Minnesota Department of Natural Resources Podcasts
Glacial Potholes Easy Walking Tour 2010
Hi. My name is Kacie, and I’ll be your guide on this easy walking tour of Minnesota Interstate State Parks glacial potholes. You may have heard of a pothole before: maybe one in the road? Well I promise you that’s not what we’re exploring, but rather some of the most fascinating features in the geologic world. Before we get started I’ll tell you a little bit about how this is going to work. While this may not be the exciting part, trust me, we’ll get there. By now you’ve downloaded this audio guide. Throughout it I’m going to offer you the opportunity to pause it in order to give you time to enjoy your walk and get to the next stop. When you get to the next designated stop, press play and we’ll continue. You might want to grab a map of the pothole trail before we head out to more easily find your way around. The time it takes to complete this tour will be different for everyone depending on how long you’d like to explore each stop, take pictures, or just take a break. You can expect it to take a half hour to 45 minutes. Ok. Are you ready? Let’s begin our tour outside the Visitor Center. Press pause now and then press play when you get there. See you in front of the Visitor Center!
Alright, you made it! From standing outside the Visitor Center, you’re probably noticing the dark grey rock all around the area. This rock is known as basalt. So where did this rock come from you might ask? Well you probably don’t know you’re standing on an ancient lava field! Right where the St. Croix River flows today there was once a series of large cracks that split open 1.1 billion years ago. As these cracks split open, lava from Earth’s upper mantle spilled out. These lakes of lava covered a huge area from Canada, through Michigan, Wisconsin, eastern Minnesota, Iowa, Nebraska, and Kansas; hence this was the scene of one of the largest volcanic events in the world. This basalt rock is a very hard igneous rock that does not erode easily. But, such erosion happened here at Interstate. Continue along the Pothole Trail to the Lily Pond to uncover the history that created the potholes you’re about to see. To get to the Lily Pond, walk down the asphalt maintenance road towards the river; away from downtown Taylors Falls. As you walk down the road, you’ll notice a railing around a group of potholes to your left. Take that concrete walking trail. The first large pothole you come to will be the Lily Pond. Ok. Press pause now and press play when you get to the Lily Pond. See you there!
Here you have it- a glacial pothole! Without reading the interpretive sign, take a moment to think. What do you think created this large hole in this extremely hard basalt rock? Did you guess water? If so, you’re right! Let’s go back 10,000 years; to the end of the last ice age. Prior to that, about 21,000 years ago most of Minnesota and all of Canada was covered by a massive sheet of ice, or glacier, which averaged a half mile thick. As that began to melt 10,000 years ago, it produced huge glacial rivers; the St. Croix being one of them. At one point the St. Croix River was over one mile wide! Now, have you noticed how jagged this bedrock is? Wherever there is a fast-moving river flowing over such an irregular surface, small whirlpools, or eddies, form. These eddies, along with the sand, silt, and rocks that were found in the river acted as liquid sandpaper and wore away these smooth, round holes that you see. As you walk past the Lily Pond, notice the smooth, bluish boulders to your left. These were pulled out of this pothole and were probably the force behind the drilling water that created it. So there you have it; the method of creating a pothole! Let’s now go out to Angle Rock, straight toward the river from where we are now. Press pause, take your time, and press play when you get to Angle Rock.
Can you see why this overlook is named ‘Angle Rock’? Here the river takes almost a 90 degree turn in its course. So, remember when I said there was once a series of large cracks in this area? Well, the change in the rivers course was caused by the glacial river encountering this ancient, inactive fault line, or crack, in the otherwise solid basalt bedrock. Because it was easier for the river to wear away this crack, a deeper river channel was created. So, when the glacial St. Croix River became smaller and was only fed by rainfall, as it is now, it took the lowest channel as the easiest course to follow, even though it meant flowing around Angle Rock. Now, back to the potholes. Wisconsin Interstate State Park, just across the river, also has glacial potholes, but they are smaller than the potholes you have seen on the Minnesota side. Can you think of why that might be? If you said this bend in the river had something to do with it, you’re right! Thinking back to the glacial river running through here, Minnesota was on the inside corner of this bend. Minnesota is also slightly lower in elevation than Wisconsin. The combination of more turbulence on the inside corner, and longer-lasting river flow over this area created wider, deeper, and overall larger potholes in Minnesota. Speaking of large potholes, let’s make our way to the Bottomless Pit; the deepest-known pothole in the world! To get to the Bottomless Pit, we’re going to have to take a few stairs. For those of you not comfortable using stairs, you can return to the Visitor Center the same way we came, but I encourage you to continue to listen to this audio tour for more information. If you’d like to go to the Bottomless pit, walk back to the Lily Pond, and take a left across the bridge just before it. The Bottomless Pit is 60 feet deep, so you’ll most likely recognize it when you get there. Watch your step on the stairs. Press pause now, and press play when you get to the Bottomless Pit. See you there!
Stop 4: The Bottomless Pit
So, have you listened to your echo in the Bottomless Pit yet? Although its name says that it’s bottomless, you might be able to see that it really isn’t. But, you have to imagine that it didn’t always look like this deep hole. As the glacial river became smaller and retreated into the crack around angle rock it left behind loads of sand, rocks, and debris that filled these potholes up. After the park was established crews came into this area and cleaned out the holes so that they could see how deep they were and we could see the bottom. The Bottomless Pit is thought to have begun excavation, or cleaning out, between 1897 and 1900. During that attempt, the crews dug 47 feet of debris out and failed to find the bottom; hence the name Bottomless Pit. It wasn’t until many years later; between 1927 and 1934 that they succeeded in finding bare bedrock bottom. A re-excavation of this pothole in 1987 removed eighteen feet of rocks, mud, and assorted litter that had accumulated over the years. Among items that were found included sunglasses, a gold railroad watch, an antique camera, and many pennies which can be seen on display at the Visitor Center. So, there you have it! The Bottomless Pit; the deepest-known pothole in the world!
Now, move just a little further down the trail and you’ll see a second pothole within this set of railings. This is the Bake Oven. Depending on the water level, you might notice that there is a smaller 6-foot wide by 9-foot deep pothole in the bottom of this pothole. It seems unlikely that this can be explained by a single event of water turbulence. So, OK, back to the Glacial St. Croix River! This mile-wide river went through at least four peak flow stages, or water levels, as glacial ice retreated and re-advanced over the area. It is likely that one of the early peaks carved a 6-foot diameter pothole from where you’re standing to the bottom of the 6-foot by 9-foot hole at the bottom of the Bake Oven. During a less powerful stage of the river, this pothole likely filled up with sediments such as sand and rocks that were within the river. During a later peak, as the river grew stronger and more turbulent, the sediments were spun out of the smaller pothole and the river carved the existing hole wider, but not as deep; leaving a pothole within a pothole! If it is open and you’d like to go down inside the Bake Oven, follow the trail down the stairs and to your left. When you’re ready, we’ll meet at our next stop just down the stairs and straight ahead between the large, cracked basalt wall and the swamp. So, press pause now, and when you’re ready, press play when you’re standing along the large, cracked basalt wall straight ahead.
Stop 5: Just down the steps from the Bottomless Pit; between the cracked wall and the swamp
Welcome back! I hope you’re enjoying your time among the glacial potholes. I have just two more stops to tell you about before I let you go to explore other areas on your own. So far on this tour I’ve spoken a lot about how the Glacial St. Croix River sculpted this area. However, even today there is another type of erosion taking place at Interstate Park- frost action. Have you ever put a water bottle in the freezer? If so, you might have come back hours later to find that it had frozen and expanded! The same thing happens within the cracks of this basalt rock. In the spring, summer, and fall water gets into these cracks. In the winter, it’s like putting a water bottle in the freezer: it freezes and expands! As a result, it breaks the rock apart creating more and larger cracks. A perfect example of this is the squeeze! Straight ahead you’ll see a large rock that, because of frost action, broke off of its cliff base. Feel free to make your way through the squeeze or meet us around the other side. You can press pause now. When you get through the squeeze, press play and we’ll continue.
Stop 6: Just past the Squeeze
Whew! You made it! As you’ve seen on this tour, a lot has been learned from this area including the billion-year-old lava flows and the mile-wide glacial river. But, there is much that we haven’t even discovered yet. Remember when I mentioned these potholes needed to be dug out in order for us to see the bottom? Well, look around. What do you suppose we might find if we were to dig out the swamp in this area, or the ground you’re standing on? There are probably potholes beneath here that we don’t even know of! Even more exciting, we can only imagine what we might find inside those potholes if they were ever dug out. Not only might we find thousand-year-old Native American artifacts, or hundred-year-old logging artifacts, but we would likely find up to 10,000-year-old leaf and forest samples that would tell the story of what this area was like so long ago! Interstate has been a State Park for over 100 years and even though there’s no place else in the world that has pothole formations as large and numerous as the ones we already know about, there’s still more to come. All of this is because of the unlikely combination of hard basalt rock, a gushing glacial river around that corner, and a farsighted group of people back in 1895 who decided that it was a landscape worth preserving.
I hope you have enjoyed this audio tour of the glacial potholes and enjoy the rest of your time at Interstate State Park. From here, you can take a left across the small boardwalk to view the Cauldron; the parks widest, but unexplored pothole, or you can follow the trail straight to the asphalt road which, to the right, will take you back to the Visitor Center where we began our tour. When you return to the Visitor Center, your comments about this audio tour are greatly appreciated and can be given on the comment cards located at the self-registration station or by contacting the park directly. Interstate, and other Minnesota State Parks, are always striving to better serve you during your visits to the great outdoors. I have really enjoyed being your guide on this walking tour of Interstate’s glacial potholes and hope you return often to Interstate and other Minnesota State Parks.