On Friday, Feb. 21, Paul Ehlers came to our school, Northwoods Community Secondary School. Mr. Ehlers came to share some information with us about the caves on the shores of the Apostle Islands National Lakeshore. He came because, as a field study, our school is planning on going to the caves. Although this could have just been a “field trip,” our teachers wanted us to be able to get more out of it than just a fun experience. Our advisors wanted us to meet academic scope and sequence. Before we went, we needed to have some background knowledge on the subject of geology.
Mr. Ehlers told us a “little” bit about “How Caves Form”, “How Sandstone Forms”, “Where Sandstone is Located”, and “How Waves Affect the Cave Formations.” The title of the presentation was “Geologic History of Wisconsin-The Bayfield Sandstones.” However, it was much more than just that.
There were quite a few things that I found interesting in this presentation that I didn’t know before. I didn’t know that there was originally a mountain range in the higher regions of Wisconsin and evidence of volcanoes in Wausau 1.6 billion years ago. At another point there was also an ocean covering Wisconsin. The oldest rock found in Wisconsin was a piece of zircon and it is 4.6 billion years old.
Plate tectonic activity has operated since the Paleoproterozoic period. The Paleoproterozoic period was when the seven continents first stabilized. About 135 billion years ago (in geological time), the land masses started shifting. Some experts in the field of tectonic plates think that the shifting of the plates is going to continue to happen and that the plates will still move left and right due to the heat radiating from the center of the Earth. Others think that the movement will change the direction in which they move. I didn’t know that the Tectonic Plates move as fast as fingernails grow.
The ice encased caves near the Apostle Islands have three layers of Sandstone. Depending on how big or small, round or jagged, smooth or rough a granule of sandstone is, a geologist can figure out why that granule looks that way. The Chequamegon Sandstone is the top layer and is 1,000 feet thick. Next is the Devils Island Sandstone that is 300 feet thick. Lastly, there is the Orienta Sandstone that is 3,000 feet thick. The Devils Island Sandstone layer is the layer that the caves form in.
According to Mr. Ehlers, the Bayfield Group of Sandstone is irregularly bedded. In order to form a sea cave, the host rock must first contain a weak zone, or soft spot. The driving force in littoral cave development, in this case, is wave action. Mr. Ehlers showed us a few pictures of the ice caves and some of the ice that makes these caves so famous was halfway up the side of the sandstone. The sandstone has water in it. The clay doesn’t let the water through, so the water drips through the sandstone layer and as it keeps flowing, the end keeps freezing; creating the icicles. Sandstone has a porosity of 30 percent. Porosity is when something, in this case sandstone, has the ability to absorb water. Also in the pictures that Mr. Ehlers showed us, there was ice that was differently colored. Some of the ice was blue and some was brown. Mr. Ehlers said that the blue ice was blue because it was reflecting the sky. He said that the brown ice was brown because of the dirt that was washed/pulled down the cliff by water. Mr. Ehlers also said that almost all rocks have some amount of iron in them. That may be another reason why some of the ice is colored brown.
On March 7, we went to the ice caves and experienced one of the most amazing sites ever; the Apostle Islands National Lakeshore. The ice around and in the caves was very slick. Falling was basically unavoidable. The background knowledge of the caves that I was given didn’t really change the experience that I had at the caves, but the knowledge helped me with the project that we did about the ice caves later. I believe that the caves were a great treat to see. Hopefully, all who were able to see the ice caves with us enjoyed their experience.