Dr. Hoge`s Rock Walk

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Transcript Dr. Hoge`s Rock Walk

Dr. Hoge’s Rock Walk
(Houston’s Main Street Geology
Field Guide)
Start on South Deck
The purpose of this “Rock Walk” is to learn about the rock cycle. We will see
examples of each type of rock; igneous, sedimentary and metamorphic. As we
go from stop to stop, keep in mind the story of these rocks, and of the earth
itself. We will be traveling to the core of a batholith and out. We will ride a
subducting continental plate as it is subsumed into the earth’s mantle. We will
be resting calmly in seas and lagoons, lakes and hot springs, surrounded by
sea life. We will be buried and heated and pressed into new forms. And, we
will be lifted up from the depth of the abyssal plain to the peaks of the
Appalachian Mountains.
Along the way, we will also see another lesson from geology, stratigraphy. We
will discuss the earth’s history in the rocks we see, but we can also explore
Houston’s “City Stratigraphy”. We will walk over tiles once adorning the floors
of banks and hotels, now paved over for parking lots. We will see the
regression of store facades from the pressures of widening streets. We will
see pieces of Texas’ history imbedded in Houston’s building like the xenoliths
we see in the rocks adorning them.
I hope you are ready for the ride.
Stop #1: Our first stop is the South Deck,
overlooking Buffalo Bayou
A bayou is a body of water that flows inland during high tides and towards the sea at low tide. It is common in
coastal areas where the elevation increases gradually. These types of coasts are typically “saliciclastic coasts” and are
formed from floodplains and deltas of larger meandering rivers which deposit sediment as they flood and approach
the sea.
A meandering river has a life span. They start as a trickle through the lowest part of a water shed. As they flow and
gain water, they broaden and form channels. A river channel carves the land it flows through creating features we are
all familiar with. The flowing water forms cut banks on the outside edges of turns where the energy of the flowing
water is greatest. They form point bars on the inside of turns where the energy of the water is lower. Levees are
piled up on either side as the river floods. The heaviest sands are dropped along the banks of the river as the flood
water flows into the flood plain. These flood plains are filled with finer silt and clay. Forests that grow in these flood
plains are called riparian. Rivers meander because they follow the path of least resistance. As the river approaches
the ocean, it must slow down. It deposits the last of its sand into deltas and its silt and clay onto the continental
shelf. These broad flat layers of sediment are called beds, and over time are buried, compacted and cemented into
sedimentary rocks.
Buffalo Bayou displays many of the features of a meandering river.
Stop #2
SW corner of Main and Franklin
This rock is commonly called Texas Pearl. It is a diorite. It is closely
related to granite. It was formed from magma hotter than that which
forms granite, however. We will discuss the entire history and
structure of the batholith this diorite came from later in this guide (it is
the Town Mountain Batholith, formed in the Texas Hill Country).
Diorite forms from mid-temperature magmas, so it is more mineral rich
than granite, but still less than basalt.
This particular diorite comes from Central Texas, near Marble Falls.
Stop #3
NW corner of Main and Franklin
This rock is granite, though you can tell by the size of the crystals that it is close to a diorite. Another
feature of this rock is its xenoliths. They are pieces of shale metamorphosed into schist by the heat
and pressure of the intrusion of this magma into the surrounding country rock. Shale is a
sedimentary rock that forms from fine-grained, organic rich sediments.
You should notice that the quartz in this rock is smoky. Smoky quartz is smoky due to carbon
included in the crystals as an impurity. Other forms of quartz can be pink, blue, purple, yellow and
orange, all due to different impurities.
Because this granite formed from lower temperature magmas than the diorite we saw at stop #2, we
know that it formed nearer to the edge of the pluton (the intrusive plume of magma.)
Because there are xenoliths, we know that this rock was formed at the contact margin with the
overlying or surrounding country rock.
Stop #4
SW corner of Main and Congress
This rock is another diorite, and is most likely from the same quarry as
the previous sites. In fact, in this diorite you can see the contact
between the diorite and the granite we saw in stop #3. There are
quartz veins in the contact zone which indicate the cooling of the
magma closer to the country rock.
So, all three stops so far are likely from the same quarry, from different
parts of the pluton. This may seem surprising, that each of these
rocks which appear so different are in fact from the same place, but
doesn’t this make economic sense? Consider the expense of shipping
stone into Houston for construction.
Stop #5
NW corner of Main and Congress
(Bee Hive sculpture)
This is another granite, though it is closer to being a porphyry. This granite is white due to the potassium in its major
mineral, feldspar. We will see this mineral in all of the various granites on this walk, but they will differ in color.
Notice the large crystals. This indicates the lower temperature of the magma that this granite formed from, relative to
the ones we’ve seen so far. The white phenocrysts are sodic feldspar. The pearly phenocrysts are potassium feldspar.
Rocks with large phenocrysts surrounded by a matrix of smaller crystals are said to be porphyritic. A porphyry would
be a rock with a matrix of crystals too small to see without a hand lens.
There are also xenoliths in this rock, but they are the diorite we saw in the previous site. This tells us that this granite
was a second intrusion into the first one.
It is also from the same quarry as the others, but younger. How do we know that?
This granite is part of the Grenville Orogeny, and is ~900 million years old, while the older granite is from the Kenoran
Orogeny, about 2.5 billion years old. The Kenoran Orogeny established the margin of the craton that underlies most
of Canada and eastern North America. The Grenville Orogeny marks the last Precambrian activity in Texas, and
includes the intrusions at Enchanted Rock.
Stop #6
Between Congress and Preston
This is a good place to stop and discuss Houston’s “City Stratigraphy”.
Stratigraphy is the study of the layers of the earth’s crust. We can decipher the earth’s history by studying the rocks
in these layers. By using the Law of Superposition, the Law of Original Horizontality and the Law of Cross-Cutting
Relationships, we can determine the waxing and waning of oceans and the tectonic history of continents.
These tiles are Terrazo tile made from pieces of rock. The pieces in this tile are:
brown – chert
red – jasper (flint)
white – limestone
What’s interesting about these rock fragments is that they all came off of the Ozark Plateau in Arkansas. They are of
Mississippian age, meaning about 325 million years old. They formed from Si-rich, low pH ocean water (Question –
our current ocean pH is trending down due to global warming. At what point might we see chert forming in our
oceans?). Limestone is made from CaCO3, while the chert and jasper are made from silica. They are all sedimentary
Stop #7
NW corner of Main and Preston, sculpture
The rock in this sculpture is also sedimentary. This is a coastal carbonate forming from the lime that precipitated out of ocean water, fine silts
and sands, and shell fragments. The fact that these shell fragments are small and mixed indicates that they formed in the surf zone. This is
called a shell hash. You can also see algal mat fragments in this rock.
The sand layers indicate a low energy of deposition, while the shell layers indicate higher energy. Notice how they are “stratified”. The
pattern of these layers of high energy and low energy beds indicates that this rock formed in a tidal flat, or possibly a bay inlet similar to
Galveston Bay.
There are Crinoid fragments in this hash, however, indicating that this rock is of at least Mississippian age. Crinoids went extinct at the end of
the Permian, and did not appear in the area this rock formed in since the Mississippian.
This rock was also formed in the Texas Hill Country, near what is now White Horse Crossing. There have been three major oceans covering
Texas throughout the Phanerozoic, during the Mississippian, the Permian and the Cretaceous.
We should also take some time to look at the river stones at the base of this sculpture. These stones look similar, but likely include limestone,
granite, porphyry, migmatite, sandstones, etc. Some show veins and xenoliths. Others may possibly contain fossils. They were rounded and
polished by tumbling along a river channel. Considering economic interests, these were probably accumulated by the Colorado River and also
quarried from the Texas hill country.
Are you beginning to appreciate the scope of Texas’ geologic history?
Stop #8
Between Preston and Prairie
Ah, now we see more a familiar rock. This is Texas Pink Granite, commonly
called Texas Rose. It is the granite that forms Enchanted rock. This again is
the same batholith that produced the white granite we saw earlier. This
granite formed from yet lower temperature magma and so has a slightly
different mineral composition, which accounts for its more vibrant color.
The black mineral in this rock is horneblende (as it was in the white granite).
The pink mineral is feldspar, but with a higher potassium content. The redder
the color, the cooler the magma (more Ka rich, with some added Fe).
This part of the batholith cooled around 1 billion years ago.
Stop #9
SW corner of Main and Prairie
One of the prettiest rocks on the tour displays the most violent history. This is a Serpentine Breccia Schist, commonly called greenstone.
Let’s break its name down a little.
Serpentine is a metamorphic rock formed from deep sea oozes rich in Fe and placed under extremely high temperature and pressure. This
indicates major tectonic activity, usually the subduction of an ocean plate. Calling it a breccia indicates that it has been broken up into pieces
and moved somewhere other than where it formed. A schist is another type of metamorphic rock that forms when there is enough heat and
pressure to partially remelt the crystals in the rock so that they begin to migrate into separate layers. This is called foliation. Look closely, the
large serpentine inclusions in this rock are foliated.
Saying that this is a serpentine breccia schist, therefore, indicates that this rock started as an ooze on the ocean floor, was metamorphosed by
tectonic activity, was weathered and transported and then metamorphosed again, probably by an intrusion. Whew!
The white swirls in this rock are asbestos. Asbestos is a mineral that forms at high temperatures. The white inclusions are marble veins,
indicating lower temperature and pressure metamorphism of CaCO3, incorporated into this rock during the second round of metamorphism.
This rock probably formed in the Appalachians, though it is possible that it formed in the hill country. I’m not aware of any exposure here,
but the ages and processes between the hill country and the Appalachian mountains are similar.
This rock formed early to mid Precambrian (2.5-4 billion years ago).
Stop #10
NW corner of Main and Texas
More Texas Pearl, but higher temperature. How do we know? The tiles are
Texas red granite, so same age, and from the same area of Texas, the same
batholith in fact, but different in color due to more sodic feldspar compared to
Ka feldspar. Notice at the base of the red columns that the granite looks
different (whiter). It’s the same granite, so why does it look different? Go to
the end of the block and look at the swirls in the granite in the square posts.
Imagine how this rock formed. It was molten rock, swirling like convection
currents in a pot of boiling chocolate.
We should also notice the drain covers on the street, which say – “Drain to
Galveston Bay National Estuary – Please Keep Clean” You mean trash we
throw in the street here can end up in Galveston Bay? You bet it can. How?
Stop #11
NW block of Main and Capital
We are at the end of our story of the batholith. This black
rock is gabbro, a high temperature igneous rock (the
highest we’ve seen so far). It is most likely from the same
batholith that was the source of the array of granite and
diorite we have seen along Main St. (I’m not sure, but
since it could be I’ll include it in the story). As you walk
the block, notice the increase in phenocryst size. This
indicates decreasing temperature and gives me a good
place to tell the whole story.
Let’s Recap the Story
Imagine a huge upwelling of magma from the earth’s mantle, into the Precambrian seas bordering the North American
shield. There was no Texas at this point, only the layers of limestone and shale covering this ancient sea. The oldest
rocks in Texas are ~ 2.5 by old, the Packsaddle Schist, formed from the metamorphism of this intruding plume.
This magma boiled up over many millions of years, never quite reaching the surface, cooling into layers like an onion.
The innermost layers were the hottest and therefore the most mineral rich. These are the gabbros. As the layers
cooled from the outside in, they formed granite and then diorite. We have not only seen these rocks, we have seen
the transitions of one type of rock into another (remember the changes in crystal size and the inclusion of xenoliths).
This is truly amazing. From one quarry in central Texas we have walked through space and time. By looking closely
at the sides of these buildings, we have seen the earth from the inside out.
Now these rocks are exposed, but remember, there was another ocean covering them during the Cretaceous. This
thick sequence of rocks can be seen in other parts of the Texas Hill County, but these younger sedimentary rocks have
been removed over the last 100 million years by the erosion that has formed all of Texas south of the Balcones
escarpment (Wow! That’s another story, but we’ll have to look at the geologic map of Texas on the ninth floor for that
one). We will see a piece of this history across the street, however.
Walk two blocks west and one block south
to the corner of Milam and Texas
Enter the Travis Tunnel at the Calpine Entrance
Stop #12
Lobby of the Calpine Building
There is a blue marble here that is truly amazing. It is beautiful, and also difficult to figure out. The
blue color is probably sodalite, a mineral that forms when a volcanic mineral called nepheline is
invaded from below by sodium chloride. Since this is marble, the original rock was limestone. The
volcanic sodalite may have come from deep sea volcanos and mixed with the CaC03 in the ocean
abyss, or it may have percolated through the limestone as it was being metamorphosed. The
intricate patterns suggest the second origin, but the darker streaks are fractures filled by the sodalite
and might suggest the first origin. It could have squeezed the sodalite out of the limestone under
The green color is a mineral called grossularite (a form of garnet). You can also see some red
grossularite in this rock, and some of mixed color. The green indicates high iron in the garnet while
the red indicates low iron. It is believed that these garnets form from the metamorphism of impure
siliceous limestones. What does this suggest about the origin of the sodalite?
Stop #13
Calpine Tunnel entrance
As you take the steps down to the Travis Tunnel you will see another amazing rock. This is a gneiss
filled with adradite (the blood red garnet which is January’s birth stone). Look at the wall to your left
as you go down to see the foliation characteristic of a gneiss.
At first look, this gneiss appears to have formed by the metamorphism of a granite, but there are a
couple of clues that suggest this rock may have in fact been a meta-granite. A meta-granite is one
that forms from deeply buried sediments metamorphosed very slowly by heat and pressure. This is
not regional or contact metamorphism, but the excruciatingly painful pressure of burial.
How do we know this? One clue is the absence of feldspar. The grossularite is another clue,
remember it forms from siliceous limestones. The black mineral is horneblende which forms from
shale. So, by looking at the mineralogy we can surmise that this rock was originally a very low
carbonate sandy shale deposited in a shallow marine environment then slowly buried and
metamorphosed into a meta-granite and then a schist and then a gneiss over a long period of time.
Stop #14
Follow the Travis Tunnel through the JP Morgan Building lobby (notice the town mountain
granite) into the part of the tunnel that leads to the Houston Club
This is a travertine, but not your typical one. Most travertines form from mud oozing from hot
springs. This one formed from mud flows from a geyser. This is happening today in Yellow Stone
National Park, though different minerals give the flows different colors and viscosities.
The oolites and pisolites (the fish egg and pea sized round structures, remember this has been cut
smooth) indicate ground water percolation after the travertine was laid down. The banding is from
algae which grew in mats on the surface of the mud flows. You can count the number of mud flows
by counting subsequent algal mat surfaces. Some of these blocks have as many as seven mud flow
So, the geyser erupts, mud flows, algae grows on the surface as ground water percolates through the
mud from below, the geyser erupts again, the algae grows again, and the story continues . . .
Stop #15
The Esperson Building elevator bank where the Travis Tunnel
meets the Tunnel Loop
This is marble (a medium to highly metamorphosed
limestone). It also has evidence of organic rich mud
layers, which indicates lagoonal origin, possibly behind a
reef. We know it is pretty highly metamorphosed by the
schistose bands of orange and black. See how wavy they
are. The orange is from iron in the mud. There are also
large sections of calcite indicating the original limestone
was very fine-grained.
Stop #16
Go east in the Tunnel Loop to the BankOne elevator bank
Wow, I love this rock. This is a low grade metamorph limestone with stromatolites (algal mats) and
veins of glauconite. This rock has been put under pressure, but not enough to destroy the fossil
The green veins are glauconite, a sedimentary rock which forms from the fecal material of organisms
living in a lagoon or shallow water. The red algal fragments, along with occasional pieces of coral,
snails, bivalves (oysters and clams) and cephalapods (ammonites, related to today’s nautaloids),
indicates that this was probably a coral reef (algae form the “cement” of coral reefs, and are actually
the most abundant organisms in a reef). There are bands of fossil hash indicating tidal energy. The
dark gray is the mud that eventually buried the reef. Though very hard to tell, the shell pieces seem
to indicate that this limestone formed during the Mesozoic (there may be crinoid stems, along with
the shelled cephalopods which both indicate an age older than the Cenozoic).
Wow, I love this rock! (did I say that already?)
Oh, well, this is our last stop in the tunnel, so you may decide to end the tour
here and eat lunch. If you do, you’ll miss the white and red marbles at the next
stop along with more travertine and metamorph limestone and even some more
Texas history.
There is some of the Oriental marble at the base of the limestone at the Esperson
Building, so if you stop for lunch, at least go back to see that. It’s quite a story.
If you continue the tour,
Return to the Travis Tunnel, but exit through the Esperson Building to
Cross Main St. at Rusk
Stop #17
NE corner of Main and Rusk
This rock is marble. The white marble with black swirls is made of
calcite, the mineral form of lime (CaCO3). The relatively large calcite
crystals result from low temperature metamorphism. The lack of
fossils indicates high pressure, however. Limestones usually contain
fossils, but they are destroyed during the metamorphic process. The
black swirls form from carbon and iron that migrate together during
This marble is probably Appalachian, formed during the Appalachian
orogeny, ~ 600 million years ago.
Just down the block is Oriental marble. This one tells quite a story. The limestone this marble formed from was brecciated, which means it
was broken up into pieces and moved by erosion. This particular type of breccia is called a contact breccia. It was formed when a cave or
cliff face collapsed.
The red color indicates a high Fe content in the water these limestone pieces fell into, so probably an arid sea, like the Persian Gulf today
(also high salinity).
The greenish inclusions are shell hash which would have been the top of the bed overlying the breccia.
The white crystal are calcite without Fe. This indicates that this body of water was shallow, but with low energy, so not a coast with wave
energy. Possibly a bay or lagoon butting up to cliffs.
Note the green veins within the large red fragments. These are veins of the original Fe-rich lime, oxidized and leeched into the red color of
the calcite. The white veins are possibly secondary to the metamorphism that formed the marble. They may have formed as hot water
seeped through pores in the marble.
The dark green inclusions are quartzite, formed from sand beds in the original sea. The lighter green inclusions are quartzite from finer silt
and shale (even though quartzite does not usually form from limey sediments, but rather from sandstone, the high aluminum content in this
sediment allowed for a slower metamorphism, so that the crystals could become large enough to form quartzite).
This marble is probably older than the white marble, but possibly not. It is probably from the Appalachians, but possibly not. These are fairly
common marbles found throughout the world, even though they tell a very elaborate story.
Farther down the block is more Texas Red Granite with Texas Pearl.
Stop #18
East side of Main between Texas and Prairie
UUUUGGHHH! All of what is described below has been plastered over!
Talk about City Stratigraphy!
(I’ll leave the description of what was here, though, just in case you’re interested.
If you want to see something similar, visit the San Jacinto Monument. The walls inside are made
of shelly travertine.)
We’re almost done, but I promised a story of the Cretaceous in Texas, the time the dinosaurs roamed. The rock on the façade of this building
is Travertine. Travertine is a sedimentary rock that forms in hot springs or areas flooded by hot springs. This rock formed in the Llano area
of Texas, during the Cretaceous period.
The clams that left the molds in this rock were probably fresh water clams indicating that this travertine probably formed in a shallow lake
flooded by a hot spring slurry.
If we take the time to notice some other evidence from this rock we will again see Houston’s “City Stratigraphy”. Travertine is a very “soft”
rock and weathers easily. It would not have been used on the outside of a building on purpose. Notice the weathering on this exposed
travertine. This was originally installed indoors. In fact, look at the tiles in front of us and the evidence of walls that once stood here. We are
most likely standing in the lobby of an old bank or hotel. This lobby was removed to make room for the expansion of Main St. Look along
Texas Ave, you will notice evidence of the curb being laid down atop older tiles. Imagine being here when this building was still opulent, and
the parking spaces were filled with Studebakers.
► Sorry
to slow us down, but I can’t resist. Notice
how the Texas Red Granite below this travertine
has larger phenocrysts than we saw before. What
does this indicate?
► You
might also notice the evidence of the
widening of Main Street on this side of the street.
What kind of evidence can you find?
Stop #19
Sports Café facade (407 Main St.)
O.K., this is our last stop. Look at the lowest section of the façade. This is limestone with shell
fragments, but it has been put under some pressure and shows some evidence of low grade
metamorphism. The black lines are styalites, produced at the point of differential pressure as the
weight of overlying sediments increased. Note that the layers are more marbleized below the
Below the limestone in this facade is a sandstone. It is sub-arkosic, which describes the fairly large
grain size. This sandstone looks real, but is probably a fake since there are no sutures to indicate
that blocks of rock were cut out of a quarry.
Notice the limestone in the next building. There are crinoid stems below and to the right of the
plaque that reads “Site of first White House of the Republic of Texas”. This tells us that the limestone
is probably from the Texas Hill Country, but of Mississippian age, rather than the more familiar
limestone from Texas, the Austin Chalk (which is Cretaceous). Where have you seen this type of
Well, this concludes or show for today. Thank you for
coming, and have a safe walk back to campus.