polymathy

Proterozoic country rock

Medford dike is Triassic gabbro, just after the first dinosaurs
intrusive
seeped into a fissure
solidified
dikes are younger than surrounding country rock

gabbro is mostly mafic
pyroxene (darker colored)

plus some plagioclase feldspar (lighter colored)

striations from glacial grooves (faint)

Lynn Volcanic Complex
felsite
lighter colored

then a dolerite dike cuts through it
dolerite, unlike gabbro and basalt, has larger grains (not fine-grained, not coarse-grained)
pyroxene
more abundant than plagioclase

it has iron, which means it oxidizes when it weathers, and looks rusty


mixed volcaniclastic sediment and pyroclastic units of a lower Middle Hill Member
(Zpmh), and lithic crystal tuff of an upper Wrights Tower Tuff Member.

back to LVC
pyroclastic felsite
tuff
Pyroclastic rocks made of ash with any of the other three components are known as tuff, which is what occurs here on Pine Hill. In the closeup view at the steps to the tower (image above), you
can see plagioclase crystals (tiny white specks), scattered lithic fragments (f, occasional round to angular clasts) that are mostly volcanic, but also include metasandstone (more on the metasandstone later), and pumice fragments (p, pinched light gray splotches). This is where a hand lens or magnifying glass can be helpful

Boston lower than the Fells because sedimentary rock, which is more easily eroded

minor fault
easily eroded
creates valley

the volcanic rock forms an upland, the gabbro a valley because of differential erosion