Anomaly beneath the waves
Beneath the waters of northeastern Lake Ontario, about 25 kilometers south of Kingston, lies a peculiar circular basin named the Charity Shoal Crater. This submerged feature measures between 1.2 and 1.5 kilometers in diameter. Its structure is a basin floor, reaching a depth of about 20 meters, surrounded by a continuous, raised bedrock rim that rises to within a meter of the lake surface in some places.
The crater was first seen as a distinct circular shape during side-scan sonar surveys in the early 1990s, and it was formally proposed as a possible impact crater in 2001 based on detailed bathymetric mapping. The structure is carved into Middle Ordovician limestone belonging to the Trenton Group, which forms the lakebed in this region. Subsequent high-resolution surveys in 2010 and 2011 provided a detailed one-by-one-meter grid model of the feature, confirming its distinct crater-like morphology. In addition to the geology, a long ridge extends 3 to 4 kilometers southwest from the crater, a formation interpreted as a crag-and-tail feature sculpted by the movement of ancient glaciers.
The impact debate
The leading hypothesis for the crater's origin is the impact of an extraterrestrial object during the Middle Ordovician period, approximately 460 million years ago. The primary evidence is the structure's pronounced circularity and raised rim, which is typical of a simple impact crater. Magnetic surveys reveal a negative magnetic anomaly over the feature, also found at impact sites. Models based on this data suggest a true depth of 450 to 600 meters, far deeper than its current appearance.
However, definitive proof of an impact origin is missing. Such proof requires the discovery of shock-metamorphosed minerals or shatter cones, which are rock formations created by the immense pressure of an impact. Because the crater is underwater and its floor is covered by up to 30 meters of glacial and post-glacial sediments, obtaining bedrock samples from the original crater floor has been impossible. While rock samples have been taken from the rim, they have been inconclusive. Alternative, though less likely, origins include a volcanic cone, a karst sinkhole, or a glacial kettle, but the geological and geophysical evidence makes the impact scenario the most consistent explanation.