GSA Today's “Geology Through the Lens” - Matera’s Timeless Geological Legacy

In the February 2025 issue of the Geological Society of America’s publication "GSA Today", I had the distinct honor and privilege of showcasing my photo of the ancient and modern city of Matera with an accompanying description. 

Taken during a recent visit to the Southern Italian regions of Basilicata and Apulia, Matera and environs is heaven on earth for geologists, anthropologists, paleontologists, botanists, historians, tourists, hikers, bikers, gastronomes, oenophiles, sociologists and cinephiles.  The text that accompanies the photo is a bit verbose (my forte) and condensed, but the editors only allow so many characters.

In the coming months, I plan to post a detailed account of the geologic evolution of the karstic carbonate platform on which Matera.  In the meantime, I offer the following view of the ancient and modern city taken from the bottom of Gravina di Matera (Gorge of Matera) and a few images from the Alta Murgia plateau (Apulian Platform) plateau on which the city resides. 

South-facing View of Gravina di Matera

Located on a karstic plateau of the Tertiary-uplifted Apulian Carbonate Platform of Mesozoic African affinity in the Basilicata region of southern Italy (“instep of the boot”), Gravina di Matera is riddled with thousands of solution caves that, in part, constitute the city of Matera’s Sassi (the “stones”) that is perched on the edge of the gorge.  Many caves have Neolithic documentation, and those of Matera have been continually inhabited since the Paleolithic with quarried Renaissance facades over their entrances.  Following a period of extreme overcrowding, disease, poverty and national shame in the twentieth century, Matera emerged as a 1993 UNESCO World Heritage Site and first-class tourist destination.

North-facing View of Matera from the Bottom of Gravina di Matera 

Matera's occupation spans a significant portion of human history, with evidence of settlement dating back to the Palaeolithic period, approximately 9,000 BC, possibly older than 10,000 BC. That makes it one of the oldest continuously inhabited settlements in the world - due in large part to the region's karstic geology!

A visit to Matera must include a moderately challenging, two or three hour hike into the gorge to the opposite rim and back.  There are trail maps available online, but the layout isn't complicated, although signage could be better. You will find that there are numerous confusing subtrails that branch off, but they eventually converge back into a main trail. Fortunately, you never lose the sense of where you are or where you're headed, so you can't get lost.

Near the top of the opposing wall you will be rewarded with a number of caves with rupestrian (located on rock walls) frescoes, painted by Greek monks seeking religious freedom and isolation in the 8th to 13th centuries. Familiarity with the gorge's calcareous stratigraphy and tectonic provenance before you head out is highly recommended. 

Lastly, don't forget to pack water, some of the locally famous Matera sourdough bread, a little local cheese and a few Leccino black olives. Molto delizioso! 

Pontrelli Quarry and Dinosaur Track Site

Located about 10 miles north of Matera and a few miles east of Altamura on the Alta Murgia, the floor of the long-abandoned Pontrelli Quarry, from a distance, looks like machined striations created by stone workers. To the surprise of paleontologists, it turned out to be an enormous Late Cretaceous dinosaur tracksite with over 4,000 footprints created by over 200 mid-sized, quadruped dinosaurs belonging to at least five different species. 

The ichnofossils (preserved traces of animal activity and behavior) were created within the shallow-water, peritidal Altamura Limestone formation (Calcare di Altamura in Italian).  It was uplifted intermittently throughout the Cenozoic by orogenic events that ultimately formed the foreland of southernmost mainland Italy and the Adriatic Sea.

The discovery is unique in several respects. Fossil dinosaur tracks from Italy are rare, and the location in the Apulian foreland sheds light as to the climate at the time of deposition, which was similar to the Bahamas. In addition, it reveals new and critical information regarding the tectonic evolution of the western Paleo-Tethys Ocean (the proto-Eastern Mediterranean). 

As a result, it is believed that the Apulia and Basilicata regions originated as part of the Adria microplate of North African plate affinity (originating perhaps as an African promontory at the time), before rifting, drifting and colliding with proto-Southern Italy.

Pulo di Altamura Sinkhole

Located about six miles NNW of the quarry and five miles north of the town of Altamura, Pulo di Altamura (an early Italian and perhaps regional term for a sinkhole with potential Indo-European origins) is the largest sinkhole (doline in European geological terms) on the highly permeable plateau of Alta Murgia. The topographic depression formed by karst solutional processes, as did Gravina di Matera in association with fluvial dissection, that progressed in a subterranean locale to the extent that collapse of overburden occurred. 

Some researchers believe that parts of the eastern Grand Canyon of Arizona may have evolved in this manner in regions of limestone stratigraphy that, over time, facilitated development of a west-flowing, ancestral Colorado River through the Kaibab Upwarp via the collapse and coalescence of sinkholes. It might have allowed subterranean flow with eventual connection between the eastern and western Grand Canyon. Such a process may have occurred on the Apulian plateau and resulted in the formation of a gorge (European for canyon). 

Karst solutional processes occur when neutral pH rainfall becomes acidic by absorbing atmospheric carbon dioxide (and other gases) and when groundwater acquires additional acidity from decomposition of organic matter in the soil and root respiration. Mildly acidic, the groundwater chemically dissolves and mechanically erodes the calcium carbonate mineralogical component of the uplifted calcareous seafloor platform that collapses when undermined. 

Notice that the walls of the sinkhole are peppered with caves, as are those of Gravina di Matera. They show signs of human habitation dating back to the Upper Paleolithic, at least 5,000 years ago through the Iron Age and into the Middle Ages. Archaeological finds include human remains, engraved pebbles and a fossilized shell, indicating the site was used for cults and resource exploitation.

I didn't encounter anyone on the wonderful 5.1 mile loop-hike to the sinkhole across the unspoiled plateau and wheat fields of the Alta Murgia National Park. 

The Altamura Man

As at ancient Matera, many of the karstic caves that developed on rocky ledges of the Alta Murgia were inhabited by an archaic form of Homo neanderthalensis that lived in the region during the Middle-Upper Pleistocene between 170,000 and 130,000 BP.  In fact, a deep karst cave complex near the sinkhole called Lamalunga, discovered in 1993 by cave explorers but closed to the general public, contains a complete Paleolithic skeleton, the most intact and oldest ever discovered in Europe that has been accurately dated and awaits DNA sequencing.

 

Dubbed Altamura Man and irretrievably inverted and encased in calcareous rock and covered with corroloids (cave 'popcorn'), caused by aerosol spray of precipitated calcium carbonate, the skeleton is in an excellent state of preservation, having been completely replaced by calcite (the most stable polymorph of calcium carbonate).  How does that happen geochemically? 

Within the cave and driven by several factors, calcium carbonate precipitates out of saturated solution in the reverse direction of the aforementioned karst chemical reaction, as carbon dioxide degasses back into the cave's atmosphere and as water slowly drips from the void's walls and ceiling.  Over time, calcium carbonate is redeposited forming speleothems - cave formations such as stalactites, stalagmites, flowstones and the Altamura Man's entombing calcareous armor. 

The Alta Murgia plateau is peppered with such caves ('grotto' in Italian), some open to the public with tours, and many await discovery and exploration. One wonders what finds will be disclosed in the future, hominins or otherwise. (Image from Wikipedia)

My Lovely (Very Hungry) Daughter

By the way, if you pay a visit to the Apulia and Basilicata region, the local cavatelli pasta ('little hollows' and for me, reminiscent of Matera's karstic solutional caves) is delicious but challenging to roll by hand, as my daughter and the rest of our family discovered in a cooking class at "Cook'n Fun at Mary's" (shameless plug) in Matera. A few glasses of the local wine didn't make things any easier. My forlorn culinary creation is on the top tray. 

Memorable Places Here and There on the Colorado Plateau: Ribbon Falls

About eight miles down the North Kaibab Trail from the Grand Canyon's North Rim, a short detour off to the right beckons sun-parched backpackers to Ribbon Falls. Its irresistible mist is near impossible to forgo on a typically hot and dry day in the canyon, making this side excursion a necessity to visit. But what’s truly fascinating is the geological structure that the falls have produced. The action of ground water, by virtue of its mineral composition, has resulted in the formation of a spectacular travertine dome that's over thirty feet tall.

How did this colossal structure form? Water from the falls makes a 120 foot free-fall landing precisely at the apex of the moss-covered travertine dome. Calcium carbonate is in solution, being made soluble by the absorption of atmospheric carbon dioxide, which makes the water mildly acidic. Its acidity allows the carbonate to be “acquired” from limestone formations at higher elevations such as the Redwall and Muav. Subsequently, carbonate is “released” from the mineral-rich dripping water when it plunges over the falls and releases the carbon dioxide held in solution. The change in water chemistry causes the re-deposition of the carbonate in the form of travertine or tufa (softer and more porous) from the mineral-laden water. Gradually, the mound grows by re-crystallization, molecule by molecule. This landform is called karst, made possible by the dissolution of soluble bedrock. The identical process forms the more familiar stalagmites and stalactites in subterranean limestone-caverns.

Ribbon Falls is located in an amphitheater bounded by dark red cliffs of Shinumo Quartzite. The falls plunge over the ledge of a resistant diabase sill. Diabase is the intrusive equivalent of basalt. This sill is part of a system of Cardenas conduits and a massive basaltic outpouring of the same name that fed magma to the Earth’s surface. These rock formations, along with three others, are members of the Unkar Group, which comprise the lower Grand Canyon Supergroup. Beginning 1.2 billion years ago, the formations of the Unkar Group were deposited over a span of 100 million years and appear to have been associated with a continental collision event that culminated in the formation of the supercontinent of Rodinia.

This view is taken from behind the falls, looking out at the top of its verdant, mossy travertine dome. Vegetation such as the moss, and golden columbine, maidenhair fern and scarlet monkeyflower thrives in the oasis of the fall’s unique microclimate. These plants are not indigenous to the hot, arid climate of the Grand Canyon only a few feet away.

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!