Billions of years of bombardment by space rocks of all sizes have left our planet remarkably unscathed, yet if one looks closely the traces of enormous ancient impact craters are all too obvious. Or are they? Hudson Bay’s Nastapoka Arc may superficially resemble the many frozen lava “seas” of the moon but its actual origins are much more down to Earth.
Great Whitewater North
Hudson Bay, known to many from its prominent place in the center of Canada (and the Canada Dry ginger ale label), was first discovered by European explorers on this date in 1610. Henry Hudson had thought he had found the fabled Northwest Passage but ended up being abandoned by a mutinous crew who didn’t share his desire to confirm the possibility.
Hudson lost out twice: this seemingly endless body of water was merely an enclosed bay, and although Hudson is memorialized by having it named for him, he received nary a farthing for the naming rights. We can possibly add a third fail, as Hudson sailed across the huge Nastapoka Arc without even realizing it. Oh Henry!
Looking at a map of Hudson Bay one quickly notices two salient features. One is James Bay, a southeastward-pointing fingerlike projection. The other is the Nastapoka Arc, a strikingly semicircular stretch of coastline that, if expanded beyond its over 155° natural arc, would form a nearly perfect circle some 280 miles (450 km) in diameter.
Knowers Arc?
The Earth bears the scars of hundreds of meteor and asteroid impacts, most of them well under a mile or so in diameter. Where are the really big impact craters, like those so plainly visible on the Moon? Most have vanished due to the actions of weathering, erosion, glaciation and plate tectonics over hundreds of millions of years, and most of the larger impacts occurred very early in the Earth’s history.
When a large crater is discovered, it’s often through the analysis of magnetic and/or gravitational anomalies that reveal subsurface evidence of the impact. If any surface features are found, time has distorted them such that their relation to an ancient impact would not be guessed at without the invisible data. A prime example is the Wilkes Land Crater in Antarctica, a 300-mile (480 km) wide basin estimated to be up to 500 million years old.
Assuming the Nastapoka Arc is the remnant of an ancient asteroid impact, it sure doesn’t act like one! Only two aspects of the area ring meteoric alarm bells: the exquisite, cookie-cutter sweep of the shoreline and the offshore Belcher Island archipelago which sits just about where a large crater’s central peak would be found. There’s no raised crater rim, though grounds could be made for repeated episodes of glaciation having shaved it flat.
Here’s another neat anomaly about the Nastapoka Arc. Early mapmakers had neither the knowledge or the instruments required to draw accurate maps, and so many early maps show geographic features in a rounded, less fractal-like style (see 1886 map, above left). As time passed and better maps became available, however, the Nastapoka Arc seemed to regress to a more rounded configuration. This led to a number of geographers, geologists and astronomers to wonder if the feature’s origins didn’t evolve, well, naturally.
Lunar Tunes
I mention astronomers because the emergence of modern maps roughly coincided with the first clear photographs of our near neighbor in space, the Moon. Our battered satellite displays an abundance of impact craters, some of which were later flooded by lava and became the Lunar Maria. The features were so named due to their dark tints, homogenous surfaces and distinct “shorelines” (actually crater walls). They looked like the oceans of Earth… but did earthly seas look like lunar maria?
They do indeed… at least, in one particular place: the Nastapoka Arc. Circular reasoning dictates the “unnatural” arc must have been formed by an impact event, a cosmic cookie-cutter as it were. These reasoners thought they had the perfect pair of pairs all lined up: the Nastapoka Arc off of Hudson Bay, and the Sinus Iridium off the lunar Mare Imbrium.
On the face of it, it’s easy to draw conclusions based on a host of circumstantial evidence. “It walks like a duck, it talks like a duck, therefore it must be a duck!” Trouble is, ducks don’t talk… and geologists now look for a number of essential clues to confirm if a crater-like feature has a celestial origin. Above is Crater Lake in Oregon, USA… not the crater you were thinking of.
Derp Impact
Following the 1968 publication of On the possibility of a catastrophic origin for the great arc of eastern Hudson Bay by C.S. Beals, a 1972 investigative expedition headed by Dr. Robert S. Deitz and J. Paul Barringer failed to find any of the now-recognized markers of an interstellar impact: shatter cones, unusual melted rocks such as suevite, pseudotachylite or mylonite, radial faults or fractures, signature injection breccias, or other related evidence of what geologists call “shock metamorphism”.
Even the Belcher Islands didn’t fit the mold of an impact’s central peak (or remains thereof), being instead composed of ancient rocks of many types – none of them unearthly or with a direct or indirect impact-related origin. Sorry folks, show’s over, nothing to see here, just plate tectonics at work. You can fool the casual eye but you can’t fool Mother Nature. Then again, maybe we just haven’t found the right evidence yet. As someone once said, “the truth is out there.”
It’s somewhat ironic that early astronomers once thought the impact-related Lunar Maria were the seas of the moon while the Nastapoka Arc – a sea of the Earth – formed though tectonic processes occurring deep within the Earth. So much for WYSIWYG… geology sometimes takes a long and complex route to an ending that only seems obvious to us.

