Flying High Above Boston’s West African Harbor Islands
On an Appalachian-Derived Beach at Fort Lauderdale
Living Cretaceous Fossils in Bloom in Boston’s Backbay
Luxuriating in the Grenville-Age High Peaks of the Adirondacks
A Summer’s Wade in the Late Cretaceous Marl of Big Brook
Monster Mushrooms in Chestnut Hill, Massachusetts
My Lofty Visit to an Alpine Bog in New Hampshire
High Atop Laccolithic Katahdin in the Remote North Woods of Maine
The Remnants of Historic Fort Bowie within the Apache Pass Fault Zone
Oblivious to most passersby alongside Beacon Street, a major thoroughfare out of Boston, is a cross-section of an ancient streambed embedded within a cliff wall. The stream bed appears as a semi-circular channel outlined perfectly by fallen leaves. The transected bed and its banks consist of fine-grained, thinly-bedded, fissile (easily split along its planes) siltstone (mud rock) that displays a large infill of conglomerate rock over its entirety. The siltstone preserves the contours of an ancient landscape that was buried by subsequent deposition.
Upon close inspection, laminations within the streambed display whorls of sediment indicative of stream turbidity currents and slump features indicative of settling. The manmade wall at the top is composed of stacked conglomerate boulders.
The flat-lying rocks of the entire assemblage, being sedimentary, were deposited horizontally under the action of gravity. Subsequent to their deposition, compaction, cementation and lithification (conversion to solid rock), the assemblage and the rocks in the region were tilted by tectonic forces, which accounts for the angulation seen in the photo. These rocks belong to the Roxbury Conglomerate, a 2,000 foot thick formation of coarse arkosic sandstone with small to medium-size, rounded clasts (rounded fragments of stones). In 1830, the American poet Oliver Wendell Holmes likened the Roxbury to puddingstone, its common name, since it reminded him of raisins in English bread pudding.
The puddingstone's sandy matrix and rocky inclusions indicate they were deposited in a high-energy depositional and/or transport system such as a cascading mountain stream or a massive submarine flow. The Roxbury is exposed almost everywhere in the neighboring towns to the west and southwest of Boston. The channel's siltstone is a facies change, a clastless sediment within the Roxbury Formation. Along with the Cambridge Argillite (or Slate), the Roxbury Conglomerate comprises the sedimentary strata of the Boston Bay Group. As mentioned in the first vignette at the top of this post, the group was deposited on the microcontinent of Avalonia in an extensional regime, such as a faulted rift basin in Late Proterozoic-time between 595 and 540 million years ago.
Avalonia originated as an elongate volcanic island chain along the edge of the megacontinent of Gondwana, possibly of West Africa cratonic provenance in the southern hemisphere. Avalonia’s deeper basement is volcanic in origin, and, in the vicinity of the Boston Basin, they include the Brighton, Dedham, Mattapan, Lynn and Westwood granites, which underlie the rocks of the Boston Bay Group. During the Acadian orogeny, Avalonia welded to the continent of Laurentia about 370 million years ago. Can't get enough of the Roxbury Conglomerate? Check out my previous post here.
The "unnoticed" streambed is an example of my masthead statement at the top of my blog. "Geology is all around us, scarcely thought of as we go about our lives." Perhaps I should add, "but not by all of us!"
|High Dynamic Range digital photograph|