Guest post: Interstate State Park’s bedrock tells a billion-year-old story

Follow a geologist through the Minnesota park as he explains the landscape’s long and fascinating history.




9 minute read

Justin Tweet documents fossils for the National Park Service. In his spare time, he writes about paleontology for his blog, Equatorial Minnesota.

I’m standing where a tropical sea pounded volcanic rocks. Not bad for Minnesota!

I. Cambrian island-hopping at Taylors Falls

The bedrock formations around Taylors Falls are an unusual snapshot along the coastline of a shallow sea that existed 500 million years ago, during the Cambrian Period. That’s when life “exploded” on Earth, with most of the creatures we know today — birds, mammals, fish, etc. — first appearing. It’s also when shallow seas covered the area, depositing sediment and minerals on the bottom.

In the south part of Interstate State Park on the Minnesota side of the St. Croix River, for an investment of a little more than a mile of walking round-trip, you can go from ancient islands of billion-year-old basalt, to the rocky rubble surrounding them, to sandy beach. Geologists love to try to paint verbal pictures of vanished settings, but rarely do the modern outcrops cooperate so nicely.

I wrote about the more northerly part of Interstate Park previously, following a visit to the basalt and potholes. The park is justifiably famous for its glacially sculpted Precambrian rocks. What is less well-known to the public is that there is a long history of paleontologic exploration in the St. Croix Valley, as far back as the 1840s (see Owen 1852 for some early mentions).

Charles Walcott, in his quest for Cambrian fossils, designated the Cambrian rocks of the St. Croix as the Croixian series, representative of the Late Cambrian. The Croixian series encompasses three biostratigraphic divisions, from oldest to youngest Dresbachian, Franconian, and Trempealeauan.

These stages are all named after locations in eastern Minnesota or western Wisconsin. The Cambrian rocks of Taylors Falls represent the Franconian stage, currently thought to date to between about 497 to 492.5 million years ago. To put that in perspective, that’s about 600 million years younger than the basalt, and about 40 million years older than the Platteville Formation capping the Mississippi River bluffs in the Twin Cities.

In other words, there’s more time between the Cambrian sedimentary rocks and their basement than there is between the Cambrian rocks and us, and the amount of time between us and the deposition of the sediments that became the Badlands of South Dakota and Nebraska (about 37 million years old at the base) would fit with some room to spare between these Cambrian rocks and the Platteville. (Of course, what *really* fits between them is the St. Lawrence Formation, Jordan Sandstone, Prairie du Chien Group, St. Peter Sandstone, and Glenwood Formation.)

And also toads. (Justin Tweet)


So, what was going on in those 600 million years?

We can’t say too much about what was happening, due to the absence of evidence. Presumably some significant part of that time was occupied in erosion of the basalt and any rocks, sedimentary or volcanic, that happened to have been above them.

Because the Cambrian sandstones are quartz-rich, with lesser amounts of feldspar minerals and dolomite, there must have been source rocks somewhere in the region that weren’t plain old basalt. When Cambrian seas came pounding, the basalts of Taylors Falls proved to be made of stern material. If you pull up the bedrock geology of Chisago County (Setterholm 2010, PDF), you’ll see that the town and surrounding area are within an area mapped as “Pcf”, the Precambrian-age Clam Falls Volcanics.

All around this green blob are areas mapped as units starting with “C” with a line through it, meaning Cambrian rocks. It’s a little simplistic to describe it as such, but that green blob was a large island of basaltic rock during the Cambrian. The geology was a little confusing to the first geologists; we see in Owen (1852) that they thought the volcanic rocks had come up through the sedimentary rocks.

Imagine this, only without the plants, and an ocean instead of land on the other side of the river. The rubble becomes the Mill Street Conglomerate.

As the sea arrived in the area, it would have battered at the basalt, producing rubble that was then pounded and rolled in the surf. The eventual product of this action was conglomerates.

One example is the Mill Street Conglomerate, which can be found in the park.

The Mill Street Conglomerate is much beloved by regional geologists. It is a rare example of one of these initial conglomerates, and as such is a reasonably frequent subject of field trips. It is composed of rounded chunks of basalt up to a couple of feet (a little more than half a meter) across, with very fine sandstone as the matrix.

Although conglomerates are not known for their fossils, small fossils of brachiopods, trilobites, and snail-like mollusks are preserved in the matrix, and show the Late Cambrian age. (More below.)


Sufficiently conglomeratic?

Away from and above the conglomerate is the remains of the beach and shore, in the form of thick sandstone beds. The sandstone has traditionally been assigned to the Franconia Formation, but of late geologists are shying away from using the Franconia as a formation.

(The problem is the Franconia is also used widely for biostratigraphy, which was once permissible but is now avoided, if for no other reason than to reduce confusion. Again, see Mossler 2008.)

The Minnesota Geological Survey has imported the Tunnel City Group name from Wisconsin for these rocks. The sandstone in the park is divisible into two units, a less resistant lower unit and a thicker, more resistant overlying unit. Under the old terminology, these were the Birkmose Member and overlying Mazomanie Member of the Franconia Formation (Berg 1954). Under the new terminology, these are called the Birkmose Member of the Lone Rock Formation and the Mazomanie Formation, both of the Tunnel City Group.

Incidentally, the eye may interpret “Mazomanie” as “mah-ZOH-man-ee”, but it is actually pronounced “may-zo-MAY-nee”. The Birkmose tends to be finer-grained and is gray to green in color, while the Mazomanie is lighter colored and tends to be coarser-grained.

There isn’t a lot of Birkmose exposed in the park, so most of the namesake sandstone of the Sandstone Bluffs trail is Mazomanie.

Bedding in the Mazomanie.
Curtain Falls, currently dry. Much like the falls in the Twin Cities, a more durable bed (in this case part of the Mazomanie) is being undercut by erosion of softer underlying rocks.

Fossils have been found and described from up and down the St. Croix valley, including the Taylors Falls area and outcrops that are within the park. Because most of the Cambrian formations in the area are sandstones, preservation can be so-so at best, and fossils can be very subtle and difficult to observe. Most body fossils pertain to trilobites, inarticulate brachiopods, and snail-like mollusks, but burrows and other trace fossils can be abundant.

These rocks represent energetic, fluctuating environments, so you can guess that the animals probably had some durability and tolerance for changing conditions.

Inarticulate brachiopods in a rock from about a mile upstream from the park, near the dam.

II. Life on Mill Street

C. P. (Charles Peter) Berkey (1867–1955) was a geologist who did extensive early work in the St. Croix Valley. (If you’re curious, I’m paraphrasing from a short biography available online (PDF), Kerr 1957.) Berkey was a student at the University of Minnesota in the 1890s; in fact, he got his bachelor’s degree (1892), master’s degree (1893), and doctorate (1897) from the university.

For his doctoral work, he described the geology in and around Taylors Falls and St. Croix Falls; his findings were published in several issues of The American Geologist (Berkey 1897, 1898a, 1898b), which was run by Newton Horace Winchell’s family.

Among his contributions was the naming of the Franconia Formation, from outcrops at Franconia. A significant part of his doctoral work was devoted to an unusual fossil assemblage he found in a conglomerate at Taylors Falls. Paleontology, however, was not his primary interest.

As things turned out, Berkey was really an engineering geologist. He cut his teeth on the Catskill Aqueduct project and worked his way up to dams, being involved in the construction of Grand Coulee Dam, Hoover Dam, Hungry Horse Dam, and Shasta Dam. He also eventually became the president of the Geological Society of America and was part of Roy Chapman Andrews’s Central Asiatic Expedition to the Gobi. All in all, a full career.

A geologic map of the Taylors Falls/St. Croix Falls area, from Berkey (1897); some of the colors for the geologic units are difficult to distinguish.

Dams, aqueducts, and the GSA were all far in the future for Berkey in the 1890s when he went to work in Taylors Falls. Berkey found several areas with outcrops of the basalt conglomerate, although only the Mill Street location and another along the river were of any size. At the time, the Mill Street Conglomerate was located very near a railroad; although Mill Street remains, the railroad itself is long gone, but Interstate State Park uses the rail bed for a nice level trail.

It was in the sedimentary matrix of the conglomerate that Berkey found fossils of various tiny animals living on and around the basalt boulders of the ancient sea cliffs. These fossils went beyond the typical brachiopod/trilobite-dominated assemblages of the other Upper Cambrian rocks in the area; instead, the fauna included abundant small mollusks, generally limpet-like or conical-shelled snaily things (monoplacophorans).

Most of these animals were completely new to science, and he ended up naming about a dozen species and varieties of mollusks and two species of trilobites from the fauna. As happens sometimes, Berkey’s collections have been divided and whittled down over the years; a lot of the material went with Berkey to Columbia University and eventually to the American Museum of Natural History, while a small group of fossils at the University of Minnesota which he might have collected have been donated to the National Museum of Natural History (Yochelson and Webers 2006).

Two fossil taxa described by Berkey (1898b) (plus an early example of photos being used in a paleontological publication). Cheilocephalus st. croixensis is a trilobite (you’re looking at a glabella, kind of like a forehead if you want to anthropomorphize), now known as Cheilocephalus stcroixensis because punctuation is not permitted in taxonomic names. Hypseloconus recurvus var. elongatus is a monoplacophoran, now known as Hypseloconus recurvus (Yochelson and Webers 2006).

Today, the outcrop no longer produces mollusks, suggesting Berkey’s timing was just right to exploit what had just been affected by railroad construction (Yochelson and Webers 2006). The brachiopods proved more enduring, and at one time the site was extensively collected by commercial outfits for the educational trade, allegedly to the point of undercutting the outcrop (Yochelson and Webers 2006).

I can’t speak to the history, having not been there, but the outcrop does indeed have an undercut profile, and there hasn’t been much for fossils either time I’ve visited. We did find one piece of sedimentary matrix with small brachiopod fragments, which has gone to the park for their use.

Click to embiggen and see the brachiopod fragments, highlighted by white arrows.

Adapted from material originally published on Equatorial Minnesota. Printed with permission.


Berg, R. R. 1954. Franconia Formation of Minnesota and Wisconsin. Geological Society of America Bulletin 65(9):857–882.

Berkey, C. P. 1897. Geology of the Saint Croix Dalles. Part I.—Geology. American Geologist 20:345–383.

Berkey, C. P. 1898a. Geology of the Saint Croix Dalles. Part II.—Mineralogy. American Geologist 21:139–155.

Berkey, C. P. 1898b. Geology of the Saint Croix Dalles. Part III.—Paleontology. American Geologist 21:270–294.

Kerr, P. F. 1957. Charles Peter Berkey 1867—1955. A biographical memoir. National Academy of Sciences.

Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.

Owen, D. D. 1852. Report of a geological survey of Wisconsin, Iowa, and Minnesota; and incidentally of a portion of Nebraska Territory. Lippincott, Grambo & Co., Philadelphia, Pennsylvania. Available at here (plates not included), here (full plates) or here.

Setterholm, D. R. 2010. Geologic atlas of Chisago County, Minnesota [Part A]. Minnesota Geological Survey, St. Paul, Minnesota. C-22. Scale 1:100,000 and 1:200,000.

Weiss, M. P. 2000. Frederick William Sardeson, geologist 1866–1958. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 48.

Yochelson, E. L., and G. F. Webers. 2006. A restudy of the Late Cambrian molluscan fauna of Berkey (1898) from Taylors Falls, Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 64.


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Guest post: Interstate State Park’s bedrock tells a billion-year-old story