The Alsen formation of NY state is a member of the Upper Helderberg Group, which also includes the Becraft and Port Ewen formations.  Rocks of the Upper Helderberg represent the second in a series of onshore-offshore depositional cycles which occured in the Devonian.

The Alsen formation connsists of lower Devonian (~400 mya) rocks found in an outcrop belt trending north-south from southeastern New York to Albany and shifting to an east-west trend near Albany.

The image at the left shows a stratigraphic column for the Helderberg and Tristates Group.  The Alsen formation is circled in red.  Click the image to enlarge for better resolution.

The lower Devonian strata of New York state were deposited within a single basin and provide good representation of the onshore-offshore enviornments of this time period.  The Alsen formation represents a Siegennian aged middle shelf environment, lithologically similar to the Kalkberg formation of the lower Helderberg Group which was deposited in the previous transgressional cycle.  A middle shelf environment is a transitional marine region of moderate, increasing depth and decreasing light penetration just beyond the inner shelf and grading into the deeper regions of the outer shelf (Bordeaux, 1999).

The Alsen formation has been described as "a nodular, finer grained dark-gray limstone alternating with layers of calcareous and agrillaceous shales which drape over the nodular material below".  Black calcareous shale beds are present at the top of the formation near Catskill (Mazzo and LaFleur, 1984).  Fossils most commonly found in this formation include articulate, fragmented brachiopods, bryozoans, and corals.  Though not pervasive, bioturbation is also found in the Alsen, with Chondrites and Planolites among the most prevalent traces.  As previously noted, the Alsen and Kalkberg formations are lithologically similar, and in regions of intense folding or faulting, fossils are the basis for distinguishing these two units (Bordeaux, 1999).

Throughout the Hudson Valley and southeastern New York State, the Alsen formation grades into the Port Ewen formation which can be distinguished by its great abundance of quartz nodules, though in some localities the Port Ewen lies unconformably on the Becraft.  On average, the Alsen is 20-30' thick, though in Austin Glen it reaches its greatest thickness at 35'.  The Alsen formation appears to be restircted to the Hudson and Rondout Valleys.  It thins to the north and is absent west of Ravena, NY.  (Bordeaux).

Field Trip Guide to the Glaciated Karst and Helderberg Stratgraphy:
http://www2.oneonta.edu/~zrsge/SGEFIELDTRIP.html

Geologic Map of the State of New York:
http://geology.about.com/library/bl/maps/newyorkmapmid.jpg?once=true&

New York Paleontology - Fossils of the Empire State and the Strata In Which They Are Found
http://bingweb.binghamton.edu/~kwilson/home.htm

The Fossil Hall of Fame (for species gone the way of the dodo):
http://www.extinctions.com/extinctions.com/home/extinctions.fossils?home

Do you know your official state fossil?
http://www.intersurf.com/~heinrich/statefossil.html

Bordeaux, Yvette.  "Field Trip to the Hudson Valley Region, New York State, Fall 2001".

Mazzo, Carl and Robert G. LaFleur.  "Stratigraphy of the Port Ewen Formation (Lower Devonian), Eastern New York".  Northeastern Geology, v.6, no.2, p.71-82, 1984.

Suzi is a senior double majoring in Geology and Environmental Studies and hoping to pursue graduate study in planetary science.