America's Volcanic Past -
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Volcanic Highlights and Features:
[This list is just a sample of
various Maryland volcanic features or events and is by no means inclusive.]
Maryland is part of six distinct physiographic provinces:
Excerpt from: Maryland Geological Survey Website, 2001 |
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Atlantic Continental Shelf and Coastal
Plain:1
Piedmont Plateau:1 Maryland's Piedmont Plateau Province is composed of hard, crystalline igneous and metamorphic rocks and extends from the inner edge of the Coastal Plain westward to Catoctin Mountain, the eastern boundary of the Blue Ridge Province. Bedrock in the eastern part of the Piedmont consists of schist, gneiss, gabbro, and other highly metamorphosed sedimentary and igneous rocks of probable volcanic origin. In several places these rocks have been intruded by granitic plutons and pegmatites. Several domal uplifts of Precambrian gneiss mantled with quartzite, marble, and schist are present in Baltimore County and in parts of adjacent counties. Differential erosion of these contrasting rock types has produced a distinctive topography in this part of the Piedmont. The rocks of the western part of the Piedmont are diverse and include phyllite, slate, marble, and moderately to slightly metamorphosed volcanic rocks. In central Frederick County the relatively flat Frederick Valley is developed on Cambrian and Ordovician limestone and dolomite. Gently undulating plains underlain by unmetamorphosed bedrock of Triassic red shale, siltstone, and sandstone occur in three areas in the western Piedmont. Blue Ridge Province:1 Unlike the Coastal Plain and Piedmont Plateau Provinces, the Blue Ridge, Ridge and Valley, and Appalachian Plateaus Provinces are underlain mainly by folded and faulted sedimentary rocks. The rocks of the Blue Ridge Province in western Frederick County are exposed in a large anticlinal fold whose limbs are represented by Catoctin Mountain and South Mountain. These two ridges are formed by Lower Cambrian quartzite, a rock which is very resistant to the attack of weathering and erosion. A broad valley floored by Precambrian gneiss and volcanic rock lies in the core of the anticline between the two ridges. Ridge and Valley Sediments:1 The Ridge and Valley Province between South Mountain in Washington County and Dans Mountain in western Allegany County contains strongly folded and faulted sedimentary rocks. In the eastern part of the region, a wide, open valley called the Great Valley, or in Maryland, the Hagerstown Valley, is formed on Cambrian and Ordovician limestone and dolomite. West of Powell Mountain, a more rugged terrain has developed upon shale and sandstone bedrock which ranges in age from Silurian to Mississippian. Some of the valleys in this region are underlain by Silurian and Devonian limestones.
Appalachian Plateau:1 The Appalachian Plateaus Province includes that part of Allegany County west of Dans Mountain and all of Garrett County, the westernmost county in Maryland. The bedrock of this region consists principally of gently folded shale, siltstone, and sandstone. Folding has produced elongated arches across the region which expose Devonian rocks at the surface. Most of the natural gas fields in Maryland are associated with these anticlinal folds in the Appalachian Plateau. In the intervening synclinal basins, coal-bearing strata of Pennsylvanian and Permian ages are preserved.
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| Maryland's Volcanic Rocks |
Maryland's Igneous Rocks:1
The Piedmont Plateau Province is composed of hard,
crystalline igneous and metamorphic rocks and extends
from the inner edge of the Coastal Plain
westward to Catoctin Mountain, the eastern
boundary of the Blue Ridge Province.
Bedrock in the eastern part of the Piedmont consists of schist,
gneiss, gabbro, and other highly
metamorphosed sedimentary and igneous rocks of
probable volcanic origin. In several places these rocks have been
intruded by granitic plutons and pegmatites.
Deep drilling has revealed that similar
metamorphic and igneous rocks underlie
the sedimentary rocks of
the Coastal Plain.
Baltimore Gneiss:2
Baltimore Gneiss:
any of several similar looking banded gneisses in a
variety of colors, texture, and composition that
were quarried along the Jones Falls and Gwynns Falls in Baltimore City.
It is thought that the
first buildings of Baltimore in the 1700's
used stone quarried from the Jones Falls gneiss,
near the old Mount Royal railroad shops. Many of
the tone buildings, foundations, roads,
and curbstones in Baltimore were built of rock
from both the Jones Falls and Gwynns Falls quarries.
The blue-gray color of fresh stone caused the term
"blue stone" to be used by the quarrymen.
The last building stone quarry in Baltimore
closed in 1958.
Ellicott City "Granite":2
Ellicott City "Granite":
a porphyritic gneiss that was first
quarried in the late 18th century.
It was used in building the Basilica of the
Assumption in Baltimore, which was erected during
the period 1806 to 1821. Material was hauled from the
Ellicott City area to Baltimore by
huge wagons drawn by nine yoke of oxen.
After 1892, the stone was used primarily in
foundations and as paving stone.
Little building stone
was produced from this area after 1896.
Soldiers Delight Serpentine Barrens:3
Soldiers Delight Serpentine Barrens is
located in the Soldiers Delight Natural
Environmental Area (NEA) in western Baltimore County. The barrens
are underlain by serpentinite,
a rock that contains very little quartz and
aluminum-bearing minerals and consists mainly of serpentine. When
serpentinite weathers most of the rock
dissolves leaving behind a thin, sand- and clay-poor soil
which is easily eroded. Therefore the land surface
over serpentinites is stony, unfertile and
sparsely vegetated - hence the term "serpentine barren."
Typically a serpentine barren contains scrub oak
and pine, cedar, grasses and some unique and rare wildflowers.
Serpentine is valued as a decorative building stone, road material,
and, in two Maryland localities, a
historic source of chromium ore. During the
19th century Soldiers Delight and the Bare Hills district of
Baltimore City were the largest producers of chrome in the world.
In these two locations,
chromite is a significant accessory mineral in the serpentine
and was mined up until 1860. Several old
mines and quarries are still visible in these
serpentine barrens.
Sykesville Gneiss:2
Sykesville Gneiss:
a dark gray, often schistose gneiss with a roughly rectangular
fracture pattern is currently being quarried near Potomac
in Montgomery County
and was at one time used
extensively in the Washington, D.C. area for
foundation and rubble stone.
The stone was first worked commercially as "granite" about 1850.
The early settlers in the Washington area used
many of the schists and gneisses found
along the Potomac River. This Sykesville stone may be
what was once known as "Potomac Bluestone."
Woodstock Granite:2
Woodstock Granite:
a pinkish-toned, coarse-grained, gray granitic rock
that was quarried near the town of Granite in Baltimore and
Howard counties.
This rock was first quarried in 1832
and was used intermittently until the 1920's.
It was employed as a monument stone as
well as being used for building exteriors.
Buildings made from this stone include: parts of the
Capitol building and the Library of Congress in
Washington, D.C., the Baltimore Customs House,
and the old Baltimore County Court House.
Many curbstones, paving blocks and bridges
are made of this stone.
| Great Falls of the Potomac |
Great Falls of the Potomac, Montgomery County, Maryland:1
The Great Falls of the Potomac are located west of Washington, D.C.
in Great Falls Park. This series of cascading falls has developed on the
Potomac River in the Wissahickon Formation.
The Wissahickon consists of resistant metamorphic rocks
including schists, gneisses, metagraywackes
and metaconglomerates.
These rocks date from the
late Precambrian and are about 750 million years old.
The Falls formed when sea level dropped
during the last Ice Age, causing the Potomac to downcut its valley.
| Soldiers Delight Natural Environmental Area |
Soldiers Delight Serpentine Barrens:3
Soldiers Delight Serpentine Barrens is
located in the Soldiers Delight Natural
Environmental Area (NEA) in western Baltimore County. The barrens
are underlain by serpentinite,
a rock that contains very little quartz and
aluminum-bearing minerals and consists mainly of serpentine. When
serpentinite weathers most of the rock
dissolves leaving behind a thin, sand- and clay-poor soil
which is easily eroded. Therefore the land surface
over serpentinites is stony, unfertile and
sparsely vegetated - hence the term "serpentine barren."
Typically a serpentine barren contains scrub oak
and pine, cedar, grasses and some unique and rare wildflowers.
Serpentine is valued as a decorative building stone, road material,
and, in two Maryland localities, a
historic source of chromium ore. During the
19th century Soldiers Delight and the Bare Hills district of
Baltimore City were the largest producers of chrome in the world.
In these two locations,
chromite is a significant accessory mineral in the serpentine
and was mined up until 1860. Several old
mines and quarries are still visible in these
serpentine barrens.
Excerpts from:
1) Maryland Geological Survey Website, Pamphlet Series,
A Brief Description of the Geology of Maryland, 2001
2) Maryland Geological Survey Website, Pamphlet Series
Building Stones of Maryland, 2001
3) USGS/NPS Geology in the National Parks Website, 2001
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