The Geology of Newton

by James W. Skehan, S.J. and Catherine W. Barton

STOP 2. NEAR NEWTON SOUTH HIGH SCHOOL

Outcrops of the Brighton Volcanics occur extensively in the wooded area across Brandeis Road from the school. From the first set of parking lots north of the school and to the left (east) of Brandeis Road, a path leads to a break in the fence and up the hill. The outcrops form two steep cliffs, one low, and the other higher which are separated by a rather flat bench.

Important features of the upper cliff are:

1) The rock itself is a fragmental volcanic material, certainly pyroclastic and probably a welded tuff. The rock is characteristic of a near vent deposit.

2) The rock is felsic rather than mafic and has a blocky fragmental character. The blocks range from fine ash to blocks inches to feet across.

3) Collapsed, possibly recrystallized pumice and lapilli make up most of this deposit, although amygdular lavas may constitute a certain part of the volcanic sequence. These rocks are white and rather plastery in appearance.

4) Amygdules or gas hole fillings and small lapilli can be seen in the felsite along with some vug quartz.

5) The "nubbles" which can be found at certain spots may be subaerially deposited volcanic bombs or lava droplets which initially solidified as glass beads and have since undergone recrystallization. Notice the differential weathering here.

6) Farther toward Route 9 from the "nubbles," numerous quartz veins can be found which extend in varying directions indicating that these veins may perhaps all radiate out from a single source. These veins were fissures filled by quartz solutions.

7) At certain points a definite foliation or lamination of the volcanic rock can be seen. This is interpreted as a layering probably developed as the tuff or the ash accumulated on the flanks of the volcano. This type of layering is characteristic of the early formed ash deposits of Surtsey Volcano, in Iceland as the island was being built up by eruptions above sea level.

Important features to notice on the middle bench: Large perched boulders, glacial erratics, are of Roxbury Conglomerate. Some deformed pebbles can be noticed in the conglomerate.

The conglomerate contains rounded to subrounded pebbles of quartzite, granite and felsite (Fig. 7).

Features on the lower cliff: Notice the red color of the rock which is a welded rhyolite tuff. There is a second Brighton Volcanics outcrop along the west side of the school.

A number of features can be observed on this hill, including the fragmental nature of the volcanic rocks, perched glacial erratics and a distorted quartz vein.

STOP 3. OAK HILL

Drive up Ober Road which runs off Brookline Street. Oak Hill is a drumlin (elevation 291') which was formed by glacial action as described generically above.

Notice:

1) Sandy till in the outcrop at the top of the hill.

2) Rocks of varying sizes from sand to cobble size intermixed in the till.

3) These rocks are of varying compositions and textures.

4) As you walk down the road a bit you can see the Blue Hills, which form the southern border of the Boston Basin in the distance.

5) From the top of Oak Hill you can look across to Bald Pate Hill (elevation 302'), another drumlin. Note the typically rounded elliptical shape of the hill.

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