Rocks of the Area

Types of rocks

Most rocks of the Adirondacks are of metamorphic nature: metasedimentary, metavolcanic, and metaplutonic are all present .  It is important to mention what happened to these rocks to make them metamorphic.  During the mountain building process that took place in this area, rocks were buried many kilometers underneath the surface.  At these depths, the rocks underwent great pressures.  Along with this pressure, the rocks experienced tremendous amounts of heat generated by the Earth.  This caused the rocks to undergo change.  A metamorphic rock was produced with different characteristics.  These characteristics were driven by the orginal composition of the rock and the temperature and pressures that it experienced (Roecker).

Metasedimentary and Metavolcanic:

Most of these rocks are contained in the Northwest lowland region.  They include marbles that were metamorphosed from limestones, quartzite, and gneisses which originated as shales and sandstones.


These rocks are mainly located in the Central Highland region.  They are made up of granite gneiss (the most common), metanorthisite, and olivine metagabbro.

Geologic map of Adirondack area 
(Rogers)used by permission


Glacial and Alluvual Sediments

Middle Proterozoic Age Rocks

Leucogranitic Gneiss







Interlayered Hornblende Granatic Gneiss and Amphibole

Tonalitic Gneiss

Metamorphosed Sedimentary Roacks




There has been extreme deformation of the rock of the Adirondacks.  Two major types have occured: brittle and ductile.  Brittle deformation occured at shallow, cool depths.  Ductile deformation occured at greater depths where there was greater amounts of heat.

Ductile Deformation:

Folding is the most common form of deformation in the Adirondacks.  Very large folds, tens of kilometers across, making an east-west are, make up the southern region.  In the Northwest region, the folds generally run in a northeast direction.  Some areas, have been folded sveral times.  Foliation and lineation can be seen in the folds, revealing clues about the direction of the folds.

Besides folding, rocks that were buried at great depths have experienced ductile shear.  This happened when one block of rock slid past another.  Because of the intense heat at such depth, the rocks caught in between have been deformed and stretched.   This action has resulted in a narrow, deformed section caught between the blocks.  These sections have been strectched and flattened.  These minerals have recrystallized in the form of a fine grained rock called mylonite.  Mylonites are very common in the Adirncacks, especially in the southeastern and Carthage-Colton Mylonite Zone.

Brittle Deformation:

In the long, straight valleys that occur in the eastern half of the region, brittle deformation is dominant.

The Lake George and Schroom Lake are examples of such valleys with steep faults and a central block that has depressed at least 400 m.  These blocks that have been down faulted are called grabens.  Many such places are in the southern area and contain flat-lying sedimentary rocks from the Cambrian and Ordovician ages.

Other valleys have resulted from erosion of rock that have already been broken.  Since the raock has been broken and fractured already, it tends to erode much easier than the surrounding rock.  These fractures are differnent from faults in that the rocks have not moved,  but simply broken in place.

Joints are also present.  Similar to fractures, a joint does not need any movement of the rock.  They simply look like a long slice through the rock.(Roecker)

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