to hold up the structure.
Men of Robust Physique
But make no mistake: using caissons was expensive and dangerous. In 1912, The New York Record and Builder’s Guide commented:
Builders sometimes ask why foundation work costs so much in New York. The answer is: The risk is so great no matter how well trained the units of a foundation company’s working organization may be, mistakes are sure to happen at times. No matter how carefully the site may have been bored, boulders or quicksand are liable to appear….All perilous work is expensive, because men of robust physique and of sufficient bravery hourly to risk death are hard to find.
Skyscrapers and Bedrock in Lower Manhattan
But, upon further inspection, the “perilous work” to get to the bedrock in the worst areas was not a barrier. How do we know? Because, using geological maps, we can ascertain the bedrock depths below the earliest skyscrapers. And what do we see? A few of the city’s tallest skyscrapers, in fact, were built over some of the deepest bedrock; suggesting that skyscraper builders were able to overcome the geological obstacles put before them, and were not scared off, as the myth implies.
The figure below shows the depth to bedrock below a sample of buildings in Manhattan as of 1915 (the vertical axis) versus its location relative to the southern tip of the island. The triangles are skyscrapers (80 meters/242 feet or taller) and the circles are (randomly chosen) low-rise buildings. The hump shows that as we go from south to north the bedrock depths increase into a valley. Then, continuing north, the rock moves closer to the surface.
We can also see that a few skyscrapers, most notably the Woolworth Building (1913, 55 stories) and the Municipal Building (1914, 40 stories)–the highest triangles in the figure just above the City Hall indicator–were constructed over some of the worst geological conditions. The graph shows that as lower Manhattan’s bedrock got deeper, high-rise builders were not deterred. But, just as importantly, on the north side of the valley (between Canal and 14th Streets), there are no skyscrapers where the bedrock is relatively accessible. If bedrock depths were key, we would likely see them where it was easy to build. Rather, we see none. The reason was the lack of demand for such projects (discussed below).
The Construction Costs
The bedrock myth ignores the cost of construction in lower Manhattan. But the terrible conditions below the surface made them high, in both blood and treasure. Then, why did builders persist? Why was the Woolworth Building—the world’s tallest building when completed in 1913—built over some of the deepest bedrock in the city? Because the rewards were so great.
Once the decision to build a skyscraper was made, the foundation costs, as a fraction of the total construction costs, were not that high. The bulk of the caisson costs, for example were fixed and associated with planning, building the boxes, setting up the air compression equipment, hiring medical professionals to help workers from the bends, and so on. Once the box was placed on the ground, the difference in costs for going from say 20 feet (6 meters) to 40 feet (12 meters) down was relatively small.
Costs versus Land Values
Given the low costs of sinking the caissons further down, we can do some back-of-the-envelope calculations to see where a skyscraper developer would have built if he were only concerned about cost minimization. During this period—in 1905, for example—average land values were about $72 per square foot ($775 per square meter) south of City Hall, and $23 ($248 per square meter) just north of it. Say we take a plot of land suitable for a skyscraper of 250 feet by 100 feet (2,323 square meters). Using these average prices, would yield a land cost of $1.8 million downtown, but only $575,000 north of City Hall. This means that a developer could have saved a bundle on land costs by moving his project northward. The question then becomes: do the extra costs of getting to bedrock in the north exceed the savings in land costs? The answer is a definite no.
A statistical analysis reveals that once a developer decided to use caissons, the additional cost of sinking the boxes another foot was about $3,000 (about $9900 per meter). So, let’s say, in an extreme case, the difference between the bedrock depths on the southern and northern lots was 50 feet (15 meters), this would yield an extra cost of about $150,000—far below the difference in land costs! An enterprising developer could have saved substantial sums by buying a plot along the bedrock valley at a much lower price and paying the additional costs of digging to the bedrock. But none did so. Why not?
The Densest Place on Planet Earth
Just north of City Hall were the densest immigrant neighborhoods on Planet Earth. In 1900, one block held 1200 people per acre! These residents were mostly newly arrived European immigrants, who were unwilling and unable to pay for high-quality space. The majority of them could only afford a tiny tenement apartment with few amenities. For the landlord, the most profitable form of real estate was the five-story walk up tenement. They were easy to construct from off-the-shelf blueprints. The sand below the ground was strong enough to hold up a low-rise, without creating additional expenses.
But, most importantly, the tenement was the structure with the most demand, and thus had the highest return on investment. A skyscraper developer could have put up a 20-story high-rise for a lower total cost there, as compared to Wall Street. But who would pay the rents needed to make it profitable? Nobody. Immigrants couldn’t afford them, and banks and brokerage houses would not move to a crowded residential district far away from the business action.
So next time someone tells you the skyline is the way it is because of the bedrock, here are the responses you can give. First, depth to bedrock is irrelevant if there’s a bed of quicksand floating on it. Lower Manhattan, below the waterline, is a viscous lake of wet sand, which required expensive foundation technologies—most notably caissons—to anchor the buildings to bedrock. So Downtown was not, in fact, some geological paradise, but rather presented a treacherous journey for the adventurous builder.
Second, a few of the world’s tallest buildings at the time—most notably the Woolworth Building (193, 55 stories)—were built over some of the deepest bedrock in the city. Bedrock depths were not, in fact, a true barrier to construction. The high costs of construction were willingly paid to get the reward of high market rents where businesses were willing to pay them.
Developers avoided building over the deepest bedrock just north of City Hall due to a coincidence. Given Manhattan’s particular economic and demographic history, the area with the deepest rock also just happened to have the most crowded immigrant neighborhoods on Planet Earth, where there was no demand for tall buildings. If there was, a developer could have paid the extra costs of digging the caissons deeper to earn the extra revenue. However, there was no money in skyscrapers, just in five-story tenements.
But Why Midtown?
But we still haven’t addressed the question of why skyscrapers appeared in Midtown at the dawn of the 20th century. The reason, though, has nothing to do with bedrock, and everything to do with economics and demographics. This story will be taken up in the next post.
I would like to thank Maria Thompson for valuable editorial assistance.
 The average apartment rent in New York City is about $3000 per month for a one-bedroom apartment. Assuming the average one bedroom unit is 750 square feet (70 square meters) then the average renter pays $4 per square foot per month (or $43 per square meter).