Transcript Understanding Maryland Geology

Understanding Maryland’s Geology
Supplemental Diagrams
Photo by Martin F. Schmidt, Jr.
ALWAYS!
My
Story
My
Story
My
Story
My
Story
Reasons to study geology
So our decisions today can be affected by events that
happened hundreds of millions of years ago!
Land use by humans
are
usually
the result
of
is often
controlled
by
Landforms of an area
Underlying geology
Events of geologic history
was
produced
by
Directions to study geology
Science
builds
fromtodata
we canif we
observe,
moves
But
sometimes
it's easier
understand
look at and
the big
picturetoofmaking
what has
happened
in the past,sothen
see from
how this
in what
we can
observe.
interpretations,
goes
topreflected
to bottom
of this
pyramid.
Land use by humans
Landforms of an area
Underlying geology
Events of geologic history
How have the plates moved to create
Maryland's geologic story?
Source: http://earth.rice.edu/mtpe/geo/geosphere/topics/2plate_tectonics.html
Best example of the result of the difference in erosion rates as
streams flow from the Piedmont into the Coastal Plain:
Great Falls of the Potomac, NW of DC
Photo by Martin F. Schmidt, Jr.
For conclusions of Part B:
How do structure and rock
resistance interact to make the
landforms of the provinces?
What makes the high ridges in the Plateau?
Winding Ridge
A resistant sandstone in the unit mapped here as the Pottsville Group
makes a mountain each time the sandstone appears at the surface.
Dashed lines shows where it has worn away.
Cross-section from the Geologic
Map of Maryland, 1968.
Structure (folds) doesn’t always match the
topography; here are anticlines that are valleys
due to non-resistant rocks in the center of the fold.
How do structure and rock types interact in the Valley & Ridge?
Sometimes the structure and the topography match.
An anticline of resistant
sandstone that makes a
mountain.
A syncline of nonresistant rock that makes a
valley.
Or they don’t match; here are 2 synclines with resistant rock in
the center, so they make mountains. Sideling Hill is the
syncline on the first slide of this PowerPoint.
Cross-section from the Geologic
Map of Maryland, 1968.
Resistant rock layers of the Valley and Ridge province.
Originally from Maryland Geological Survey Bulletin 19.
How do structure and rock types interact in the Blue Ridge?
The Blue Ridge is a large anticline with sub-folds (an anticlinorium).
Where the very resistant quartzite of the Weverton Formation crops
out on the surface, it makes the ridges of the Blue Ridge province:
South Mtn. & Catoctin Mtn.
The Catoctin Metabasalt show volcanoes & rifting occurred here
when the supercontinent Rodinia broke up, and the Frederick Valley
rift occurred when Pangaea broke up – about 400 million years later!
Cross-section from the Geologic Map of Maryland, 1968.
Another way of seeing the complex folding in the Piedmont
Complicated folding like this made the “dome” structures in the
Baltimore area. The folded rocks are also folded in on themselves.
Cross-section from the Geologic
Map of Maryland, 1968.
For conclusions of Part C, section 3:
As we look at the geologic structures
across Maryland (west to east), what
do they show about how the
amount of forces varies?
The Appalachian Plateaus are underlain by sedimentary rock layers
with broad, gentle, folds, sometimes described as “warped”; this
contrasts with the tighter folds in the Ridge & Valley, as shown in this
cross-section across most of the western part of the state. Sandstone
layers then make the ridges in both provinces. Click for labels. Border
between
Broad folds in Appalachian Plateau
provinces
A’ B
A
Tighter folds in Ridge & Valley
Cross-section from the Geologic
Map of Maryland, 1968.
C’ D
D’ E
B’ C
This is a cross-section of the rocks in the western Piedmont, showing
many folds and faults (faults are the heavy lines with arrows on
them). This is typical of the complex geology of the Piedmont. This
complexity formed when the area which is now the Piedmont was a
mountain range; what is left today is the geology at the roots of those
mountains.
Source: Union Bridge Quad Geologic Map, Jonathan Edwards, 1986.
Piedmont.
Source: Phoenix Quad Geologic Map, Stuart Moller, 1979.
Piedmont folds/nappe diagram; shows how deformed layers can be.
Piedmont: complex folding & intrusions + metamorphosed.
Plateau: broad
folds = “warped.”
Valley & Ridge:
tighter folds than
Plateau.
Blue Ridge: a
large fold +
metamorphosed.
Which end of this car got hit? How
So from which side did the continental collision
can
you
tell?
occur
for Maryland?
Coastal
Plain: layers
of sediment
Conclusion:
gently
continental
collisions
dipping
came from this
toward the
direction.
ocean.
Added after
continental
collisions.
A Wilson Cycle very
much like the history of
the Appalachians.
Author: Lynn Fichter,
James Madison University.
Source:
http://csmres.jmu.edu/geollab/Fichter
/Wilson/Wilsnall.pdf
More descriptions:
http://csmres.jmu.edu/geollab/Fichter
/Wilson/Wilson.html
The history of the
Appalachians – and
therefore Maryland in one set of crosssections.
Ordovician: 450 mya
Devonian: 375 mya
Mississippian to Permian: 350-250 mya
Source: Earth - Portrait of a Planet, by
Stephen Marshak
Precambrian: 1 Bya
Complete Maryland Geologic Cross-sections west of the Coastal Plain
– Slide 1.
Border
between
provinces
Appalachian Plateau
A’ B
A
Valley & Ridge
C’ D
Cross-section from the Geologic
Map of Maryland, 1968.
D’ E
B’ C
Complete Maryland Geologic Cross-sections west of the Coastal Plain
– Slide 2.
Valley & Ridge
Piedmont
Cross-section from the Geologic
Map of Maryland, 1968.
Blue Ridge
Geologic Cross Section of the MD Coastal Plain, NW to SE
Choptank & St Marys
Cross-section from the Geologic
Map of Maryland, 1968.