If any of you have wondered where the Commonwealth’s beautiful gray granite curbstones come from, today’s photo is the answer: The ~375 Ma (millions-of-years-old) Chelmsford granite. This aerial photo is of the Fletcher granite quarry in Westford, which has been in operation since the mid 20th century. Quarrying of the Chelmsford, and other granites, in Massachusetts has been occurring since the 1600s, according to various town histories. What is of particular interest in this photo is how well the Chelmsford granite naturally splits at 90 degree angles— something historic granite workers immediately noticed and have taken advantage of over the centuries . The old quarry worker’s term for this is “Rift and Grain”. In New England, the rift and grain of granites, in many places, is oriented in vertically at 90 degrees to one another: in North-South and East-West striking planes: Very convenient for quarrying dimension stone. The origin of this rift and grain is even more fascinating, and has been the subject of many academic papers over the years including ones by the famous Richard Jahns, Don Wise, and Terry Engelder. The rift and grain are parallel to regional fracture systems in the granite that occurred in response to tectonic stresses in the geologic past— the orientation of those stresses are different than the present day stress in the continental crust of New England (created by pushing from the spreading Mid-Atlantic Ridge). In order to take full advantage of the older rift and grain, quarry operators have to isolate large blocks of the granite and let them “relax” for a few decades so the present-day crustal stress can be removed from the rock. This minimizes wastage and ensures nice, 90 degree angles when the stone is quarried. Photo credit: Bing Maps. (via Massachusetts Geological Survey)
Pop-upView Separately

If any of you have wondered where the Commonwealth’s beautiful gray granite curbstones come from, today’s photo is the answer: The ~375 Ma (millions-of-years-old) Chelmsford granite. This aerial photo is of the Fletcher granite quarry in Westford, which has been in operation since the mid 20th century. Quarrying of the Chelmsford, and other granites, in Massachusetts has been occurring since the 1600s, according to various town histories.

What is of particular interest in this photo is how well the Chelmsford granite naturally splits at 90 degree angles— something historic granite workers immediately noticed and have taken advantage of over the centuries . The old quarry worker’s term for this is “Rift and Grain”. In New England, the rift and grain of granites, in many places, is oriented in vertically at 90 degrees to one another: in North-South and East-West striking planes: Very convenient for quarrying dimension stone.

The origin of this rift and grain is even more fascinating, and has been the subject of many academic papers over the years including ones by the famous Richard Jahns, Don Wise, and Terry Engelder. The rift and grain are parallel to regional fracture systems in the granite that occurred in response to tectonic stresses in the geologic past— the orientation of those stresses are different than the present day stress in the continental crust of New England (created by pushing from the spreading Mid-Atlantic Ridge). In order to take full advantage of the older rift and grain, quarry operators have to isolate large blocks of the granite and let them “relax” for a few decades so the present-day crustal stress can be removed from the rock. This minimizes wastage and ensures nice, 90 degree angles when the stone is quarried. Photo credit: Bing Maps. (via Massachusetts Geological Survey)

Fuente: facebook.com

The photo above showing a delicately balanced boulder was captured in the ablation zone of the Matanuska Glacier near Anchorage, Alaska. This angular rock, about 10 ft (3 m) across, was transported by the glacier from far up the valley. Eventually, the remaining bit of ice will melt out from underneath it leaving behind a large, isolated rock — a glacial erratic. Photo taken on July 29, 2009. Credit: Mark Meyer. (via EPOD)
Pop-upView Separately

The photo above showing a delicately balanced boulder was captured in the ablation zone of the Matanuska Glacier near Anchorage, Alaska. This angular rock, about 10 ft (3 m) across, was transported by the glacier from far up the valley. Eventually, the remaining bit of ice will melt out from underneath it leaving behind a large, isolated rock — a glacial erratic. Photo taken on July 29, 2009. Credit: Mark Meyer. (via EPOD)

Fuente: epod.usra.edu

kinks, bends, fractures, fillings.. oh my.. (by Ale*) Many of the iron-rich layers in this outcrop show extremely fine lamination. In this example there are four couples of pink and brown bands at the base, followed by a few whitish bands and then an overall lighter pattern before ending up in the black cherts. If one looks to the right, it is possible to see a discontinuity, a microfault, that breaks the bands. Across the fault, the bands have different thicknesses, but they are consistent in number and order. It looks like that when the bands were folded, they expanded towards the outside the bend. The cracks have been filled likely with calcite, but they stop at the edge of the iron layer: you can not follow the cracks into the chert, indicating that the iron was already a solid rock when the chert, above and below, was plastic enough to flow following the deformation. Banded Iron FormationsSoudan, Minnesota
View Separately

kinks, bends, fractures, fillings.. oh my.. (by Ale*)

Many of the iron-rich layers in this outcrop show extremely fine lamination. In this example there are four couples of pink and brown bands at the base, followed by a few whitish bands and then an overall lighter pattern before ending up in the black cherts. If one looks to the right, it is possible to see a discontinuity, a microfault, that breaks the bands. Across the fault, the bands have different thicknesses, but they are consistent in number and order. It looks like that when the bands were folded, they expanded towards the outside the bend. The cracks have been filled likely with calcite, but they stop at the edge of the iron layer: you can not follow the cracks into the chert, indicating that the iron was already a solid rock when the chert, above and below, was plastic enough to flow following the deformation.

Banded Iron Formations
Soudan, Minnesota

Fuente: Flickr / greenriver

Top