Lake Champlain Thrust Fault: Geologic History

The Lake Champlain Thrust Fault extends south from Canada to the Catskill Plateau, a distance of approximately 199 miles. It consists of Lower Cambrian Dunham Dolostone, a resistant carbonate rock formed 542 to 500 million years ago and Middle Ordovician Iberville Formation, a dark gray shale with white calcite veins which formed 460-440 million years ago. This thurst fault is a "reverse fault" in which the older Dolostone is found on top of the younger Shale. This fault was active around 450 million years ago and marks the most westerly thrust of the Taconic Orogeny, a mountain building process taking place 500-400 million years ago in which Maine pushed into New Hampshire which pushed into Vermont, resulting in the formation of the Green Mountians and many faults, including the Lake Champlain fault.



Around 500 million years ago the land mass that contained what would become Vermont was found on the equator. It was surrounded by the Iapetus Ocean, a warm, shallow sea. Around 400 million years ago the Iapetus Ocean closed and the sedimentary rocks of the shoreline and continental shelf folded and faulted to form the Green Mountains. The stresses of mountain building altered the older sedimentary rocks by heat and pressure into metamorphic rocks and also caused portions of the Earth's crust to break and move as large fault blocks. 20,000 years ago a glacier covered all of Vermont and when the Earth began to warm 12,500 years ago Lake Champlain was formed from melted glacial waters. 10,000 years ago the land was so depressed by glaciers that the Atlantic Ocean flowed into the Lake Champlain basin for a short amount of time before the land rebounded and the Lake took the appearance it has today.



As the Dolostone moved over the Iberville Shale during the Taconic Orogeny, the CaCOin the Shale dissolved out under pressure, resulting in pieces strewn all along the shore, smoothed by the water. The Shale present on Lone Rock Point is powdery and crumbling into layers due to the pressure exerted upon it by the Dolostone. Chunks of Dolostone are seen all around the beach and far into Lake Champlain. These chunks exist because as the Shale eroded underneath the Dolostone the pressure on the Dolostone became too great, causing large chunks to break off.




Dolostone: CaMg(CO3)2


Dunham Dolostone was formed in ancient seas, often in coral reefs. Corals, sponges and other animals that lived in a shallow sea more than 400 million years ago accumulated on the sea-floor and could have formed limestone, but sometime after the burial the "lime" (calcite and aragonite) in the limestone changed into a new mineral-Dolomite. Dolomite replaces the calcite in limestone and for dolomitization, or recrystallization, to occur, Mg2+ is required. No one is entirely sure how magnesuim entered the limestone, but one popular theory is that the calcium within the CaCO3 was replaced by magnesium. The Mg2+ could have been derived from the dissolution of skeletal components or from marine pore water. 

2CaCO3 (solid) + Mg2+(aq) = CaMg(CO3)2 (solid) + Ca2+(aq)



Iberville Shale:

 A gray, thinly layered rock deposited in deep ocean waters as mud (small particles of silt and clay). This Shale capped the sedimentary sequence of the Champlain Valley. Shale is a class of fine-grained, clastic (inorganic) sedimentary rock, although the Shale present at this site contains some organic materials.

Vegetation growing on/around the Dolostone:



Vegetation growing on/around the Shale:

Cedar Tree Age:

Tree #1 growing on Dolostone: Circumference: About 9 in.
               Age: About 45 years.


Tree #2 growing on Dolostone: Circumference: About 6.5 inches.

               Age: About 35 years.

Tree #3 growing on Shale: Circumference: About 20 inches

              Age: About 82 Years.