The other day someone asked me if I could explain to them exactly what a glacial erratic was. So here goes:
The people who study geology and glaciation reckon that over the last two million years, the planet has undergone probably five periods of continental glaciation. That means that in an area like the Keewatin, it just kept snowing and snowing until it got kilometres deep and the downward pressure started to push the glacier out to cover most of the continent.
As the glacier started to move outwards, it plucked at the bedrock and picked up rocks. This included large blocks of rocks that had fallen off cliffs or been fractured. These rocks were moved tens and even hundreds of kilometres. Then when the ice age ended and the glacier melted, the rocks were left behind on the landscape.
So a glacial erratic is a large rock found in the middle of nowhere that is often a long way away from its place of origin. Moved by continental glaciation. All across the NWT, one can come across glacial erratics and there are certainly a bunch around Yellowknife.
In Yellowknife, there are also some very rounded smooth boulders and deposits of sand that indicate they were transported by water. A very fast-moving river can carry or roll rocks along and they get eroded round and smooth. You can find such rocks particularly around the Bristol Pit and sand deposits at the sand pits around Long Lake and Frame Lake.
When the ice age ended, the thick sheets of ice began to melt, particularly in the summer. Not only that, but they also got hit by thunderstorms and heavy rain. All that water had to go somewhere and it went into crevices and formed some mighty rivers running under the glacier. The abundance of round rocks and sand deposits show that one of these rivers passed over what is now Yellowknife. So we have not only glacial erratics, but also river rocks from glacial rivers.
Another way rocks get move around is, of course, by humans. People pick up rocks and move them to make rock gardens or for landscaping. A lot of round rocks from the sand pits and Bristol Pit were moved by people to places around town. But the moving of rocks goes much farther. Red sandstones slabs with ripple marks were brought from Great Slave Lake to form walkways.
Also, for a while we had a city administrator who thought that big chunks of blast rock were ideal for landscaping. So one sees blast rocks scattered all over city medians, beside roads and in the oddest places. I am really not a fan of using blast rock for landscaping, but some of our blast rock was apparently loaded on trucks and sent to Alberta landscapers.
Rocks which are picked up and moved to another location by humans can be called manuports, or I suppose humanports. Anthropic is also a term for rocks made, altered or moved by humans.
Humans have been moving and altering rocks for thousands of years. That is what the Stone Age is all about. Stonehenge in England was created. It was built starting around 5,000 years ago, or 3,000 BC, and the rocks were moved possibly 240 kilometres. One can look at sites all around the world where humans have been moving around some mighty big rocks and even building pyramids and cities out of them.
It has always bothered me that Yellowknife has some great geology and geography that the city either ignores or destroys. We really should highlight our rocks for residents and tourists.
