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Yesterday afternoon, I had some time to visit the Hoppin Hill Reservoir, in North Attleboro. At my first trip, apparently the water level was extremely low, as this time it was ~4-6 feet above the previous line. Some apparently extensive outcrops were now covered. In the path before reaching the exposures, I picked up a piece of shale from an outcrop, which contains limonite?-stained trilobite fragments, as well as a possible brachiopods. Pleura of a Strenuella strenua? I found near the end of my first trip, a fragment of drift containing a small trilobite cephalon. That area is now covered by water. However, knowing the character of the rock in which the trilobites are found, and the general area, I fortunately found the source exposure above water. Unfortunately, the rock was somewhat heavily metamorphosed, so most of the rock turned into a sort of quartzite, where I found very few trilobite fragments. Some rock turned greyish-white, probably due to the heat. Although most of the rock was like this, some of it was not metamorphosed. Some rock retained its shaly-limestone form. Shaw (1950) states that when the limestone is weathered, and the calcium carbonate leaches out, the rock is reduced to a soft siltstone that “can be crumbled in the fingers.” This was unfortunately the case for most of the rock. I managed to recover some small amount of material that had not completely weathered away. Here is a single thoracic segment preserved in the metamorphosed material. It appears to be silicified. Some fragments of trilobites were recovered from the soft siltstone. I found that upon drying completely, they become slightly more solid. Thoracic segment and free cheek, probably Strenuella. At the end of the day, I took a block of what seemed to be the fossiliferous layers, to split. I found a partial Strenuella strenua, exhibiting the stouter type, with a shorter anterior “flange.” Also, in that same block, I found this quite nice specimen, though it was very fragile. It has a more prominent “flange,” and the occipital spine is rather well preserved. Also, I prepared the trilobite cranidium from the first trip. I believe it is a Dipharus attleborensis, with a nice palpebral lobe preserved. The specimen was much more stable than the ones found on this trip, so I think the layers probably are much better below the water-line. Here is I think Anstey’s map of the reservoir, showing Shaler’s locality 2, as well as where I found the trilobite fragments. I believe the shale layers are a few meters south to where they are represented. Also, I think Landing’s (1988) revision of the formation is unneeded. He puts the Weymouth and the Hoppin formations into one, and names the lower quartzarenite layers the North Attleboro formation. As I have seen the quartzarenite layers grade into the shales and slates, I think this is unnecessary. Also, the species in the sub-trilobitic layers of Hoppin Hill are found throughout, so I think they do not need a separate name. The Hoppin Formation (with the exception of the unusual “Paradoxides walcotti” ) is a preolenellidian fauna, and the Weymouth at Mill Cove has a distinct Callavia fauna with many olenellids. Shaw suggests that the Mill Cove locality is younger in age. I agree with this conclusion. In fact, the only trilobite present in both layers, is Strenuella strenua. Several papers imply that the Pearl Street (Mill Cove) locality does not conformably overlie the sub-trilobitic layers. Correlations have been made between Mill Cove, and Nahant and Cohasset in the faunas found in the sub-trilobitic layers. I propose the Hoppin stay seperate from the Weymouth Formation, and the sub-trilobitic layers in the Weymouth be given a different name, or perhaps be a seperate member of the formation. This may only be a local thing, and somehow the two faunas were separated in some way and they could be of the same age. This is how I think the Formations are aligned. *Not to scale! Overall, I had a fun trip, though the water level at the reservoir was somewhat high.
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A rangeomorph holdfast trace fossil from the Ediacara formation, Rawnsley quartzite of the Flinders Range, South Australia. This specimen is Medusina mawsoni, so called because it was until recently thought to be a jellyfish, but is now believed to be the attachment point of a fractal rangeomorph as Charniodiscus is the point of anchorage for Charnia sp. This one may have been the holdfast point for some species of Rangea. The diameter of the outer circle is 1.5 cm and the fossil is estimated to be 555 million years old.
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