Glacial Erratics along the highway

Dear Doc Rock,
I sure hope you can help settle an argument between me and my sweetie Betty. We drove our motorhome through BC and Alberta last summer. She insisted we take a side trip to see a giant rock beside the road a little south of Calgary, Alberta. I wasn't so red hot about going out of our way to see some stupid rock, but she said she'd walk there herself if I didn't go along. She gets the funniest notions but I love her anyway.

Not wanting to spend yet another night in the doghouse (that's another story), I said 'Okay' and we went. Wow! It sure was big! I thought everything really was bigger here in Texas where we live, but I don't think we have rocks that big! Anyway, the fight is over what these kind of rocks are called, because neither one of us can remember. Betty says they are called glacial erotics but I sure do know what that means and that isn't it.

So, Doc, I hope you can settle our lovers quarrel about what these rocks are called. And, where did the glacier go? There isn't one there now, not that I could see anyhow. Are these kind of rocks in Texas? Where should we avoid traveling next summer so we don't get into arguments over glacier rocks?
Thanks a heap,
Deloris R.
Midland, TX

Dear Deloris,
I hate lovers quarrels, so let's get this settled right now. It is a bummer to sleep in the doghouse all winter. Betty was close with 'erotic' but the word is 'erratic'. For those of you who aren't as sharp with Latin as you once were, that word comes from 'errare', which means 'to wander'.

Glacial erratics are rocks that have been moved by glaciers from their original location to wherever they are now. By definition, an erratic has to be made of rock that doesn't match the type of rock where it now sits. The Okotoks Erratic is quartzite, a type of metamorphic rock. The local bedrock in this part of Alberta is sandstone, a sedimentary rock.

The big rock you saw is called the Okotoks Erratic. Look close at the photo to see people around it. It is the world's largest known glacial erratic. Since it is in Canada, I better give the dimensions in metric first (don't worry, I'll translate for Texans). The rock is 9 meters high (that would be 'metres' in Canada), 41 meters long, and 18 meters across. It is estimated to weigh 16,500 metric tons. If it was in the US, it would be 30 feet high, 135 feet long and 60 feet wide, and it would weigh 18,150 tons or 36,300,000 pounds. It is broken into a few smaller pieces, but looks like this happened when it was left here about 10,000 years ago.

The giant block of quartzite comes from the Rocky Mountains. There is similar rock at Mount Edith Cavell in Jasper National Park, 210 miles to the northwest near Jasper. But it is likely that the ancient glacier first flowed east out of the mountains onto the plains of Alberta, then joined up with the huge ice sheet that covered central Canada and extended all the way down into Iowa.

Sorry Deloris, the giant glaciers didn't make it to Texas, so no erratics there. You are safe if you never leave the state. The last of the great glaciers melted in the area of Okotoks 10,000 years ago, and that's when the big rock got dumped onto the ground.

If you want to avoid erratics and still travel stay in the southern half of the USA. If you want to make Betty happy and go see some other huge erratics, here are two you can visit on a looooooooong road trip from Midland.

Madison Boulder in New Hampshire is granite, and is 23 feet high, 83 feet long, and 37 feet wide. This was at one time considered the largest erratic in the US, until yours truly measured one in Lake Stevens Washington in 2011. The Lake Stevens erratic is 34 feet tall, 78 feet long, and 210 feet around. We don't know how much of the giant is buried in the ground.

Directions to the Lake Stevens erratic on my Northwest Geology blog. The Madison erratic can be found by an internet search.

Directions to Okotoks: 7.5 miles south of Calgary on Highway 2, turn right on 522- 2A for 7 miles through Okotoks, then turn right on Highway 7 for 4 miles and you can't miss the huge rock on your right. There is a parking area and then a flat path for 200 yards to the rock. You can see it in Google Earth, just search for Okotoks Alberta, and mvoe a bit to the west.