Avalonian Rock Samples

Westwood Granite

(Westwood, MA)

This sample shows the characteristic orange tone and small amounts of dark minerals (biotite?)

Ash Flow Tuff with Volcanic Glass

Mattapan Volcanics, Hyde Park, MA

Extrusive ash flow tuff with embedded glass beads, which would have formed as silica-rich magma was expelled from volcanic vents.

Ash Flow Tuff

Mattapan Volcanics, Newton, MA

Rock shows welded ash from volcanic eruptions. Probable layering of the ash can be seen at lower right.

Roxbury Conglomerate

Brookline Member, Newton, MA

Small conglomerate clasts (largest size about one inch) show alignment with the bedding of the rock. Matrix of the conglomerate is rather coarse sandstone compared to many Roxbury Conglomerates featuring a fine mud matrix. A layer of sandstone is at the top, showing a gentle curvature in the bedding.

Sandstone

Brookline Member, Newton, MA

Fine sandstone representative of the sandstone that interlaces with conglomerates in Brookline Member of the Roxbury Conglomerate. The bedding can be seen in the banding along the side.

Mudstone with Crenulation

Brookline Member, Newton, MA

Metamorphosed mudstone (William O. Crosby's "Boston slate" in the Chestnut Hill Reservoir area) showing the fine bedding of the mudstone layers (left image upper left corner) and crenulation along the stress axis (left image front). Crenulation axis is about 45 degrees off of bedding layer (right image).

Roxbury Conglomerate with Quartz

Brookline Member, Newton, MA

Conglomerate showing an ash-flow clast (top right) embedded in a dark mudstone matrix. The sample has significant quartz inclusion from post-depositional modification.

Mudstone with Ripples

Brookline Member, Newton, MA

Mudstone from near the top of the Brookline member shows multiple layers bearing ripple marks suggestive of persistent, quiet depositional environment. Ripple marks show interference pattern that suggests cross currents in the environment of deposition.

Intrusive Basalt

Brighton Igneous Suite, Brighton MA

Chunk of a dark, massive basalt that was part of a dike intruded into the Brookline Member conglomerate.

Extrusive Basalt

Brighton Igneous Suite, Newton MA

Part of an extruded basalt deposited on Brookline Member showing gas bubbles formed in the magma. Larger bubbles generally about 3/8 inches. Larger bubbles were filled with inclusions that were later eroded away (on back, not shown here)

Volcanic Flow Breccia

Brighton Igneous, Brookline, MA

Jagged chunks of volcanic rock embedded in matrix that appears to be lava flow. Dark matrix contains tiny light minerals (quartz? feldspar?) showing the flow of the magma. Although not visible in this view, the breccia fragments have been flattened in the orthogonal dimension from this view, and the sample was shed from the larger rock sample along the nearly-vertical cleavage plane.

Ash Flow Tuff with Volcanic Bombs

Brighton Igneous Suite, Brookline, MA

Extrusive ash flow tuff with embedded volcanic bombs. Average size about 2-2.5 inches. Bombs show some layering in the rock, with modification of the outer layer of some of the bombs (color change).

Ash Flow Tuff

Brighton Igneous Suite, Brookline, MA

Finely bedded layers of welded ash can be seen in this sample. Some of the layers have been replaced by quartz after deposition and consolidation.