Shells, Sand, and Sea Lilies – When Playing in the Sea Raises Questions

The writing of this post epitomized the dilemma of an amateur venturing into these subjects – composing a sentence seems only to raise thorny questions, technical or otherwise, that keep me from writing that next sentence. Well, I have relied on that timeless stratagem, make the questions the content. [Another aspect of the amateur's dilemma: After initially posting this piece, I went back and tried to correct the modifiers I used to describe time-based divisions of geologic periods -- e.g., if I'm talking time, it should be Early Devonian, not Lower Devonian.]

The Sea

Driving in the early morning in the very late summer, sun rising in a deep blue sky, a massive collection of Beach Boys songs blasting from the car radio – all’s right with the world and I’m on my way to the sea. (I wish it really were that simple.)

Northwest out of the Washington, D.C. area, the roadway rises and falls and rises again. Makes sense, since I’m driving into the area known geologically as the Allegheny Ridge and Valley province. I cross over South Mountain and skirt Fairview Mountain, moving into Washington County, deeper into Western Maryland where, well, there actually is no sea. No, not today.

But, there was . . . on occasion at least during the Devonian Period (416 to 359 million years ago). That sea encroached on the interior of the continent – an epicontinental sea – and I’m fully equipped for playing in it, my backseat is loaded with steel-toed boots, hammer, and chisels. Stones and fossils, a great day at the beach.

Maps

This area, from the beginning of the Early Devonian until the Middle Devonian (416 to 385 million years ago), apparently was underwater. I have stronger questions about where the water was in this area for the rest of the Devonian because of contradictions I find in what others have written and mapped for it. Safest assertion – sea incursions and retreats (geologically termed transgressions and regressions) marked this general area in the Devonian.

All three sites on this foray’s itinerary are Early to Middle Devonian in age. Each is in a very different rock formation: I’m in pursuit of marine fossils in Needmore Shale (part of the Hamilton Group), Keyser Limestone (part of the Helderberg Formation), and Ridgeley Sandstone (part of the Oriskany Group).

The colors used for different rock formations give a superficial beauty to geological maps. But, the maps’ inherent beauty is really in the story they are telling. Consider the small portion of a Maryland Geological Survey map reproduced above. These rock formations are like many thin fingers lying across this pinched portion of Maryland, their thinness testament to the incredible pressure brought to bear on these formations during their geological history. Notice, too, that the fingers run from the northeast to the southwest.

Maps created by state geological agencies are predictably parochial. In this map of Maryland, there is nothing, literally nothing, beyond the state’s borders. These rock formations do stretch well to the northeast and to the southwest. For example, the name Oriskany comes from the tiny town of Oriskany Falls, New York.

In describing all three of the sites I played in during this trip as Devonian, I’m following the lead of the Maryland Geological Survey in its organization of rock types and formations for its geological maps of this part of Maryland (Washington County -- link here). But, of course, things are not always as they might seem – according to other references, the Keyser Limestone element of the Helderberg Formation was laid down at the very end of the preceding period, the Silurian (see description of stop number 12 of the U.S. Geological Survey’s geological field trip found at this link).

And, in this tale, there is certainly more that falls into a gray area. For instance, the vagaries of the naming of geological formations are challenging and often frustrating – names change over time, names for the same formation change from geographic location to location. According to some geologists, Ridgeley Sandstone is the name for the rock at one of my sites; for others, it’s appropriately called Oriskany Sandstone. All apparently the same stone.

Returning to maps for a moment. I ask (of myself) one of those context setting questions that typically spins off many more questions than it answers. What did this part of the globe – the North American part (with Western Maryland in it) – look like in the Devonian Period?

Dr. Ron Blakey, professor emeritus of geology at Northern Arizona University, generously provided the map that appears below showing the North American continent 395 million years ago, near the end of the Early Devonian. (With his permission, I have circled the area of my sea excursion – look for the little red circle that doesn’t really show up unless the picture is clicked and enlarged. See note below for link to his site and other wonderful maps.)


The continent straddled the equator at the time. The equator cut through the upper left quarter of the continent, running from the upper right hand corner of the map across to the middle of the left side. The United States was oriented on an angle toward the equator – if the equator were shown, it would appear to be balancing on the northwestern tip of Washington State with the rest of the States stretching off to the south. Given its location in the mid-latitudes, the land masses here had a climate that was warm and arid.

Outcomes of the Seafaring

But, what, after all, came of this sea outing? In terms of their age, from oldest to youngest, the sequence of the sites I visited runs from Keyser Limestone to Oriskany (or Ridgeley) Sandstone to Needmore Shale. At each site, there were delicate and small (often very small) signs of ancient marine life, but not an overwhelming amount (after all, I am just the most recent of nearly innumerable visitors to each).

Shells

The Keyser Limestone site is a sloping wooded creek shoreline, with one of the largest and densest expanses of poison ivy I’ve seen – a wrong turn took me down a very narrow footpath through the bulk of it (I think I made it through unscathed). The floor of the woods is littered with pieces of limestone, many fringed in green moss. Isolated brachipod molds turn up as in this first picture. (Brachipods have appeared elsewhere in this blog; they are bivalves, each valve is symmetrical but the two are of different sizes.) Identify its genus? Perhaps later.

The second picture below shows two perspectives on what I think was the same kind of brachipod. It’s clearly a shell on the left. As for the object on the right, I’ve used Moore’s Invertebrate Fossils (1952) to identify it as also a brachiopod partial, showing the internal structures of the hinge between the two valves. But, it remains a question.










Sand

The Oriskany Sandstone site left no doubt that I was dealing with sandstone. Indeed, once I suppressed the initial thought that I’d, somehow, stumbled onto an arctic microclimate complete with snow drifts, I realized that here, as the sandstone weathers and the carbonate “cement” holding it together dissolves, it leaves behind brilliantly white quartz sand.


Still newly exposed sandstone can be tough stuff and some fossil imprints repelled all efforts to break them out of their matrix. Where chunks had broken off or developed some cracking due to weathering, progress was easier. Brachipods turned up fairly often. And then there was this.



Is this the septal (apex) end of a coral?







Sea Lilies

This Needmore Shale site defines the adjective “fissile” – the propensity of rock to split into thin sheets.



Sometimes, as the shale is split along its natural cleavage lines, fossils are revealed when you flip one shale plate off another (think turning pages in a book). Here, the fossils are likely to be crinoids, so-called “sea lilies,” an invertebrate animal often looking like a lush flower on a tall stem. In the Indiana Limestone I explored in downtown Washington, D.C. (a previous posting), the little circular sections (ossicles) of the stem often showed up. My best find at this site was a stem with several ossicles in place. As an added bonus, on the top side of this piece of shale are the faint traces of crinoid arms, the elements that extend from the crown or calyx – perhaps part of the same animal, I don’t know. So, the trip ends with questions.












Sources

For such a little rambling post, I had lots of help but only I can be blamed for errors of fact or imagination.

For an inspiring excursion, I highly recommend spending some time on Dr. Blakey’s website (link here) where he has posted many examples of the beautiful paleogeographic maps he has created. These depict not just North America over time, but the world as well.

In addition to other sources cited in the text, I would add the following: Martin F. Schmidt, Jr.’s Maryland’s Geology (1993) is superb for taking the reader step by step through the geological history of the state. The chapter on the Devonian Period in Lynne Clos’ North America Through Time (2008) sets the Devonian scene vividly. There are several online and in print guides to fossil sites in this area. A dated guide from the Maryland Geological Survey offers some guidance – Collecting Fossils in Maryland by John D. Glaser, 1979 (revised 1995).