Thursday, April 2, 2009
Importance of the Red Beach Granite
The Red Beach Granite holds a special place in the geology of eastern Maine because of its relationships to surrounding rocks. It intrudes into deformed sedimentary and volcanic rocks of the Eastport Formation, so it must be younger than the Eastport Formation. The Red Beach Granite is overlain, in turn, by sandstone and conglomerate of the Perry Formation, which contains cobbles that were eroded from the Red Beach Granite. This relationship demonstrates that the granite must have intruded, cooled, been uplifted, and eroded before the Perry Formation was deposited.
The sequence of events including eruption of volcanic rocks, deformation of the earth's crust, and intrusion of Red Beach Granite is a part of the formative process of the Appalachians called the Acadian orogeny. The subsequent, relatively quiet time of erosion and accumulation of the Perry Formation marks the end of this mountain-building episode. Therefore, the Red Beach Granite dates the climax of the Acadian orogeny in eastern Maine. Fossils indicate that the Eastport Formation formed near the beginning of the Devonian Period and the Perry Formation formed near the end of the Devonian Period. So the Middle Devonian age implied for the Red Beach Granite is generally taken as the age of the Acadian orogeny in eastern Maine. Recent calibrations measuring the decay of natural radioactive elements (Tucker and others, 1998) date the Devonian time period to span 362 to 418 million years ago.
Some of the blocks remaining here have clean surfaces where the texture of the granite can be seen. Close examination shows that, as for all true granites, three predominant minerals make up the rock. Quartz is light gray and translucent. One type of feldspar (plagioclase) is milky white. The other type of feldspar (alkali feldspar) is orangey-red. It is the red feldspar that gives the rock its distinctive color. In addition there are very tiny flecks of black mica, found mostly with quartz, that comprise a few percent of the rock.