Introduction to the Area:
Wellesley Island State Park is located on Wellesley Island, in the St. Lawrence
River. It is part of the Town of Orleans, in Jefferson County. One of the main attractions
of the park is the Minna Anthony Common Nature Center, which includes a museum,
varied habitats such as wooded wetlands, 3 miles of shoreline and open granite outcrops,
and miles of trails for hiking, cross-country skiing and nature education (Fig. 1).
Fig. 1. Map of Wellesley Island State Park (Google Maps).
Lithology:
Wellesley Island is located on the Frontenac Arch, which is the exposed
connection between the Adirondacks, NY, and the Grenville Provence of Ontario (Reitz
& Valentino, 2006). This area is made up of mostly phaneritic pink granitic gneiss.
There is also sandstone, limestone, amphibolite, and quartzite. Wasteneys et al. (1999)
has found the age of the leucogranite on Wellesley Island to be around 1172 Ma.
The granites are well exposed on Wellesley Island (Fig. 2). Some of the pink
granite is slightly foliated or gneissic recrystallized granite and has contact with country
rock quartzite and calcsilicate. Both of these occur as xenoliths in granite (Wasteneys et
al., 1999). Wasteneys et al. studies also show that the ages of the pink leucogranite are
consistent with the age of Rockport granite and must be part of the same suite (1999).
Fig. 2. Granite found on Wellesley Island (Bratek).
The sandstone that is present on Wellesley Island is a part of the Potsdam
sandstone. It is apparent in a thin belt around 3.5 km long and about 500 m wide
(Wallach, 2002). It is separated from the pink granite on both sides. Due to the linearity
of the belt and the change from the much older granite to the 600 Ma quartz sandstone,
the sandstone occurs in a fault-bounded block that has dropped down relative to the
granite. Wallach observed breccia zones with second-order fractures, which suggests that
there are normal faults and bounding lineaments which define a small graben (2002).
Reitz and Valentino have found kinematic indicators that suggest ductile deformation in
this area which is possibly associated with the Ottawan orogeny (2006). Also, it has been
noted that this right lateral ductile shear zone may be a conjugate to the sinistral shear
zones located in the central Adirondacks.
Concretions can be found in the sandstone, ranging in diameter from 2 to 15 cm
(Pair, 1997). See Fig. 3. This shows that at one time, Wellesley Island and the area
around it was covered with a glacier. The rock present around some of the concretions
has been much worn and has produced furrows with sharp rims about 1 cm deep. Some
extend as lateral furrows for 10-20 cm parallel to the regional ice flow direction, towards
the Ontario Basin. These concretions are evidence of subglacial conditions at the former
bed of the ice sheet (Pair, 1997). Erosion by small amounts of sediment-filled subglacial
meltwater could have produced these forms. There are many ways in which these forms
may have been produced, but there is no way of knowing the actual way. Pair notes that
however it occurred, the degree of separation of the glacier sole from the bed was very
small and there only needed to be a very small water depth of a few centimeters (1997).
Fig. 3. Concretions found in the Potsdam sandstone, located on Wellesley Island in the
channel of the St. Lawrence River (Pair, 1997).
Discussion:
At Wellesley Island, I mostly saw the pink granite. There were many exposed
outcrops along the trails. I also saw the Potsdam sandstone. I did not see any of the
concretions, however, I did see the glacial potholes. I saw two of them, one six feet deep
and twelve inches wide and the other fifteen feet deep and thirty-six inches wide (Fig. 4).
The possible cause of these potholes would be by the glacier providing the height needed
for water and rock debris from the melting ice to rush downward through channels with
enough power to scour holes in the bedrock. That process is really amazing to think
about, but the actual pothole was hard to see and if the signs were not there, I would have
overlooked it.
Fig. 4. Glacial pothole found on Wellesley Island (Bratek).
I took a look at the pond also, before going on the Granite Trail (Fig. 5). This
trail was probably my favorite part because I got to walk up the hill and the trail led right
over the top of the exposed granite. I saw many joints and fractures in the granite and
saw nothing but granite all around me. I measured the strike and dip of foliation and
joints. I have done this before but this time I now had knowledge of what type of rock I
was actually looking and what it was made up of.
Fig. 5. Granite outcrops on Wellesley Island (Bratek).
Summary:
Wellesley Island is made up of 1172 Ma pink granite with xenoliths of quartzite
and calcsilicate and 600 Ma Potsdam sandstone. The entire area was once completely
covered by the Laurentide Ice Sheet and evidence of that is found in the glacial potholes
observed as well as the concretions studied by Pair (1996). It is a dextral ductile shear
zone and part of the St Lawrence fault zone.
References:
Pair, D. (1997). Thin film, channelized drainage, or sheetfloods beneath a portion of the
Laurentide ice sheet: An examination of glacial erosion forms, northern New York State,
USA. Sedimentary Geology, 111(1-4), 199-215. doi: 10.1016/S0037-0738(97)00015-8
Reitz, K., Valentino, D. (2006) Structural analysis of a ductile shear zone at Wellesley
Island, Thousand Islands, New York. Geological Society of America, 38(2), 67.
Wallach, J.L. (2002). The presence, characteristics and earthquake implications of the St.
Lawrence fault zone within and near Lake Ontario (Canada-USA). Tectonophysics,
353(1-4), 45-74. doi: 10.1016/S0040-1951(02)00285-8
Wasteneys, H., McLelland, J., Lumbers, S. (1999). Precise zircon geochronology in the
Adirondack Lowlands and implications for revising platetectonic models of the Central
Metasedimentary Belt and Adirondack Mountains, Grenville Province, Ontario and New
York. Canadian Journal of Earth Sciences, 36(6), 967-984. Retrieved September 18,
2009, from Platinum Periodicals. (Document ID: 46826934).