Consequences of Bedrock Geology in the Adirondacks

Marble and gneiss make up much of the bedrock geology in the Adirondacks.  Both are metamorphic rocks, but while marble is quite susceptible to both physical and chemical weathering (erosion), gneiss is not nearly as susceptible, being a stronger, more resistant rock.  This is because of the rocks’ mineral compositions.  Marble is metamorphosed limestone, meaning it consists almost entirely of calcite but contains small amounts of dolomite as well.  Both are minerals that are very easily weathered by solution, a process of chemical weathering that takes place when water runs over a mineral surface.  The water grabs ions from the marble, which means that slowly and eventually, there is a noticeable difference in the mass and volume of the marble.  Marble is also quite soft and easily shaped by physical force, a reason it is so popular in sculpture.

Gneiss, on the other hand, is much more compositionally complex.  It is metamorphosed granite and consists of hornblende, biotite, plagioclase feldspar, muscovite, potassium feldspar, quartz, and garnet.  Hornblende and biotite are easily weathered chemically, muscovite and plagioclase feldspar are less easily weathered chemically, potassium feldspar is hard to weather chemically, and quartz and garnet are incredibly stable and hard to weather chemically.  This makes for a rock that is pretty difficult to erode, especially from physical means that transpire during glaciation.

This difference in resistance causes fascinating geographic trends that are especially visible in the northeastern Adirondacks, where marble and gneiss are the primary building blocks of the area.  Because of marble’s susceptibility to erosion, glaciers and other erosive processes were able to carve valleys and slopes down through it.  They were less able to erode the gneiss, however, leaving gneiss ridges and cliffs overlooking marble lowlands.  Maps containing marble and gneiss bedrock geology with rivers added are particularly intriguing to survey; the correlation and overlap between marble lowlands and rivers is hard to miss and rarely diverges, except for where the river sources appear in the gneiss-covered higher areas.  Waterways will always take the least inhibited route down to lower elevation due to gravity and the characteristics of fluids, so it makes sense that they occur where marble has been deeply scoured.  It’s thought-provoking, however, to consider that rivers and other geographic features exist where they do because of bedrock geology.  This means that so much of human history has been shaped directly by bedrock geology.

Although much of the Adirondack bedrock geology is in the form of gneiss and marble, the High Peaks region is mostly anorthosite, a relatively resistant rock, which is likely the cause that the High Peaks occur where they do.  If the High Peaks region mostly consisted of marble instead of anorthosite, there could be valleys and very topographically mild country as opposed to the peaks that arise.

A surprising geologic event in the Adirondacks is rapid (relative to other geologic activity) crustal rising, especially located in the High Peaks.  Nobody knows the reason for this ascension, and nobody even seems close to understanding the causes behind it.  I find that very strange considering how much we do know about geology and the Earth, but it’s comforting to me that humanity doesn’t understand everything yet.  It’s exciting that there’s more yet to be discovered about the nature of our world, especially in that there’s so great a mystery so close in proximity to us.  It succeeds in making the Adirondacks even more alluring than they already are.

P.S. sorry about the wait!  There were some technical issues.