The Hoppin Formation named for Hoppin Hill in
North Attleborough, Massachusetts, contains
Early Cambrian fossils associated with the explosion of multi-cellular life, beginning 539 million years ago at the start of the
Phanerozoic Eon.
The bottom rock units of the formation are
arkose
Arkose () or arkosic sandstone is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar.
Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose.
Quartz is c ...
quartzite,
conglomerate
Conglomerate or conglomeration may refer to:
* Conglomerate (company)
* Conglomerate (geology)
* Conglomerate (mathematics)
In popular culture:
* The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes
** Co ...
and
sandstone. The quartzite lies on top and was sourced from the underlying Dedham Granite. Upper units in the formation include green and red
slate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. ...
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
, calcareous nodules and layers of red,
argillite limestone.
The Hoppin Formation contains fossil
trilobites
Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...
,
hyolithids and
volborthellids. It is capped off by non-fossiliferous red and green slate, 186 meters thick, followed by an angular
unconformity with the younger
Early Pennsylvanian
The Pennsylvanian ( , also known as Upper Carboniferous or Late Carboniferous) is, in the International Commission on Stratigraphy, ICS geologic timescale, the younger of two period (geology), subperiods (or upper of two system (stratigraphy), s ...
Pondville Conglomerate.
In total, a 244-meter thickness of the formation is exposed. In 1969, Boston College geologist, James Skehan, proposed that the unit might be related to the Manuels Brook section in southeastern Newfoundland.
Paleoenvironment
The Hoppin Hill Formation records a
marine transgression and a near-shore shelf environment on top of
Paleozoic granites and matches up with the
Weymouth Formation, near Boston.
Other outcrops
In addition to the namesake exposure at Hoppin Hill, limestone and slate belonging to the formation are exposed beneath a power line, east of Cumberland Street and immediately north of the Rhode Island state line in Wrentham.
References
{{reflist
Natural history of Massachusetts