While Petit Portal – also called Petit Arch and Arch Rock by some – is often confused with the partially caved in Grand Portal, it’s a smaller and as yet intact formation. You can read about some of these from the Pictured Rocks National Lakeshore website. The two arches (and many cave structures) are formed by the powerful action of Lake Superior waves on the soft sandstone that underlies the harder layers above.
You can read more about the geology of Pictured Rocks from Oh Ranger! (a cool site I just found today) and also this PDF on geology from the Lakeshore. If you want to see them up close, the boat tours from Munising are worth every penny.
Donald took this 5 years ago. Check it out background bigtacular and in his stunning Pictured Rocks Nat’l Lakeshore slideshow which includes some jaw-dropping shots of Chapel Rock and even a look from inside Petit Portal.
Much more on the Pictured Rocks on Michigan in Pictures.