Ayer, Middlesex Co., Massachusetts, USA
|Latitude & Longitude (WGS84):||42° 33' 40'' North , 71° 35' 25'' West|
|Latitude & Longitude (decimal):||42.56111,-71.59028|
|Köppen climate type:||Dfb : Warm-summer humid continental climate|
Originally part of Groton, the town of Ayer was incorporated in 1871.
3 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
|Wenlock - Llandovery|
427.4 - 443.8 Ma
|Ayer Granite - Devens-Long Pond facies|
Age: Silurian (427.4 - 443.8 Ma)
Stratigraphic Name: Ayer Granite
Description: Equigranular to porphyritic gneissic biotite granite and granodiorite. Ayer Granite is divided into the Clinton facies and the Devens-Long Pond facies (Gore, 1976). In addition, there are some masses not assigned to either facies that intrude Berwick Formation west and northwest of Lawrence, and that intrude Paxton and Oakdale Formations south of Worcester and west of probable southern continuation of Wekepeke fault. Radiometric ages obtained for facies of Ayer pose problems in assigning ages to unfossiliferous sedimentary rocks they intrude. Clinton facies has a well-defined U-Pb zircon age of 433 +/-5 Ma (Zartman and Naylor, 1984) that authors cite as Early Silurian; Devens-Long Pond facies has a similar age. This age greatly compresses the time available for deposition, burial, deformation, and metamorphism of Berwick and Paxton if these units are truly Silurian. Some of the Ayer not assigned to a facies may have been more properly correlated with Early Devonian Chelmsford Granite and muscovite-biotite granite at Millstone Hill. Bodies south of Worcester may be more properly correlated with Canterbury Gneiss of CT, which lies on strike with Ayer and has Early Devonian age of 329 +/-9 Ma (Zartman and Naylor, 1984). Zartman and Naylor (1984) believe Ayer Granite has same age range as Newburyport Complex. It is quite possible, based on textural and mineralogical differences that the two facies should be separate units, representing different magmatic events (Wones and Goldsmith, 1991).
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052.