2012 Geology Posts That Never Quite Made It

Puddingstones in Brookline, Massachusetts; Pleistocene Coral in the Bahamas; Dinosaurs Tracks in Connecticut; Monster Sea Scorpions in Upstate NY; Diatreme Volcanoes in New Mexico and Deadly Poisonous Mushrooms in Chestnut Hill, Massachusetts

Every blogger knows the challenge. What shall I blog about next? What photos should I use? By the time the end of the year rolls around, there are always a few posts that never quite made it. And so, with this final post of the year, here they are from here and there.

January

This massive, foot-long clast of Westboro Formation quartzite is embedded within an arkosic sandstone matrix of the Late Proterozoic Roxbury Conglomerate, one of two surficial rock units that comprise the Boston Basin. The Roxbury arrived in (better stated to have participated in the formation of) New England within the terrane of Avalonia, having rifted from the supercontinent of Gondwana in the middle latitudes of the southern hemisphere. Avalonia and its accompanying Roxbury made the tectonic journey across the closing Iapetus and Rheic seas during the Early to Middle Paleozoic. This puddingstone initiated my personal geological journey some twenty years ago.

Brookline, Massachusetts

February
A paper-thin veneer of new ice supports a bevy of gulls.
Chestnut Hill Reservoir, Newton, Massachusetts

March
Evidence for changing sea levels exists around the world including the Bahamas.
Low tide has exposed "shore rocks" along the island's north coast which are in reality
150,000 year old fossilized star, starlet and brain coral. This former patch reef was once covered by water considerably deeper during the last interglacial period. During the ensuing glacial period, the sea floor became exposed on land and covered by a limestone-derived soil. The crusty soil is eroding and can be seen on the coral, that is if you can take your eyes off the Caribbean's incredibly blue-green water.
Cable Beach, New Providence Island, Bahamas

March
This is a positive (upper member) cast of a portion of a trackway of a bipedal theropod in shallow-water, arkosic sandstones of the Lower Jurassic Portland Formation. This brownstone, the building stone that shaped America during the late 1800's, was deposited in an aborted rift basin called the Hartford Basin in response to the opening of the Atlantic Ocean. The foot-long footprint is likely that of a Dilophosaurus or Coelophysis, early carnivors of the Mesozoic. Not too far from here in South Hadley, Massachusetts, in 1802 a farm boy named Pliny Moody discovered the first trackway in North America. That was in the Deerfield Basin, a failed rift basin almost identical stratigraphically to the Hartford. The local preacher, seeing the print's three-toed anatomy, called it Noah's Raven, a prophetic analysis considering the evolutionary relationship between reptiles and birds.
Meehan Quarry, Hartford Basin of the Connecticut Valley, Portland, Connecticut

March
This hexagonal tholeiitic basalt, with its characteristic geometry of extremely regular polygonal joints, formed as a consequence of its cooling history. These erratics fractured from a colonade of the Lower Jurassic Holyoke Basalt Flow, the middle of three flood basalts that were generated in 1,200 miles of Mesozoic rift basins along the eastern margin of North America (and across the Atlantic as well) during early rifting of the Atlantic Ocean. This trap rock, as it's called colloquially, has its name derived from the Swedish word for stairs ("trappa") referring to the step-like pattern the extrusive igneous rock assumes once cooled and contracted. Interestingly, the generation of massive volumes of this flood basalt is cited as a possible cause of the Permo-Triassic extinction event.
Tilcon Trap Rock Quarry, North Branford, Connecticut

  

April
Preserved in the famous Bertie Waterlimes of Central New York, these are exoskeletal molts of Eurypterus remipes, also known as a "sea scorpion," a necessity of growth for all body- and limb-jointed arthropods. Classified as a chelicerate (along with spiders and horseshoe crabs) based on the morphology of its anterior appendages, it was a marine creature actually related to a similarly marine scorpion. Both plied the hypersaline seas that formed cratonward within the foreland basin of the Taconic Orogeny during the Late Silurian. Eurypterids went extinct at the end of the Paleozoic during the end Permian extinction along with up to 96% of marine species. Scorpions survived the Great Dying and now enjoy a terrestrial existence.
Bertie Waterlimes, Lang’s Quarry, Passage Gulf, Ilion, NY

May
I have been jogging around this reservoir for thirty-five years. It was constructed in 1870 to supply the fresh water demands of growing Boston and its environs but is now a haven of tranquility in the heart of the city. I’m continually astounded by the diversity of the wildlife that one finds here: geese, ducks, swans, gulls, hawks, falcons, turkeys, heron, egrets, fox, coyote, raccoons, muskrats, mice, snakes, frogs, fish, and the usual collection of squirrels, rabbits, dogs and humanoids. And it's decorated with fantastic ledges of the Roxbury Conglomerate!
Chestnut Hill Reservoir, Chestnut Hill, Massachusetts

May
...and even turtles.
Chestnut Hill Reservoir, Chestnut Hill, Massachusetts

June
It's the world's tallest freestanding stone structure, standing sentinel over our nation's capital since 1884. The Washington Monument is incredibly photogenic. It virtually begs to be photographed. The challenge is to capture it in a uniquely individual way. Architectural geology can be a lot of fun especially if you're familiar with the quarry of origination.  The obelisk's exterior is marble from Maryland, Texas and Massachusetts, while its interior backing is composed of sandstone and crystalline rocks (glassy intrusive igneous rocks) from Maryland. The Massachusett quarry is named the Lee Lime in my home state. Its carbonate rocks were part of a coastal shelf along the then, southern seaboard of the supercontinent of Rodinia over a billion years ago. They were subsequently metamorphosed into marble by the collisional events of the Taconic and Acadian orogenies during the Paleozoic. Knowing the geology seems to give greater depth (no pun intended) to any subject.
National Mall, Washington, District of Columbia 

July
My colleague and I, while traveling through northwestern New Mexico, spotted the stone edifice from a distance. Not intending to stop, we became overwhelmed by its mystical presence and stayed for a day. Unlike our conventional perception of volcanoes that exude lava and build up a conical, vertical structure, Ship Rock emplaced within the Earth's crust phreatomagmatically, gas-charging its magma when it hit the water table. Its maar-crater at the surface and over 3,000 feet of overburden have eroded away in the last 25 million years, give or take. That left the erosion-resistant diatreme as testimony to the fury, topping out at 1,583 feet. The wall-like linear structure off to the left is a radial dike, one of three major feeder-conduits that emanate from Ship Rock.
Ship Rock, San Juan County, New Mexico



 July

Between the San Juan Mountains on the west and the Sangre de Cristo Range on the east is an eight mile-long, 700 foot-high sand sea where you'd least expect it, in western Colorado. In fact, it's the tallest dune field in North America! Although its shifting sands rejuvenate with the whim of the wind, the erg remains in one place

in a perfect balance of sediment supply (from the only-true-desert-in-Colorado sands of the San Luis Valley), means of transport (wind and water) and accommodation space (embraced within the Sangre de Cristos). Although cast in the shadow of the late day sun, the dark color of the sand is due to quartz and the volcanic rocks of the San Juans. 

Wind-driven sand drifts up the windward slopes of the dunes and then cascades down the leeward slopes. The wind will sculpt the dunes until its windward side slopes gently and the leeward side is short and steep. Can you tell the direction of the prevailing wind?

  Great Sand Dunes National Park and Preserve, Colorado

July

I couldn't resist one more view.

Great Sand Dunes National Park and Preserve, Colorado

July
Volcanoes to the west in the Thirtynine Mile volcanic field and the Sawatch Range periodically filled the air 
with volcanic ash 35 million years ago. Carried by the wind, ash rained down on the region of ancient
Lake Florissant in Colorado, and along with mudflows, preserved a diverse Upper Eocene ecosystem of fish, insects, mammals and plant material. Silica derived from the ash, in a scenario remniscent of Pompeii, and its interaction
with planktonic blooms produced biofilms that retarded organic decomposition. Perhaps most remarkable
to be silicified are the VW-size tree stumps of Sequoia's, members of an ancient redwood forest
that blanketed the lake region. Notice the two, rusted ends of a saw embedded within the "Big Stump,"
a vestige of wanton and destructive fossil collecting in the late 1800's.
Florissant Fossil Beds National Monument, Florissant, Colorado

July

This amiable little fellow actually tried to sell me some auto insurance.

Florissant Fossil Beds National Monument, Florissant, Colorado

August
Minutes from Lake Placid in northern New York State, we're viewing the High Peaks Region
across a dry, pro-glacial lakebed drained by an active Holocene stream. Both formed 
after the retreat of the Laurentide Continental Ice Sheet at the end of the Pleistocene.
The bedrock throughout the region, unless buried below glacial erratics, till and outwash,
is Middle Proterozoic Grenville metanorthosite, final vestiges of the supercontinent of Rodinia.
North Elba, Adirondack State Park and Reserve, New York State

September

This over three-inch monster was spinning its web on my patio. Its the largest spider I've seen outside of the zoo. I've found the web-sheathed dens of tarantulas in the Grand Canyon but never any inhabitants. Taken at night, I illuminated the critter with a flash light to try and photograph its web.

August
For the second consecutive year, this brightly-colored, orange-yellow cluster of mushrooms arose from exactly the same location and at precisely the same time of year in my neighbor’s yard. They fruited on the stump of an aging Maple tree following a week of humid, soaking rains. Their scientific name is Omphalotus but are commonly known as the Jack O’Lantern mushroom. Under suitable conditions of day length, heat,
humidity and nutrition, spores in the soil germinate to produce hyphae. When hyphae of the opposite mating type meet (a romantic love affair made in the soil rather than in heaven), a fruitbody is produced, in this case a mushroom. Mushrooms possess the spore-shedding organs of a new generation. The mushroom and its spores is analogous to an apple and its seeds. The hidden mycelium beneath the soil is the "tree" (sort of). Mushrooms are fungi, nature’s morticians in the natural environment, beneficially biodegrading and nutrient-recycling. As we all know, not all of them are edible. These delectable-looking delicacies are deadly poisonous (as in difficulty breathing, drop in blood pressure, irregular heartbeat and respiratory failure). They also exhibit bioluminescence by glowing in the dark. I returned the following day to harvest a few and observe that peculiar property in a dark room, but my neighbor unfortunately excavated his crop before I could. Based on my calculations, next August there’ll be new specimens to collect. Lesson learned? Don't eat mushrooms that glow in the dark, and you never know what’s growing in your neighbor's yard.

November

Back in D.C. again, I couldn't resist one more shot of the Monument illuminated by the setting sun.

National Mall, Washington, District of Columbia

November

This was my very first try at High Dynamic Range (HDR) photography.
Taken at sunrise, the autumnal colors are totally natural.
This pond is in the heart of town next to a parking lot at the back of a shopping center.

Hammond Pond, Chestnut Hill, Massachusetts



 December

The last snow storm of 2012 was a mild nor'easter in Boston. It gets its name from the direction the wind is coming from. Regardless of the site of origin of the storm, the nor'easter has a low pressure area whose center of rotation is just off the east coast of New England and Atlantic Canada. Its counter-clockwise rotation produces leading winds in the left-forward quadrant onto land from the northeast. That usually translates into heavy snow or rain depending on the time of the year along with high winds, pounding surf and coastal flooding. By the way, "down east" refers to coastal New England and has its origins as a Maine term for sailing down wind to the east. Can you tell which direction is northeast from the accumulation of snow on the trees?
Chestnut Hill, Massachusetts

 That's it for 2012. Happy New Year!

From Doctor Jack (and Franklin the Border Collie)

Lithified Sand Dunes of the Ancient Bahamian Landscape

On a recent vacation to the Bahamas, Paradise Island in particular, while the rest of my crew was swimming, reading and kicking back, I did some exploring down beach and out onto a narrow "rocky" spit of land. I was surprised to find that the spit was a platform composed of sand dunes. Not only were they lithified but cross-bedded, reminiscent of the eolian Coconino and Wingate Sandstones on the Colorado Plateau, but on a vastly smaller scale.

A LITTLE BACKGROUND ON THE BAHAMIAN ISLANDS
Positioned a mere 50 miles off the coast of Florida at its nearest point, the Bahamian Islands, of which there are 700, form a northwest-southeast trending archipelago. The climate of the region is sub-tropical with hot summers, warm temperate winters and an average yearly rainfall of about 30 inches. The islands of the Bahamas rest on a shallow carbonate platform, which during the Pleistocene, had been intermittently exposed and submerged in conjunction with glacially-induced high and low sea level-stands. Glacial maxima favored lower sea levels that exposed bank sediment. In turn, this favored eolianite deposition which possessed the capacity for lithification under the right circumstances.


This is a Google Earth image taken from about a 15,000 foot-altitude showing the location of the lithified dunes on Paradise Island, and showing the relationship to much larger New Providence Island and its populated capital city of Nassau. The total length of the platform measured about 1/5 of a mile and the greatest width was 150 feet.
It's highest elevation above sea level is perhaps 20-25 feet. 

Location of the lithified dunes on Paradise Island. The spit is connected to the main body
of Paradise Island by a narrow neck of a sandy beach.



This extreme close-up is taken from a distance of about one foot. It provides a good view of the lithified dune's macroscopic structure. Although the surface of the dune is severely weathered, you can clearly make out its bedding planes and its oolitic composition.

INTRIGUING QUESTIONS
Interestingly and totally unanticipated (as an avocational geologist), the dune’s composition wasn’t the typical silica-sand variety (in the form of quartz) but instead a carbonate (a limestone). Upon close inspection, the sand grains had an oolitic (egg-shaped), spherical shape, like fish roe. Indeed, silica sand-dunes are typical of inland continental and non-tropical coastal settings, while tropical coastal settings possess sands of eroded limestone. How did the dunes lithify, while above ground (subaerially) or did they? And, how did the sand acquire its oolitic shape? Here’s the intriguing answer.

GENESIS OF THE LITHIFIED DUNES
The Bahamas are not of volcanic origin, typical of many of the Caribbean islands. There are no igneous and metamorphic rocks to be found. Shallow-water carbonates are ubiquitous, having formed near the surface for 200 million years. The Bahamas are a vast “carbonate factory,” producing sediment at a fairly rapid rate on a slowly subsiding crustal platform (keeping the water deep enough for the process to continue). Oolitic limestone is precipitated directly from sea water, although containing carbonate forms from other sources such as skeletal remains.

The sand dunes formed on land when global sea level fell during the Pleistocene Ice Age. As sea level rose and fell during each of four glacial-interglacial periods, new sediments washed up onto new beaches forming a new line of dunes with classic bedding planes and erosive bounding surfaces. Cementation of the dunes with calcium carbonate occurred both during interglacial-period, marine submergence and glacial-period, rainwater exposure by both crystallization and recrystallization. The process of converting the sediments to the rocks is called diagenesis.

Looking down the coast, it appears that several “fossil platforms” are on higher ground. During the Ice Ages, continental glaciers tied-up great quantities of water making global sea levels lower. This exposed more shoreline to undercutting-erosion. During interglacials, the melted glaciers freed-up water making global sea levels rise. This created wave-cut platforms above the normal level of the sea. Since the region exhibits no folding, tilting or faulting, we can safely assume that glaciation-induced subsidence rather than geological uplift is the only causative explanation for the “elevated” platforms.



 A fossilized tree and root structure preserved within the lithified dune
adds testimony to its origin as a terrestrial sand dune.

PHYSICAL AND CHEMICAL WEATHERING
Weathering is the breaking down of the Earth's rocks, soils and minerals through direct contact with the atmosphere. Weathering occurs in situ without "movement" and is not to be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind and gravity. Physical weathering involves "breakdown through direct contact with atmospheric conditions such as heat, water, ice and pressure," whereas, chemical weathering involves the direct effect of atmospheric chemicals or biologically produced chemicals (Wikipedia).

The spit is essentially a narrow, rocky carbonate platform forming a small portion of the coast. It is evident here, in contrast to the neighboring beach itself, that morphologic change is a slow and gradual process dominated by physical,  biologic and chemical weathering processes. Tide, current and wave processes all yield change but not on temporal scales of hours and days compared to the beach. Both types of weathering can be found on the coastal carbonate platform but in varying degrees and at differing locations. The mechanisms yielding the various morphologies appear to be controlled by factors such as the position relative to sea level, the interface-distance between water and land, and the porosity and degree of cementation of the rock (which is undoubtedly directly proportional to its age) .  

PHYSICAL WEATHERING
On the oceanic margin of the spit, it has been eroded into cliffs which have been undercut everywhere by wave action forming wave-cut platforms that extend outward toward the sea. The most highly-dissected terrain was to be found in a zone that developed closest to the sea. In fact both physical and chemical weathering decreased as a function of distance from the edge of the platform.

An additional type of physical weathering includes haloclasty or salt crystallization which causes the disintegration of rocks when saline solutions seep into cracks and joints in the limestone. When the water evaporates, it leaves a residue of salt crystals behind. The salt crystals can expand up to 3 times their volume when they become heated, exerting pressure on the confining rock. It's reminiscent of the 9% expansion of water when it freezes. Salt crystallization can also occur when solutions decompose rocks, which likewise leaves a salt residue that can expand. This phenomenon is common in arid climates and along coasts.

It can readily be seen that physical and chemical weathering go hand-in-hand. On the platform, the delicately etched textures of the rocks were seen to develop within reach of frequent salt spray and are absent amongst identical rocks further away from the influence of the sea.  



Undercutting of the platform by wave action. The surface exhibits solution weathering.

 Further evidence of physical weathering and chemical dissolution contributes to the dramatic beauty.
The Pleistocene and Holocene-age limestones of the supratidal, coastal platform
are undergoing surficial meteoric diagenesis from weathering yielding "eogenetic" karst.



Watch the waves relentlessly breaking and eroding the coastal platform on the video below.

CHEMICAL WEATHERING

Rainfall is inherently acidic because of atmospheric carbon dioxide (although other atmospheric gases can be absorbed which may increase the acidity additionally). This produces a weak carbonic acid which leads to solution weathering on highly-susceptible rocks such as limestone. In addition, coastal platforms such as these are in the spray-zone. Over considerable time, the limestone undergoes chemical dissolution to the extent that its appearance becomes sharply-jagged with numerous voids, small excavations and holes, and razor-sharp edges. The holes tend to link up and gradually enlarge which gives the surface a pitted, honey-combed and drilled-out appearance. Also, kamenitza or solution pans tend to form which are shallow, rounded relatively flat-bottomed basins on exposed surfaces that develop via dissolution of limestone by standing water. These surface phenomena are generically known as karst. Subterranean karstic landforms (not the subject of this post) exist in the Bahamian tropics but  differ somewhat from traditional karstic landscapes formed in temperate climates.

Digressing briefly, classical karst terrains have distinctive landforms and drainage arising from greater rock solubility in natural water that is derived elsewhere. They are characterized by numerous caves, subterranean caverns, sinkholes, solution valleys, fissures and underground rivers and streams. Karst topography usually forms in regions of plentiful rainfall (cold and humid mid-latitude, temperate climates) where the bedrock consists of carbonate-rich rock such as limestone (CaCo3)  and dolomite (MgCaCO3), which is easily dissolved. Examples of classical karst terrains are the Dinaric Kras region (the type locality) of the Adriatic between Slovenia and Italy, and the Appalachian mountainous regions of the Mid-Atlantic States).

Most karstic features are created by carbonic acid (carbonation) which forms from the absorption of carbon dioxide (CO2) by rain (meteoric) water. Biological activity (such as plants, algae and lichen) can secrete acids that dissolve soluble bedrock. In addition, blue-green algae can produce a plant-generated surface karst (called phytokarst)  characterized by pitting and a sharp-edged, spongy lattice of ridges and pinnacles.    

The following is the main mechanism of calcium carbonate dissolution in limestone: Rain passes through the atmosphere picking up CO2 which dissolves in water. Once on the ground, the water containing the weak carbonic acid in solution passes through the bedrock and dissolves calcium carbonate.

Further attacks on the landscape occur as a result of fossilized plant-roots called rhizomorphs that once grew in the dunes long ago. Their roots may harden the soil around them via their secretions. Upon weathering, the resistant limestone can form thin, jagged edges. Biological weathering (biokarst or bioerosion) can further add to the jagged, etched and honey-combed effect from boring blue-green bacteria and invertebrate grazers (mainly molluscs such as gastropods), especially along the regions that are regularly wetted by waves and sea spray. Such plants produce acids and their filaments penetrate the rock promoting its disintegration.

It appears that distinct geomorphic zones exist on the platform that are discernible by their color, degree of  weathering and proximity to the land-marine interface.

 An extreme close-up of the razor-sharp, jagged and honey-combed surface on the platform.
This surface was virtually impossible to traverse safely in bare feet.

LET THERE BE LIFE
Not surprisingly, many creatures make their home in the intertidal zone of the platform which was teeming with life. Here are just a few inhabitants that I stumbled upon.

On the platform, a female polyplacophora (chiton) with 8-articulating shelly plates and her associated eggs are attached to the bottom of a shallow pool. Interestingly, the male chiton releases his sperm into the sea which finds (hopefully) a receptive egg-release. Various colorful gastropods (marine snails) were everywhere. Both chitons and snails are members of the Mollusc Phylum along with clams, mussels, oysters, squid and octopus.

A prickly-looking sea urchin, a member of the Echinoderm phylum (eg. starfish, sand dollars and brittle-stars),
bides its time.

A crab, a crustacean and member of the Arthropod Phylum (along with insects, spiders and extinct trilobites)

In summary, the Bahamas are largely a depositional landscape, unlike the more common, eroded landscapes of the continents, with their own unique carbonate signature. Both physical and chemical weathering can be observed on the platform that appear unique to marine coastal environments.

My casual stroll down the beach at Paradise Island turned out to be an unanticipated lesson for me in Bahamian dune composition, formation, lithification and weathering.  

P.S. Bahamian Landscapes by Neil Sealey is a great introduction to the geology and geography of the Bahamas with tremendous photos and illustrations!