at Altona Flat Rock
The Champlain Valley in northeastern New York showing
the location of the "Flat Rocks" in Clinton County (above).
The Cobblestone Hill ice margin during deglaciation (below),
about 12,500 years before present.
Much of the upper Little Chazy River watershed lies on Altona Flat Rock, the largest of a discontinuous 30 km-long, 5 km-wide belt of sandstone pavements created more than 12,000 year ago by the catastrophic drainage of proglacial Lake Iroquois and younger post-Iroquois proglacial lakes (see adjacent figure). The breakout flood originated near Covey Hill, P.Q. and flowed southeastward along the margin of the waning Laurentide Ice Sheet in northeastern New York. The torrential outflow eroded the mantle of glacial sediment in its path and created the pavement areas or "Flat Rocks" in the process.
The flood water stream ultimately emptied into glacial Lake Coveville (Franzi et al., 2002) in the Champlain Valley creating Cobblestone Hill at its mouth. The Cobblestone Hill ridge is more than 15 meters high, 500 m wide, and 2.5 kilometers long and is composed of angular boulders, almost exclusively Potsdam Sandstone, that range from 0.5 to about 3 meters in diameter. The average size of surface boulders decreases to the southeast. The position of the ice-front at the time of the breakout is marked by large kettle holes on the northern flank of Cobblestone Hill. The ice front extended northwestward toward Covey Hill where it corresponds closely to ice-front position No.11 of Denny (1974) (see figures). The Cobblestone Hill deposits occur in crude terraces at elevations of approximately 230 m and 205 m, which lie close to the projected water planes of glacial lakes Coveville and Fort Ann (Chapman, 1937; Denny, 1967, 1970). The highest deposits on Cobblestone Hill correspond to similar deposits at Bear Hollow, approximately 1 km south across the present valley of the Little Chazy River. We believe that these data indicate that the ice front lay near the location of Cobblestone Hill as the Lake Iroquois breakout began. The flood discharge initially deposited large boulders of Potsdam Sandstone, most of which were quarried locally from the sandstone pavements, into Lake Coveville. Glacial Lake Coveville dropped during the later stages of the breakout flood and the lower portions of Cobblestone Hill boulder deposit were graded to the Lake Fort Ann level. It is possible that the large influx of floodwater from the Lake Iroquois breakout overwhelmed whatever dam was impounding Lake Coveville and initiated erosion of the outlet to the Upper Fort Ann threshold. We have also observed that the southern extent of the Ingraham Esker, which extends 27-km northward from Beekmantown to Champlain, lies close to the reconstructed ice margin at the time when the ice front stood at Cobblestone Hill. We believe that it is also possible that the 30-meter drop in lake level between lakes Coveville and Fort Ann steepened the hydraulic gradients of meltwater within the ice mass and initiated the formation of the Ingraham Esker tunnel system. Cobble and gravel terraces on the northeast flank of Cobblestone Hill represent beach ridges formed in Lake Fort Ann (Woodworth, 1905a; Chapman, 1937; Denny, 1974) following retreat of the ice from the Cobblestone Hill ice margin.
< References >
< Acknowledgements >
for Earth &
Little Chazy River
|Research Experiences for Undergraduates (REU) Program||Lake
D.H., 1937, Late-glacial and postglacial history of the Champlain valley: Am.
Jour. Sci., 5th Ser. V.34, No.200, p.89-124.
C.S., 1967, Surficial geologic map of the Dannemora quadrangle and part of the
Plattsburgh quadrangle, New York: U.S. Geol. Surv. Misc. Geol. Inv. Map I-630
1970, Surficial geologic map of the Mooers quadrangle and part of the Rouses
Point quadrangle, Clinton County, New York: U.S. Geol. Surv., Geological
Quadrangle Map GQ-635.
1974, Pleistocene geology of the northeastern Adirondack region, New York:
United States Geological Survey, Professional Paper 786, 50p.
J.A., Yansa, C.H., Franzi, D.A., and Knuepfer, P.L.K., 2002., The biology and
hydrology of the northern Lake Champlain basin before the Holocene: Geol. Soc.
Amer. Abstracts with Programs, V.34, A-15.
J.B., 1905a, Pleistocene geology of the Mooers Quadrangle: New York State Mus.
Bull. 83, 67p.