Castle Rock

Last weekend, I hopped on my bike and made the 30 mile round trip trek (personal record for me :D) to nearby Castle Rock in order to spend some summer somewhat within my educational pursuits. Castle Rock is an exposed piece of the St. Peter Sandstone, a very mature, very pure quartz arenite formation that most geologist and students of the Midwest are likely familiar with, located between Dixon, IL and Oregon, IL on Illinois Route 2 in what is now known as Castle Rock State Park.

The St. Peter Sandstone is a Middle Ordovician formation dated between 465 and 460 million years ago, that is widespread throughout the Midwest. It’s deposition coincides with the beginning of the Tippecanoe Sequence (A Sloss sequence or cratonic sequence; a sequence that describes the transgression and regression of sea levels, and consequently deposition and erosion across a craton), a period of relatively higher sea levels covering the craton in a shallow sea; a perfect environment for the deposition of the sand that would become the St. Peter Sandstone.

The distribution of the St. Peter Sandstone formation (I live a bit north of Ottawa); taken from Mostly Maps (

Approaching the park, your first glimpse of the St. Peter Sandstone is the rock exposed by the road cuts that follow Route 2 as it twists through the bluffs that line the Rock River. Unfortunately, rather terrifyingly, and much to my dismay as I had apparently forgotten, the well-traveled Route 2 loses it’s shoulder here, and the blind curves make a cyclist a rather large roadside hazard. I am happy to report that I survived, however.

There are few things of note in the next photo (I also apologize for the quality, I feel like there was something wrong with my settings this day); first, the obvious cross-bedding or more accurately cross-lamination, and secondly how loosely cemented the rock is. It’s very friable (a good word for geology students to know), meaning it very easily crumbles at the touch. It’s not very well cemented together.

The cross-lamination provides an insight into the history of the deposit; you can determine the direction of flow that deposited the sands, and you can rule out certain depositional environments based on the angles of the laminations. For instance, the cross-laminations observed at Castle Rock are very low angle; these are associated with deposition by water as opposed to deposition in the dunes of a desert, where cross-bedding is much more prominent and much higher angle.

A short way down the road from the road cuts is the Castle Rock area itself. A set of trails and some river side recreational areas. The really unfortunate thing about Castle Rock now is that the Illinois DNR  has covered it with a bunch of wooden walk ways and makes it really less than ideal for people interested in the rocks to examine them. Sample taking and climbing on the rocks is strictly prohibited in the interest of preserving the site.

It’s still possible to see some of the features of the formation however, note the lamina and bedding, and more cross-lamination’s visible here.

Below the Castle Rock, along the river there are plenty of exposures too, but most of them are inaccessible unless you have a boat, but there are some along the shores that you can approach and that have some interesting features.

In the lower right of this next photo, you can see a dark layer in between the sandstone, something that hasn’t weathered at the same rate as the surrounding rock. I have no idea what it is, and it’s so localized that it’s hard to find any other spots with the layering present and again, it’s very illegal to take samples from the area so if you have any ideas, shoot them my way. The area is clear of brush though, as if someone has spotted it before and has also examined it.

If you follow the shore south, you come across a nice face you can look at, but like I said, most of this stuff is only accessible by boat. The river is a little low right now, so I was able to get this by compromising with muddy shoes.

…and here is a nice shot of the scenic view from atop Castle Rock. It’s really a nice place to check out. It’s not a geology mecca by any means, but it’s local and a nice place to have a picnic.


It was quite lovely here this weekend. My understanding is that this may have been our last taste of summer before summer officially arrives as it’s suppose to drop to the 50’s tomorrow. That’s alright I suppose, as after this week, my next few weekends will be consumed by field stuff with my Volcanology class (St. Francois Mountains in Missouri) and Structure class (Baraboo, WI). I would really like it to not be 90 degrees those weekends.

I spent my weekend mostly outdoors being an active young adult and when I wasn’t outdoors I was working on a new set of cross-sections for Structure lab. I took the above picture this afternoon on a nice 20 mile ride around the area.

I took a nice hike the day before and took some more pictures.

I am consistently impressed with the quality of photo produced by the camera in my phone. It’s definitely not a sure thing, but particularly when you can get a nice focus on something up close… It just looks decent. I’ts unfortunate that the only way to get the colors to something worthwhile is post-manipulation, but I guess you can’t expect much.

No moleste.

I have spent most of my day that wasn’t at school in a state of unconsciousness.

Check out these amazing pictures of volcanic things in 2011.

20 million in a hundred feet

Today, I headed out to Franklin Creek State Park. It’s right near my house and has the unique distinction of having the oldest exposed bedrock in the state of Illinois (if the internet is to be believed anyway). Unfortunately, that older bedrock is the same type of rock as the rest of Illinois, sandstone and dolomite.

Anyway, as a geology student living in a very stable, flat, boring piece of craton, you find your places with rocks and end up spending a lot of time around them. This location is one of the places that made me realize my interest in rocks as I decided that geology would be my education path. It’s been fun to return to the same places time and time again each time armed with new knowledge and applying it to what I see around me.

The first time I went there after I knew SOMETHING about geology (but not a lot), I went there thinking that I might find some fossils in limestone. I knew that the area used to be covered in oceans and assumed that there would be a lot of limestone. I would soon discover there is little obvious fossil content, and that what I thought would be limestone is actually dolomite (another, stronger, carbonate rock) and that a large amount of the rock in the area is sandstone.

Today, I know that there are 3 exposed formations of rock representing a wide layer of time exposed at the park– in descending order the St. Peters Sandstone, the Shakopee dolomite and the New Richmond Sandstone. These layers were deposited in the Ordovician (~480 million years ago) and are the types of formations that are key to our understanding that Illinois was once a warm, shallow, tropical sea.

The place in this picture is one of my favorite places to hang around at the park because here, you can very clearly see the disconformity that marks the boundary between the New Richmond and Shakopee formations. It’s hard to see and I’ve never been on top of that ridge, but I think the St. Peter Sandstone is accessible from the top of the rock face too. The stratigraphic column I used in the picture is from a paper about SE Minnesota geology, so it’s not representative of the exact geology of Northern Illinois, but a lot of the same formations make their presence known.

I spent the rest of my time taking cool pictures of the ice.

I still have no real idea of what I want this blog to be about. It would be much longer and take me much longer to write if I were to go as in-depth as I want to, but I think that I want this to be more casual. It will make returning to it day after day a lot easier anyway. If you have any geology questions, fire away and I’ll answer them to the best of my ability.