Glacial Geology of the
Stony Brook-Setauket-Port Jefferson Area

Gilbert N. Hanson


Home Glaciotectonics Tunnel Valleys Evidence References

Last updated on June 03, 2008


High resolution digital elevation models are available for the State of New York including Long Island. These have a horizontal resolution of 10 meters and are based on 7.5' topographic maps. For those quadrangles with 10' contour intervals, interpolation results in elevations with an uncertainty of about 4'.  The appearance is as if one were viewing color-enhanced images of a barren terrain, for example Mars. This allows one to see much greater detail than is possible on a standard topographic map. The images shown on this web site have a much lower resolution than are obtainable from the files directly. 

Digital Elevation Models for Long Island and surrounding area can be downloaded as self extracting zip files at

A ca. five foot long version (jpg) of the DEM of Long Island (see above except with scale and north arrow) for printing can be downloaded at this link. The file is 7 MB, so be patient.

A DEM of Long Island (shown above) in PowerPoint can be downloaded at this link.

The geomorphology of Long Island has been evaluated earlier based on US Geological Survey topographic maps (see for example, Fuller, 1914; and Sirkin, 1983). Most of the observations presented here are consistent with previous interpretations. Reference to earlier work is made mainly where there is a significant disagreement based on the higher quality of the information obtainable from the DEM's. Also, it is intended that this presentation encourage others to download DEM files and re-evaluate the geology and geomorphology of other areas on Long Island. It is also hoped that this presentation will encourage others to look for more information that will substantiate or negate some of the interpretations presented here, i.e., test the hypotheses presented. If you have any questions or comments on this presentation, send them to If you wish, the comments or enhancements may be added to this web site. This presentation will continue to be considered a work in progress. 

This presentation gives a short overview of the geology and geomorphology of Long Island with a more detailed description of the Stony Brook-Setauket-Port Jefferson area. The geomorphology is re-evaluated from 7.5'  quadrangle Digital Elevation Models (DEM) files created by the USGS from USGS 7.5 minute quadrangle topographic maps with a pixel spacing of 10 meters. Each pixel has longitudinal, latitudinal and elevation characteristics. 

These maps were originally downloaded from the Cornell University Geospatial Information Repository (CUGIR).  CUGIR is an active online repository providing geospatial data and metadata for New York State. The data images were created in Global Mapper.  dlgv32 Pro is a free limited-feature version of the commercial software Global Mapper that can be downloaded at These images show elevation as a function of color and shading as if the sun were shining on the surface (shaded relief). Relief can be variably exaggerated and the colors can be adjusted. Also, culture is not included in the DEM's. Culture on standard quadrangle maps makes interpreting the geology more difficult.  Surprisingly few features that are clearly a result of human activity are discernible in the DEM's. Railroads, highways and  gravel pits can sometimes be identified. 

Another program for viewing DEM files is MicroDEM a very capable viewing and analyzing DEM tool. MicroDEM was developed by Peter Gauth of the US Naval Academy. The web site describing MicroDem and where you can download this program for free  is at

There is a sharewareMac program for viewing DEM files (MacDEM) that can be downloaded at:

You may wish to visit GeoCommunity which has listings of programs that can be used for GIS or DEM viewing. 


Fig. 1 Map of central Long Island. Note that the Ronkonkoma Moraine is in the center of the island; the Harbor Hill Moraine is along the north shore; and two large valleys cut through the Ronkonkoma moraine. These valleys probably formed during the last glaciation. The Carmans River is in the eastern valley and the Nissequogue and Connetquot Rivers are in the western valley. The box shows location of Fig. 2 and 3.


Fig. 2 Click on image for larger image.


Fig. 3 Click on image for larger image.
SB-PO = Stony Brook Post Office
SB-RR = Stony Brook Railroad Station
SE-PO = Setauket Post Office
CVS = Location of CVS construction site
PJ-PO = Port Jefferson Post Office
PJ-RR = Port Jefferson Railroad Station


Figures 2 and 3  are identical DEM's of a portion of the merged DEMs of GG53 St. James, GG54 Port Jefferson. Figure 3 has some locations identified on it. The colors are function of the elevation. The blue is at sea level. Long Island Sound, bays and harbors are thus blue. The higher elevations become green, then yellow and finally red. The reflectance is set so that the shadows are created by a sun that appears to be shining from an altitude of 45o (the sun is  45o above the horizon) and an azimuth of 45o (the sun is shining from the north east). The maximum elevation in the area is the most intense red and has an elevation of about 82 meters (270 feet).

Geology of Port Jefferson-Setauket-Stony Brook area

For research papers on the geology of Long Island you may wish to download the Bibliography of Long Island Geology and Hydrology. It is possible to search a topic or author with this program.

The basement rocks in the Stony Brook-Setauket-Port Jefferson area are probably late Precambrian and Paleozoic granites, gneisses and schists similar to those cropping out in southeastern Connecticut (Overview of New York Geology gives some pertinent information. See the tectonic map.). Above this basement, which is probably highly weathered, are Cretaceous sediments of the Raritan and Magothy formations made up dominantly of inter-layered sands and clays. This sequence is part of the Atlantic Coastal Plain (see regional physiographic map.)  These sediments and the surface of the underlying basement rocks  dip about 1o to the southeast. These Cretaceous sediments extend several miles northward into Long Island Sound (Lewis and Stone, 1991). The Cretaceous sediments are not known to crop out in Connecticut north of Long Island Sound. Overlying the Cretaceous sediments are Pleistocene clastic sediments deposited during glacial and interglacial intervals. 

The glaciers advanced from the north across Long Island Sound where they encountered the Cretaceous sediments. Sea level was some 400 feet lower during the last glacier advance so the shoreline was some 70 miles to the southeast of present Long Island. As the last glacier advanced into Long Island Sound it may have encountered a freshwater lake that was dammed by the earlier Ronkonkoma moraine. The advancing glaciers incorporated some of these sediments, in some cases pushing them in front. The glacier(s) continued until they extended onto the present Long Island. The Pleistocene sediments consist dominantly of glacial sands and gravels near the surface. South of the sequence of moraines along the northern edge of Long Island and north of the Ronkonkoma Moraine the sands and gravels are underlain by a unit known as the Smithtown Clay.

Port Jefferson Harbor occurs immediately west of the interlobate moraine between the Stony Brook Moraine to the west and the Roanoke Point Moraine to the east. Sirkin, 1983, based on topographic maps, suggests that the Roanoke Point Moraine cuts the Stony Brook Moraine. The topographic information in the DEM appears to show that the highest part of the hill is on the interlobate  moraine between the two moraines. 


Fig. 4 Map showing position of moraine segments on Long Island and relative position of glacial lobes from Sirkin (1986). Moraine segments beginning with: 
H (H1, H2, etc.) were formed by the Hudson Lobe, 
C were formed by the Connecticut Lobe.
I are interlobate between the Hudson and Connecticut Lobes
CR were formed by the Connecticut-Rhode Island Lobe. 
N were formed by the Narragansett Lobe.

The southern moraine in eastern Long Island has been called the Ronkonkoma Moraine. The morainal segments to the north were previously all included in the Harbor Hill moraine. The moraine segments of interest in this presentation are C2 Stony Brook Moraine, C3 Mount Sinai Moraine, C4 Roanoke Point Moraine and H5 Northport Moraine.  


The glacial history of the Port Jefferson-Setauket-Stony Brook area
based on the DEM's

  • The Stony Brook sub-lobe advanced forming an arcuate push moraine (Moraine 1 in Figure 3). (Link her to a discussion of glaciotectonics)

  • The Stony Brook sub-lobe retreated

  • A lobe of the Harbor Hill Moraine advanced (Moraine 2 in Figure 2)

  • The Stony Brook lobe re-advanced almost to the position of the first advance forming another push moraine (Moraine 3 in Figure 3)

Subglacial streams formed valleys (link here for discussion of characteristics of subglacial stream valleys):

  • along main street (State Highway 25A) in Port Jefferson with the Port Jefferson fan developing at the moraine margin. 

  • from the mill pond immediately west of the post office in Setauket to the subglacial stream "roundabout" in Setauket

  •  present day State Highway 25A from Stony Brook Harbor past the Stony Brook Railroad Station to the subglacial stream "roundabout" in Setauket

  • from the "roundabout" to the south along present day Pond Path becoming subaerial after cutting the Stony Brook Moraine and cutting the Terryville outwash plain and possibly the westernmost part of the Port Jefferson fan. 

  • from Mount Sinai Harbor along Crystal Brook Hollow becoming a subaerial stream south of the Roanoke Point Moraine where it developed a channel in the Port Jefferson fan.

  • The valley along Lower Sheep Pasture Road was also a subglacial stream coming from the subglacial stream roundabout in Setauket. However, there is not an obvious place where it crossed the Stony Brook Moraine, perhaps it crossed the moraine as an englacial stream before the development of the Port Jefferson Fan. Note that there is an older valley immediately south of this valley on its eastern end.

  • some of the stream valleys north of the moraines probably were used for draining the ice after the glacier began melting.

Later subaerial stream valleys with dendritic patterns developed by headward erosion along the southern edge of the  Port Jefferson Fan.  Dendritic patterns are not common to tunnel valleys which are sinuous and anastimosing or are independent and straight. One of the dendritic valleys appears to cut the valley associated with the Pond Path tunnel valley which extends out to the outwash plain.

The elevation of the Port Jefferson Fan at the mouth of the fan is about 200 feet. The elevation on the outwash fan is about 100 feet. The radius of the fan is about 5 miles and consists of about 1/3 (120o) of a cone. In the metric system this is 1/3 of a cone 30 meters high with a radius of 8000 meters and has a volume of about 640 million cubic meters.  Port Jefferson Harbor is a trapezohedron about 750  meters wide on the south end, 1,600 meters wide on the north end and has a length of about 3,000 meters. This trapezohedron has an area of about one million square meters. The elevation of the surrounding area north and west of the moraine assuming that it does not include much of the moraine has an elevation of about 50 meters or less. The present depth of the harbor is mainly less than 10 meters, but probably contains significant post glacial fill. The volume of material removed from the harbor area is thus about 60 meters times one million square meters or 60 million cubic meters,. This volume of material removed to produce the present Port Jefferson Harbor is about one-tenth that the Port Jefferson Fan. Thus, it should be expected that the harbor was excavated to a greater depth and that the tunnel valley extends into Long Island Sound.

The following site by Danielle Mulch and Gilbert N. Hanson (2005) has a more detailed description of the geology of the Port Jefferson tunnel valley.


1 Much of the material presented here was first assembled into a web site while G.N. Hanson was on sabbatical at the Institut fur Mineralogie, Universitat Munster Germany in 2000.