|I apologize, also, for this, which inexplicably is my only mastodon photo, from Elephant Hall of the University of Nebraska State Museum. Not quite eastern Pennsylvania, but you get the idea. The Marshalls Creek Mastodon today can be seen at the State Museum of Pennsylvania.|
The Marshalls Creek Mastodon is an excellent example of the phenomenon where a good specimen of a large Ice Age mammal is found in a small to medium-sized town and becomes a point of local interest and history. Dinosaurs may get a lot of press, but for most of the country there's a better chance of finding a mammoth or mastodon than there is of finding a large dinosaur; Mesozoic rocks aren't present everywhere, while there's almost always at least a little bit of late Pleistocene all around.
Let's set the scene. Marshalls Creek is a village a few miles northeast of Stroudsburg, Pennsylvania, near Delaware Water Gap, in the Poconos. The Delaware River itself is a couple of miles to the southeast. US Route 209 passes through Marshalls Creek, and going by the coordinates in Hoff (2001), the discovery site of the mastodon is in a marshy area with some open water southeast of the road, with a Wendy's between it and the road; come for the Frosty, stay for the Ice Age, perhaps? There's a historical marker there now, too. The marshy area is known as Leap's Bog or Leap Bog, and so the mastodon is sometimes called the Leap's or Leap Bog Mastodon.
The mastodon was discovered in the summer of 1968. (Cue the '60s montage, only with elephants.) The bog was being mined for peat, and as noted in Hoff (1969), this kind of activity is a fairly typical way for Pleistocene fossils to be discovered. John Leap, owner of the Lakeside Peat Humus Company, was conducting dragline peat mining at the bog on July 5, 1968, when the drag bucket caught on what he thought was an old stump. Part of it broke off, and employee Paul Strausser was able to identify the material as bone, not wood. It proved to be part of a mastodon's skull. A crew from the State Museum of Pennsylvania (then known as the William Penn Memorial Museum) excavated the specimen August 8–22, 1968 in a combined land-water operation that took some ingenuity and improvised engineering. (After all this hassle, the bones were put in danger again when the barn where they were being stored caught fire from a lightning strike.)
When it was all over, the specimen proved to be about 90% complete, with the most notable absence being the tusks. As of Hoff (2001), it is the most complete mastodon of at least 18 found in Pennsylvania, and the only one of mountable quality. At that time only part of it was mounted due to space, but it has since been put on display in its entirety at the State Museum, with tusks borrowed from another Pennsylvania mastodon. The specimen was found in an excellent state of preservation, but not articulated or showing other evidence of miring, like upright articulated foot bones; instead, it is more likely the decaying carcass floated around for a time in what was then a larger lake. The bones were in a calcareous marl beneath the upper peat. Chunks of wood found with the bones have radiocarbon dates of 12,160 ± 180 radiocarbon years before present and 12,020 ± 180 radiocarbon years before present ("present" being 1950, due to atmospheric nuclear testing) (Buckley and Willis 1970), which Calib 7.10 tells me work out to between 14,830 and 13,580 calibrated years before present and 14,520 and 13,450 calibrated years before present, respectively; latest Pleistocene, in other words, and certainly postglacial.
A mastodon is not too surprising in eastern Pennsylvania, as the area is well within the classic stronghold of mastodon territory, the Mid-Atlantic states plus the former Northwest Territory (check out the map in Hay 1923). Mastodons tend to be most common in the eastern half of the U.S., while Columbian mammoths take up the slack in the western half. This is probably a reflection of their habits: mastodons were browsers, while mammoths were grazers. Incidentally, if you ever need to differentiate between mammoth and mastodon fossils, start with teeth: mammoth teeth have a washboard-type chewing surface, while mastodon teeth have prominent cusps that make the chewing surface look something like the underside of an egg carton. If you're considering the entire animal, mastodons had more level backs and longer, lower heads than mammoths.
Buckley, J., and E. Willis. 1970. Isotopes radiocarbon measurements VIII. Radiocarbon 12(1):87–129.
Hay, O. P. 1923. The Pleistocene of North America and its vertebrated animals from the states east of the Mississippi River and from the Canadian provinces east of longitude 95°. Carnegie Institution of Washington Publication 322.
Hoff, D. 1969. Mastodon at Marshalls Creek. Pennsylvania Game News 40(2):3–7.
Hoff, D. M. 2001. The Marshalls Creek Mastodon. Pages 146–149 in J. D. Inners and G. M. Fleeger, editors. 2001: a Delaware River odyssey [it's a healthy 22 mb download]. Field Conference of Pennsylvania Geologists, Harrisburg, Pennsylvania. Guidebook for the Annual Field Conference of Pennsylvania Geologists 66.