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.

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