Where the heck?

[I_gotta_rock]

The Delaware Bay and Delaware River shores are littered with lovely marine erratics. They aren't local. They aren't young. You have to dig pretty darn deep in Delaware just to get to the Cretaceous in some places. These are Paleozoic.  Many pieces are limestone and probably from the Mahantango FM. But, I find just as many that are definitely not limestone. They are silicified to cert and other shades of SiO2 plus a bit of dark blue/black mineral. I'm thjingking they are ordovician, based on the  Foerstiphyllum sp. corals here. The puzzle is, where do they and the other silicified corals, sponges, bryozoa etc, come from? The DE geological Survey doesn't even mention the erratics. I was told that it's been washed down from the Appalachians. Okay, there are definitely ordovician layers there that could have eroded into the river, but all I can find are formations of limestone and fine-to-very-fine grained sandstone. Been doing all kinds of searching through descriptions of geological formations in the area and I'm coming up blank. Anyone have any ideas?

 

[Fossildude19]

These look more like Devonian Favosites and horn corals, to me. 

Maybe they are from offshore deposits, or glacier leave-behinds.  

 

What looks like chert could be dolostone, too. 

[sixgill pete]

@Plax any insight on this?

 

[Plax]

 Chert occurs in limestone beds so these would not be unusual. You can file some rock off as powder and see if it reacts with white vinegar to see if it is indeed chert or limestone. Chert doesn't phizz. Being chert would help explain their survival in the transport down the Delaware River from the Paleozoic outcrops in the Valley and Ridge Province of PA. As you know the valleys are limestone for the most part and the shales, like the Mahantango, outcrop higher up. Very resistant sandstones such as the Tuscarora Quartzite cap many of the mountain ridges. I don't think these fossils are from the Mahantango.

  I remember crabbing at some of those beaches in the early 70s and collecting fossils like this. Bower's Beach comes to mind. You can find paleozoic pebbles with fossil inclusions in NJ also on the other side of the bay. The big sand and gravel pits in lower Bucks County PA had them abundantly. Sounds to me like you need to publish a note for the DE geological Survey!

Am agreeing with fossildude19.

[I_gotta_rock]

Thank for the input, everybody!  

@Fossildude19, yep, on double-checking (missing the obvious a lot lately!) most of what I was sure was not calcium carbonate actually is very drusey limestone/dolostone. 

 

What I know for sure is that these washed down by river action. They do extend inland for at least a few miles, but the river has been wider in the past. If they are glacieral, they were carried to PA and NJ and then washed further south by the river. A long, long trek, but certainly not impossible. The glaciers never reached the Delmarva Peninsula, though. (Fun fact: Delaware has no natural ponds because we never had any glacier action. Closest we have are vernal pools. Everything else is artificial.)  Yes, they are all over the river and bay banks. At least one of the pieces pictured, @Plax is from Bowers. The others are form Kitts Hummock, Pickering and Lewes. Your favorite old haunts?

 

At any rate I guess we've circled back around to "could be from anywhere."

 

Somehow I doubt DEGS will care much, however cool it would be to publish a note. Last time I tried to contact them about a rare heteromorphic ammonite with unidentifiable sutures, they didn't bother to reply. I'm sure they know the erratics are there, but as they have no economic value, it seems to be out of their field these days.

[Plax]

Woodland Beach was another spot that had them.

  During the last glaciation the the Delaware was not a bay but a river. Sea level was about 300 feet lower but the melt and run-off volume must have been huge compared to the river today. Ice rafting is used to explain boulder and cobble sized rocks being where you wouldn't expect them but gravel such as what you're collecting is usually simply alluvial. Your fossils aren't from "anywhere" but originated up stream of their point of deposition. I think you are correct in saying that they are from the Delaware drainage system. Possibly some Susquehanna thrown in there as well. You've done a great job with this research.

  Here is a good book that has a paper on the origins of high level gravel deposits.

https://www.cambridge.org/core/journals/quaternary-research/article/postmiocene-stratigraphy-central-and-southern-atlantic-coastal-plain-oakes-robert-and-dubar-jules-utah-state-university-press-175-pp-1200/156F23D4D193D632F78A4322F60A3EEC

[pinkus]

Could they be from ship ballast? Philadelphia has been a major port for nearly 400 years.

[I_gotta_rock]

1 hour ago, pinkus said:

Could they be from ship ballast? Philadelphia has been a major port for nearly 400 years.

Unlikely. The mouth of the bay is over 100 miles from Philadelphia and when I say littered with fossils I mean LITTERED. I bring home pocketfuls every trip out. They can be found all along the shore from the southern end of the peninsula up, at every stop on both sides of the waterway until you reach Claymont ( which is kind of sheltered on a crook in the river) in the tilled fields inland, and in the cretaceous dredge spoils piles at the C&D Canal.  Kinda cool to imagine when I'm out at the canal that some of the fossils in the sand were ancient fossils on the sea floor when the belemnites were swarming in the water above. But, certain beaches have larger gravel than other places and are generally more accessible to the casual rockhound. I like fossils I can enjoy without my glasses.

 

There are metamorphic rocks from the Wissahickon formation (runs through Philly, Claymont, and points west) to be found some places and not others. Those might have been ballast, but honestly, one would expect if there is a lot of ballast around that there would be more exotic stone to be found. Would they not? Most of it looks pretty familiar to the region, even if it traveled a bit. We lack things like, say, Florida's coquina or igneous rock from the Carribbean that might have travelled by ship, even though Delaware has some important ports, current or historical, to rival Philly. The port in New Castle was where William Penn stopped for a few months before finishing the first trip to his new property and all of DC's bananas probably are coming through the Port of Wilmington, which was home to ship building companies for centuries.