We are "Cop Rock" we are ScreechThe geology of Pennsylvania is fascinating and is undeservedly neglected in favor of the West's more striking volcanoes and active faults. A lot has happened in PA, but the land is old so it's worn-down so it's not as dramatic. And unlike the West there's plenty of water, so there are trees covering everything, which makes the geology even less obvious. For example, the volcanoes of Berks County.
We are Z. Cavaricci
We are laser removed
Tasmanian devil tattoos
You are the heart dotting "i"
In the word "apologize"
Scribbled drunk on a postcard
Sent from somewhere volcanoes are
- Bloodhound Gang, "Pennsylvania"
Yes, you read that right. I would've expected Berks County (and all of PA for that matter) to be limited to sandstone, some limestone, maybe some granite. But read the rocks in some of the hills of southern Berks, and you reveal the capricious fire and violence of the planet and the universe. A year or so ago, I was looking at topographical maps of Berks, and this formation in Exeter Township jumped out at me. Here are some screenshots of topo maps to give you context (I'm sure there's a better way to display this but I haven't found it; if you know how, please leave a comment. Unless otherwise noted maps are from Google.)
Comparing the bottom map to the one showing-the ridge-and-valley Appalachians at the top, can you appreciate how unlike the hills in the northern part of the county are, compared to the formation in the bottom map? Isolated, wedge-shaped, and surrounded by concentric rings of hills. Hills in Berks County, as in most of central and eastern PA, tend to be long, rounded off ridges (or at least pieces of rounded off ridges, sometimes dissected by old rivers), separated by flat valleys, like you can see on the top map there. Why is this? The Appalachians are old mountains formed during the Alleghanian Orogeny, a mountain building event that began over three hundred million years ago, well before the dinosaurs and the Atlantic Ocean, when North Africa ground against North America. This means, among other things, that part of the Atlas Mountains in Morocco and Algeria used to be continuous with the Appalachians. (As a result of my trip several months after this post to Morocco, I am able to present some Pennsylvania Appalachian and Morocco Atlas rock samples side by side, long lost cousins united at last!) At one point, these mountains were probably the highest mountains on Earth, like the modern Himalayas, which should reinforce your impression of their age.
Above, the world 350 million years ago when the Appalachians began forming; Pennsylvania is somewhere in the red circle. Below, the world 200 million years ago when these weird hills appeared in Berks. From geologyclass.com and scotese.com respectively.
The individual ridges today still run for several states in length; as you can see in the first map above, the more southern and central Appalachians trend north, then turn eastward in central Pennsylvania. The most easterly/southerly ridge (i.e., the most seaward of these ridges) sort of peters out just west of the Susquehanna. In practical terms, that means that Appalachian Trail hikers (who have been following that ridge since Virginia all the way to Halfway Springs in Michaux State Forest just east of Shippensburg) have to cross from the now dying ridge through a flat valley of small farms and towns to Dillsburg, and then attain what Berks Countians think of as the Blue Ridge - the last ridge cut by the Susquehanna (one of the oldest rivers on Earth) just above Harrisburg (which is why they put the state capital there) and which is again cut by the Schuylkill River as it emerges into Berks at Hamburg.
The most seaward ridge doesn't completely die though - it continues as a set of lower, worn-down hills like the ones around Mt. Gretna, and South Mountain in western Berks, and finally Mt. Penn and Neversink which continue as the uranium- and therefore (practical point!) radon-laden Reading Prong all the way to Connecticut. Hike up to the top of the newly-added piece of Pennsylvania's State forests, Gibraltar Hill (near the border between Cumru, Exeter and Robeson Townships in Berks County) and the unexpected view it affords may convince you that these mountains were once much higher.
Is that hill from an asteroid-caused tsunami? Many reasons against this, not least of which that impact was just too far south and there would be more and more obvious such remains closer, like in Virginia. Image from Slashgear.
So now you have a better idea why someone might be offended by the very existence of this bizarre isolated wedge-shaped half-cone in Exeter Township, Pennsylvania. Hills in PA are not supposed to be sharp wedges that change direction like the ones you see above, with little concentric ridges around them. And yet, to paraphrase Galileo, there it was. The best I could come up with is that these are a result of a tsunami. What? Tsunamis often leave behind wedge-shaped formations like this; and there was in fact a massive mid-Atlantic tsunami about 35 million years ago, that actually made it over the first (then even higher) Appalachian ridge, when a large meteor smashed into what is now the southern tip of the Delmarva Peninsula. (Practical point here: the ground is still sinking in Hampton Roads along the southeastern Virginia Coast, about 6 inches a century.) But really, that was hand-waving, and I didn't give any more thought to how I would test my theory. (If you can test your theory, better to test it and find out you were wrong but have a true answer, than let it dangle in uncertainty.)
When later I was looking at topo maps of PA again, I noticed this formation, around Green Lane Reservoir in northern Montgomery County, 25 miles from the first strange hill. It doesn't look like a tsunami remnant at all. But it is suspiciously circular, which makes it look much like something else. (Note: as of 2019 I have been to Green Lane Reservoir. While a perfectly nice body of water with charming villages around it, on the ground, the hills are not obviously crater-like - so I forgive upper Montgomery County residents for not making it a tourist attraction.)
Above: forested hills on the circular ridge around Green Lane Reservoir, northern Montgomery County, Pennsylvania. Below: Diamondhead crater, Honolulu in background, from Hawaii Division of State Parks.
At this point I decided it was getting ridiculous, and I better just look at a soil map. Sure enough, those hills are made of igneous (volcanic) rock. Red is igneous. (When I looked at this map I actually pounded my desk and shouted "I knew it!" So I'm a geek, so sue me.) Arrows point to the Jacksonwald Hill and the one around Green Lane Reservoir. Map is from Rocks and Minerals of Pennsylvania, published by the Department of Conservation and Natural Resources.
So I'm not the first person to figure out that there's something strange about those hills and that they're actually volcanic, but once you learn this, you immediately find out all kinds of interesting geological history of southeast Pennsylvania. The Jacksonwald and peri-Green Lane Reservoir formations are both part of an arc of scattered igneous rock ridges that extend to Newark, New Jersey, called the Wachtung Outliers.
Such was my obsession that when I was back in Berks for the holidays, I went to the Jacksonwald Outlier - although you really can't appreciate the strangeness of the shape while you're standing on it, even inside the back of the wedge, which in California we would call a box canyon if we were feeling generous (but again, Pennsylvania isn't supposed to have box canyons). I collected some samples from near the top, mostly eroded, loose rocks on the north half of Fabers Road, some of which I hammered off from a much bigger stone, to avoid embarrassing discussions with TSA as I flew back to the West Coast. (Thanks to Arch for loaning the hammer for this and Ringing Hill.) To my inexpert eye, these look like diabase as opposed to granite or some sedimentary stone.
Completely by accident, during this same visit I accidentally ended up on top of two other igneous intrusions on this visit - the first was Ringing Hill in Pottstown, which features diabase ringing rocks like the more famous ones in Bucks County, and I went there with a hammer to hear for myself. On the most zoomed-in rock type map above, you can see the red spur of igneous rock that extends southeast along the Perkiomen from the loop around Green Lane Reservoir, then west and northwest to the Berks County line, and passes the towns just north of Pottstown and north of the Schuylkill (Video below, but the audio doesn't do it justice. This phenomenon occurs when igneous rocks get glaciated for a while but we really don't understand why from a materials science standpoint.)
I already mentioned the second Pennsylvania volcano I accidentally visited, another newly-preserved park, Monocacy Hill. (Thank you for your commitment and hard work, Pennsylvania and Berks County conservationists!) When I got to the top, I noticed that the exposed rock looked exactly the same as what I found on the Jacksonwald Outlier - and indeed it's also diabase. And again, Monocacy Hill is another free-standing near-conical hill not part of an Appalachian Ridge. (When I found the diabase there in another free-standing, oddly-shaped hill, I thought to myself, "well duh.")
Above, diabase at the summit of Monocacy Hill. Below, Lake Manicouagan in Quebec.
So WHY is there volcanic rock in Southern Berks and northern Montgomery Counties? ("Trap rock", as geologists sometimes call it - visible on this nifty geological map of Pennsylvania.) Because there were a series of large eruptions two hundred million years ago, at the tail end of the Triassic (the first of three dinosaur ages that made up the Mesozoic). Two interesting things here involving the history of life on Earth: there was a mass extinction at the end of the Triassic. The jury is still out on the cause, but these volcanoes are the leading culprit. The competing suspect is the Manicouagan impactor 213 million years ago, another asteroid impact that produced was is now Manicouagan Reservoir in Quebec, and we know it affected far-flung locations because you can find microscopic spherules in contemporary strata in both New York and England (spherules are produced when a mist of molten rock is in free fall for significant time - yikes.)
But again, an asteroid impact cannot be the answer, because the timing is wrong - there's a thirteen million year gap. On the other hand, we're fairly sure that in other areas of the world, continent-wide volcanism has caused extinctions before, as in the Deccan or Siberian Traps in Asia, which would have made the Yellowstone supervolcano eruption look like a fourth-of-July sparkler. There have even been dinosaur footprints found in rocks of this age in the Passaic formation around this area, which is kind of cool. [Added later: there are dinosaur footprints from just before the Triassic-Jurassic boundary on paving stones in Valley Forge.]
Left, rhynchosauroides recreation from roelblog.nl, and right, footprints in Montgomery County. From Spencer G. Lucas and Robert M. Sullivan. Tetrapod footprints from the upper triassic Passaic formation near Graterfor, Montgomery County, Pennsylvania. From Harris et al., eds., 2006, The Triassic-Jurassic Terrestrial Transition. New Mexico Museum of Natural History and Science Bulletin.
Above: leg bone still embedded in rock wall, Capitol Reef N.P.. Below, dinosaur footprint in Canyonlands N.P. The ones in PA are much harder to spot but experts can still find them.
As to the question of why these erupted then and there, there still isn't a clear answer. As you can see above, they're studied enough to have a name (the Central Atlantic Magmatic Province) and there is much more evidence of these same eruptions in Morocco than in Pennsylvania. Morocco was just a hundred miles away when this happened, and the Atlantic hadn't spread open yet.
1) Are we sure about what caused the Triassic-Jurassic extinction? If it was the CAMP, was it a mantle plume, and more generally, what causes these terrifying mass eruptions?
2) Why do ringing rocks ring, and what is it about being under high pressures and freezing temperatures for long periods that makes igneous rocks into ringing rocks?
3) Should the Reading Prong be considered just the northeastern extension of the Southern Blue Ridge? Is there actually higher radon (secondary to higher uranium content) in the southern Blue Ridge, and if not why not? On this question, radon risk maps are inconsistent, some petering out with South Mountain, others continuing through the central Appalachians (see here and here)
Finally, here is a timeline of events, as of the time of writing, in what is now the Northeast/Mid-atlantic region of the United States. To keep there from being lots of empty space, I'm presenting it in a less-traditional and possibly even more useful logarithmic format.
References and resources:
1. Jacksonwald Syncline, with map
2. Szajna, M.J., and Hartline, B.W., 2003, A new vertebrate footprint localityfrom the Late Triassic Passaic Formation near Birdsboro, Pennsylva-nia, in LeTourneau, P.M., and Olsen, P.E., eds., The great rift valleys ofPangea in eastern North America, vol. 2: sedimentology, stratigraphy,and paleontology: New York, Columbia University Press, p. 264-272. Also see this paper for Triassic-Jurassic age dinosaur footprints found in Douglassville.
3. More information on the Jacksonwald syncline, with pictures of what the rock strata look like "on the ground". Good references if you want to read further.
4. Peter Martinson's Master's Thesis, "On the Magmatic Plumbing and Differentiation of a Shallow Mafic Intrusive System: Morgantown Pluton, its Birdsboro Dike, and the Nearby Jacksonwald Syncline, Newark Basin, Pennsylvania, U.S.A."
5. Billy P. Glass, Bruce M. Simonson. Distal Impact Ejecta Layers: A Record of Large Impacts in Sedimentary Deposits. (See the Olson et al 2002b figure - iridium from Manicougan impact is too early to be responsible for mass extinction, therefore more likely mass volcanism from CAMP.)
6. Spencer G. Lucas & Michael Morales, eds. The Nonmarine Triassic: Bulletin 3. (See p. 440, which is in agreement that the Triassic-Jurassic extinction boundary is much closer to the time of CAMP activity.)