In 1971, the Apollo 16 astronauts visited High Falls in Onaping as part of their geological training in preparation for a mission to the Moon. A year later, so did the Apollo 17 crew.
They went to different sites across the Sudbury Basin to examine rock formations associated with ancient meteorite impacts.
Sudbury's negative reputation from the environmental damage of mining and smelting had resulted in the mistaken belief that the astronauts came to the city due to its resemblance to the moon's surface. In reality, the visits were meant to give the astronauts firsthand experience at identifying and describing the kind of rocks they would see on the moon.
This year marks the 40th anniversary of the unveiling of the Onaping Falls Nature Trail and footbridge. While it is the perfect place to admire the natural beauty of the forest and falls, it is also well worth the trip to look at the rocks below, just as the astronauts did. High Falls in particular was an ideal place for them to study a type of rock known as fallback breccia.
Dr. David Pearson, a professor in the School of the Environment at Laurentian University, was with the Apollo 17 crew during their visit to Sudbury.
"The astronauts were interested in seeing the rock that forms following a meteorite impact. Fallback breccia is made of the debris launched into the air by an impact falls back into the crater and the hot fragments are welded together. The result is rock with distinctive, different-sized fragments suspended within it."
Most of the exposed rock at High Falls is fallback breccia that formed after a 10-km diameter meteorite crashed into the present-day Greater Sudbury area 1.85 billion years ago. The world was a much different place at the time, and Sudbury might have been beneath a shallow sea.
The impact produced a tremendous amount of heat, instantly melting solid rock that filled a 200-km wide, six-km deep circular crater. The metals in this melted rock slowly settled to the bottom of the crater, becoming the ore deposits that would make Sudbury famous.
The debris that fell back down on top of the melted rock became fallback breccia. The impact was so great that the debris also flew well beyond the Sudbury area. Fallback breccia formed from that impact has been found as far as Thunder Bay, Michigan, and Minnesota. The debris likely travelled even further, but has since eroded away.
The energy from the impact also travelled through the older rock underneath and around the crater, fracturing it and forming unique cone-shaped features known as shatter cones. These features can only be created by the tremendous energy involved in meteorite impacts or underground nuclear testing.
Visitors to High Falls won't find shatter cones anywhere in the area. Shatter cones could only have formed in the rock that existed before the impact. The fallback breccia under the falls and the surrounding area formed in the aftermath, from the debris that fell back down.
While shatter cones cannot be found at High Falls, they can be found in Sudbury, like at Science North or along Ramsey Lake Road. This rock had already existed for hundreds of millions of years before being shattered by the meteorite impact.
Incorrect signage at High Falls about the presence of shatter cones has led visitors to misidentify the marks on the rocks by and in the river, just slightly downstream of the footbridge.
What some visitors may have mistaken for shatter cones are actually marks from dynamite blasts, known to miners as “bootlegs”. Some of them even have remnants of the holes that were drilled to place the dynamite.
Compared to the real shatter cones that formed 1.85 billion years ago, these blast marks are the result of much more recent history.
The blasting was likely done by loggers to clear the waterways in the late 1800s, when logging began in the area. Loggers would float the logs down the Onaping River, which would eventually arrive at sawmills by the mouth of Spanish River.
High Falls presented a particular challenge because the jagged rocks would break the logs or cause jams. Blasting the rocks when the water level was low would have helped prepare the river.
Dr. Pearson brought up the dynamite blasts at a recent dinner presentation for the Sudbury Rock and Lapidary Society.
"Rock could have been cleared to allow logs to float more freely down the falls, but another likely possibility, brought up by the audience in the discussion, was that the blasting might have cleared the way for the construction of a chute to guide the logs as they went down," he said.
The fallback breccia and shatter cones are important pieces of evidence for the meteorite impact. Both can be seen at the surface of the Greater Sudbury area today because of the considerable changes to the land after the impact.
Additional layers of rock formed over the fallback breccia as sediment carried by the shallow sea settled over millions of years. Movement of the continents eventually deformed the crater and created ancient mountains in the area.
Millions more years of erosion transformed the landscape and scraped away much of the crater. What remains of it today is a 20-km wide and 60-km long oval, now known as the Sudbury Basin, with different rock formations exposed. The city of Sudbury presently sits on land that was actually underneath the crater when it formed, hence the presence of shatter cones.
The Apollo astronauts were not the first famous visitors to High Falls. A.Y Jackson, well-known Canadian painter and founding member of the Group of Seven, came to the falls in 1953.
The lookout looking over the falls was named in his honour with a plaque that commemorates his visit and painting, "Spring on the Onaping River". Although the painting was stolen and never found, an oil paint sketch of the fall colours from 1953 can be seen on display at the Copper Cliff Public Library.
A walk through High Falls follows the footsteps of artists, inspired by the colours of nature, and astronauts and geologists, looking to understand how rocks were formed on earth and on the moon. It is a trip through time, going back 1.85 billion years to when a meteorite impact shaped the land and history of Sudbury.
Tim Li is a science communication student at Laurentian University.