One of my absences this summer from blogging was due to a trip to see the family at our yearly retreat deep in the woods of northern Wisconsin. Well, maybe not that deep, but it is certainly an excellent place. Nice sized lakes formed during the retreat of the last ice sheet, good for swimming, canoeing (which might be spelled wrong but as it is the weirdest looking word I’ve typed in a while), or just sitting on the edge of. I know geologically that lakes are very ephemeral, especially lakes like these. I heard once that the only climates that preserve lakes well in the geologic record are very arid climates. That is kind of counter-intuitive, since I tend to associate lakes with places like the great northwoods of Wisconsin. But I suppose it makes sense, these lakes exist thanks to a high water table, and will someday be replaced by a well organized system of rivers. But, I have no reference or expertise in lakes or sedimentary environments, so I’ll have to leave any real discussion to our good friend now at Clastic Detritus. So first, a picture of the lake I took one morning. During the week, the lake is nice and quiet. On weekends, people with skidoos, ski boats, and other machismo replacement machines crowd the water, bringing the soothing sounds of a tractor pull to the quiet of the great northwoods!
So it turns out that this will be the last year our family will be staying at this particular lake. As such I was spending some time looking around the rental cabin and stumbled upon evidence that the fireplace has been overturned! Check out my picture and associated line “drawing”
As I am sure is the case with most geologists, I love to look at rocks all over the place, even restaurant counter tops, retaining walls, and my personal favorite, stone fireplaces. Especially in a place like northern Wisconsin, the rocks used are all local, which means most likely from glacial till, which means anything that outcrops north of Spooner and south of the North Pole. If you aren’t familiar with geologic “up indicators”, I found this site through a quick google search, but the basic idea is that many geologic processes create asymmetric structures, and some of those structures (like the migration of sand dunes) can tell you what direction was up when the sediments were deposited. Cross-stratification in rocks can therefore help you unravel the geologic history of a region. In the line drawing above, the heavy lines are the erosional surfaces. The arrow points to geologic up.*
Also in this fireplace, but not shown, were some nice chunks of the Wisconsin State Rock, a red granite (or here for an abstract.) This is one of my favorites, brings me back to my days as an undergraduate geology major on field trips throughout the state.
*disclaimer, yes, of course I know geologic up has no meaning for glacial boulders or fireplaces, but I enjoy determining which direction is young, just as I enjoy complaining that the “granite” counter tops are actually granodiorite or diorite and informing people that their kitchen is 90 million years old.