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

Evidence

One characteristic of a tunnel valley is that there will be till along the walls of the valley (C.'O Cofaigh, 1996). Several construction sites exposing the walls of valleys in the Stony Brook-Setauket-Port Jefferson area have been investigated. Why is till an indicator of a tunnel valley. In the hummocky terranes and the moraines in this area it is typical to find near the surface about 3 feet of till with 0 to 3 feet of loess overlying the till. In a warm based continental glacier till is dominantly at the base of the glacier and includes the shear zone between the glacier and the underlying sediments or bedrock. Till consists of a mixture of grain sizes from boulders to silt and clay with the rock types dependent on the path of the glacier. In the shear zone the clasts are abraded and broken by the movement of the glacier, the elevation of the shear zone in the till continually rises and falls. Some till advances with the glacier and some is left behind if it is below the active shear zone.   If the valley forms after the glacier has receded, one would expect that the till would not drape over the valley walls but would be cut (Fig. 1a). If however the valley is formed by water flowing through a tunnel at the base of the glacier, after the tunnel forms the till at the base of the glacier (dark layer in Fig. 1b) will drape the valley.

 

Fig. 1 a The dark layer is till. If a stream valley forms sub aerially after the glacier has deposited the till (top image), the till layer will be cut and not drape the valley (bottom image).

Fig. 2b. If the valley is formed subglacially (in top image, blue is ice, dark blue is water in tunnel and in  bottom image white is air) after the tunnel valley forms , the advancing ice of the glacier is likely to deposit till along the walls of the glacier (bottom image).

Till overlying sands and gravels along walls
of tunnel valleys in
Stony Brook-Setauket - Port Jefferson

(Click on images for a larger image.)

Section at Old Post Road and Crystal Brook Hollow behind house in Port Jefferson
on west wall of Crystal Brook Hollow tunnel valley.
This section is near the bottom of the valley.

isoclinal-fold.JPG (175504 bytes) Till with cobbles at the top of the section. It is dark colored due to soil processes .
 		 till-over-sand.JPG (171728 bytes) Close up shows till overlying sand. Note the flame structure of the sand into the till.
    till-down-slope.JPG (202240 bytes) Till at surface further down the slope.

At 25A and Bennets Road in Setauket. Behind CVS Drug Store
On east wall of Bennets Road tunnel valley.
CVS on map above.

Dcp_0504.jpg (199589 bytes) Note till with cobbles at the
top of the section overlying
sands and gravels


Sheep Pasture Road and Upper Sheep Pasture Road in Setauket on north wall of Lower Sheep Pasture Road tunnel valley.

Dcp_0396.jpg (137532 bytes) Till is red from soil development Dcp_0398.jpg (161656 bytes) Close up of till with underlying gravel and  sand

Behind Stony Brook Post Office in Stony Brook
on east wall of Stony Brook tunnel valley.
SB-PO on map above

 
Shows till overlying
sands and gravel.

West of Schomburg Apartments on Stony Brook Campus in Schomburg tunnel valley(which is now a recharge basin).
SCHO on map above

Till along wall with rotated
 block of gravel.
Till overlying gravel and sand