Cambrian Fossils of Maryland

[EMP]

Well it's been a while since I've last been on (over two months), and I know how much you all have been missing me , so I decided to finally get around to photographing some of the finds I've made over the summer. 

 

I've talked a bit earlier this year about collecting in the Frederick Limestone and other upper Cambrian-lower Ordovician units, but these finds are from rocks far, far older than those, nearly 100 million years older! These fossils are among some of the oldest in Maryland, and in the Mid-Atlantic region, which was part of the reason I collected them in the first place (because, let's be honest, most aren't that appealing). 

 

If you find these things interesting, the Araby was originally mapped as the Antietam Sandstone until about 1940ish when it became a separate geologic formation due to the strong difference in rock-type most common in either (the Antietam is mostly a quartz-sandstone, the Araby mostly a sandy and muddy shale and siltstone). When the time for the split came, the new name Araby was given to the formation that occupied a band roughly stretching from the Potomac River to the south north in a rough question mark shape to Pennsylvania as the type locality was situated near Araby Church (an interesting bit. A geologic formation from the Cambrian explosion named after a church!). Nowadays the church is gone as far as I know, but the area still bears the name with the apply named Araby Church Road. Going back further, in July of 1864, the Araby Formation would play a major role in the Battle of Monocacy. As Confederate forces under Jubal Early's command were marching east along the B&O RR, they were stopped in the vicinity of Frederick by scattered forces under the command of Union General Lew Wallace. During the day long battle (fought July 9), Wallace's outnumbered force of 5,000 men used the hills and small ridges to their east as a last line to stem the Confederate tide, strength roughly 15,000. This ridge, of course, was made up of the resistant Araby Formation, whose clastics didn't erode through time as quickly as the carbonates of the Frederick Limestone. Unfortunately for Wallace and the Union, the Confederates were able to outflank their positions, and forced them to retreat east past Urbana. Although it was a Confederate victory (the northernmost of the war), the battle delayed Early's advance for a crucial 24 hours, allowing reinforcements from the Union 6th Corps near Petersburg to arrive in Washington DC in time to stop the Confederate attacks on July 11-12 at Fort Stevens. Interesting to see how geology plays a role in how battles (and history!) are fought. 
 

I collected twice this summer, once in the early part and another time in September, from a roadcut near Frederick. This cut exposes the early Cambrian Araby Formation, which is nearly 550-530 million years old. The Araby is a nearshore clastic unit, likely deposited in a surf/beach zone on the elevated Piedmont block (a fancy term for a higher lying seabed). As such, it roughly correlates to the Antietam Sandstone further west, as well as, more roughly, the Kinzers Formation in Pennsylvania in the upper sections. Geologically speaking, the Araby is divided into coarser, almost buff siltstone and sandstone units and black, slaty-shale and siltstone (this includes the former Cash Smith Shale, which was found out to be in the middle of the Araby upon later work) ones. The darker, shale layers likely were deposited during times of deeper water, as there exists a degree of faunal differences between the two to suggest such (Olenellus thompsoni has been recorded from the black layers, but I never found any). Later, during the Taconic and Acadian Orogenies, the Araby Formation was slightly metamorphosed as were most other Piedmont and Blue Ridge units, though some parts escaped mostly untouched. These, of course, have the best fossils. 

 

Boring rock stuff out of the way, the Araby and the Antietam were formed at a special time in Earth's history called the Cambrian Explosion, which was a period when life underwent a rapid series of diversifications. Luckily we didn't miss out much here! Many beds of the Araby are filled with burrows and other traces of ancient wormlike creatures, as well as rarer edioasteroids, trilobites, and other creatures. Unfortunately little work has been done on the Cambrian units of Maryland, and less still on the Araby, so I haven't found any list of actual names for any species. As such, I'll use names from the Antietam Sandstone, as the two are time, stratigraphically, and lithologically equivalent. 

 

By far the most common fossils were the worm burrows, Skolithos linearis. These are rounded, somewhat tube shaped objects in their usual form, though they can sometimes occur as cross sections as you'll soon see. On top of this, they're also sometimes preserved in iron minerals, as is common with many other fossils. From what I've gathered, these "tubes" are interpreted to be the resting places of worms, likely annelids. 

 

Now, I'm not claiming to know 100% what some of these are so if any of you may have a better ID please let me know. 

First up are the Skolithos linearis. 

 

The first image is of a fairly typical "tube" shaped structure. 

 

The second image shows a cross section cut-away of a "tube", partially mineralized in what is likely iron (iii) oxide. 

 

The third image is of a large, albeit poorly preserved, complex of "tubes". The general way to tell where they are is by looking for the dark contours of them, and tracing them that way. 

[EMP]

Another cut-away, partially mineralized, Skolithos linearis. 

[EMP]

This is a mid-sized S. linearis, also mineralized like some of the above. In this case, the darker matrix helps the fossil pop-out more, and makes the over all piece a lot more attractive. 

 

 

This is a very large "tube", that is also somewhat curved. Initially, due to its large size, I mistook it for a trilobite fragment, but now I'm fairly confident on the S. linearis ID. This was also the first specimen I found! A great way to get started! 

 

 

This shows a couple of poorly preserved "tubes", deformed due to metamorphosism. This one in particular is interesting due to the number on the piece, with two being almost perfectly parallel to one another. 

 

 

 

 

 

[Kane]

I've removed the imgur links from your post. We discourage the use of third party sites for posting photos given that they may be temporary in nature. As we archive all posts, third party links have a tendency to expire, which would then make a post with said links of lesser value as a future resource.

 

There are a number of options for resizing images to conform to the 3.95 mb limit, be it through the native photo software of your operating system or various free online services.

 

Do try posting again once the images have been resized.  

[EMP]

This is a very cool find, as I believe it might be an arthropod fossil. It has the appearance of a shell mold in iron oxide, oddly like a trilobite. I can't ID beyond this, though. I know the Kinzers Formation has produced non-trilobite arthropods in relative abundance, and the Antietam and Araby have produced trilobites so the possibility exists. Perhaps some kind of Tuzoia? 

 

 

 

[EMP]

A thin, but long, "tube" cross section. In darker rocks like this, the number of S. linearis and the diversity of forms becomes less, adding to the idea that these were deposited at deeper depths below the nominal habitat range for the species. Also note a couple of very intriguing specimens above and below the "tube", the structures that appear almost like crinoid stems. Might they be edioasteroids or some other echinoderm? 

 

 

 

Another view of the echinoderms (?). 

Here is another "unknown" specimen. It appears similar to a crinoid, except these rocks are too old for such a fossil. Possibly it is an echinoderm or edioasteroid? A cool find regardless. 

 

Another "unknown", possibly another echinoderm or arthropod preserved in an iron oxide. 

 

[EMP]

An interesting S. linearis due to it appearing to have a branching pattern. Perhaps evidence of two animals meeting together during resting/feeding? 

This shows a large, albeit poorly preserved, mass of S. linearis. Sometimes with specimens where it's hard to see as in this one, finding darker contours can help outline the shape of the "tube", which you can then roughly follow and trace. 

[EMP]

A plate showing many, though poorly preserved due to metamorphosism, S. linearis. Most are in cross section. This piece helps show how common they can be in some layers. 

A little hard to see, but it is the dark line near the bottom. Deformed due to metamorphosism. 

 

 

A thick, but short, S. linearis. This shows just how diverse these fossils can be in size and form. This guy must have had some hardy eating! 

 

 

[EMP]

This is a small specimen, but interesting to note due to it's color and pen-shape. 

This is one of my favorites. It shows multiple S. linearis, but the number of individuals combined with it's size, color, and curved shape also add a lot of value to it. I don't know why these S. linearis are in this pattern, but my guess would be some kind of resting/feeding habit. Perhaps they liked to graze/snooze in groups to increase odds of finding food and protection? 

[Miocene_Mason]

Thanks for posting! I've never even heard of this newfangled Araby formation! I guess all my sources are outdated, which as you know is not uncommon with Maryland west of the D.C.

seen these rocks lying around Frederick and could tell they were sedimentary (albiet metamorphosed a bit) and wondered why they weren't fossiliferous (and I guess they are!). Very interesting!

[EMP]

38 minutes ago, WhodamanHD said:

Thanks for posting! I've never even heard of this newfangled Araby formation! I guess all my sources are outdated, which as you know is not uncommon with Maryland west of the D.C.

seen these rocks lying around Frederick and could tell they were sedimentary (albiet metamorphosed a bit) and wondered why they weren't fossiliferous (and I guess they are!). Very interesting!

 

Thanks! 

 

A good resource you could use for accurate maps in the Frederick Valley is this:

 

https://pubs.usgs.gov/gq/gq-1800/gq1800-screen.pdf

 

Most of the rocks used as gravel fill in the area are quarried from the Frederick and Grove Limestones, not the Araby. I have seen some places use Araby Formation rocks for landscaping (like rock gardens), though, mostly in the area of Monocacy Battlefield. 

 

[doushantuo]

recommended(might post these,if so desired and not already in Fruitbat's Library):

slightly older:

 

 

 

 

[Arizona Chris]

I see cambrian trace fossils.  That might be a good area to find some trilobite trackways.

[Fossildude19]

This item looks like it could also be a concretion/nodule.  

I find many similar to this in a Newark Supergroup plant site in CT. 

Some matrix removal might help to prove your theory.  

 

[doushantuo]

plants,Skolithos,that triggered something:

[EMP]

9 hours ago, Arizona Chris said:

I see cambrian trace fossils.  That might be a good area to find some trilobite trackways.

 

That's nice to hear! I hope there'll be more than just trackways though! 

8 hours ago, Fossildude19 said:

This item looks like it could also be a concretion/nodule.  

I find many similar to this in a Newark Supergroup plant site in CT. 

Some matrix removal might help to prove your theory.  

 

 

Thanks for the info Fossildude. 

 

Any ideas on the "echinoderms"?