Friday, March 6, 2015

Geo-Tutorial I - A Brief Minerals & Rocks Primer (Originally published 6/27/07)


Student geologists generally study (and learn to identify) individual minerals first, before we learn to recognize them as components of rocks.

After we gain a working knowlede of minerals, we usually study igneous rocks second, as those are the original sources of most minerals.

Minerals are naturally occurring, inorganic compounds that are solid at normal atmospheric temperatures. They have orderly internal structures, defined characteristics, and a defined composition (or range of compositions due to ionic substitution), with a few exceptions.

Some minerals are elemental, i.e., the consist of a single element, e.g., diamond - but most are compounds consisting of one or more cations (positive ions) and one or more anions (negative ions).

We classify minerals by their anions, e.g., minerals related to pyrite (FeS2) are called sulfides, usually the anion is S or S2. These can include iron copper sulfides (chalcopyrite), silver sulfides, copper sulfides (cuprite), zinc sulfides (sphalerite), lead sulfide (galena). When sulfur is present in the formative stages of these minerals, whether in the original molten form, or in high-temperature, high-pressure mineralized water - called hydrothermal solutions, and if there are a variety of metals pressent, it is not unusual to find several different sulfide minerals together in the same ore body. This is the nature of the "massive sulfides" that were mined in the Ducktown, TN area (see Part I and Part II).

The nature of the chemical bond between the cation(s) and the anion affect its chemical and physical and characteristics.

The four minerals in the rock above are all included in the Silicates class, wherein the anion consists of a silicon ion and three or four oxygen ions, which act as a single ion when bonded with a cation. Quartz is chemically an oxide (SiO2), but structurally, it is related to the other silicates. The vast majority of important "rock forming minerals" are silicates. Silicate-dominated igneous rocks range from the dense, iron-rich basalts found in Hawaii to the lighter, quartz/feldspar-rich granites that one sees at Yosemite National Park or Stone Mt., GA.

The term "rock" is a little less precise, as a rock is an aggregate of one or more minerals. We use texture (crystal sizes, mineral ratios, and overall composition) to classify igneous rocks. The rock pictured here is a piece of granitic igneous rock with a pegmatitic (large-crystals) texture and it contains four identifiable minerals. This particular specimen was collected along the Appalachian Trail on the north side of Springer Mt., GA. The biotite mica measures about 1-inch across, horizontally.

The presence of potassium feldspar and quartz identify the rock as granitic, while the relatively large crystal sizes identify the rock as being "pegmatitic". The large grain (or crystal) sizes of this rock are the key to this identification. Pegmatites are irregularly-shaped igneous bodies that fill fracture zones and because of the significant presence of pressurized water in the magma, larger crystals form (in a pegmatite in South Dakota, the lithium mineral Spodumene occurs in crystals 40 feet long). In a former pegmatite mine in Georgia, "books" of muscovite mica 5-feet across were mined.

In igneous rocks, the key to crystal size is the rate of cooling and the quantity of pressurized water. Molten lavas at the surface cool relatively quickly because of their exposure to the atmosphere, thus their crystals are generally small, if they are visible at all. If a lava contains some large crystals, these crystals were already solidified before the lava was erupted. Obsidian forms when a lava flow enters a lake, river, or the ocean and the lava is chilled. The ions are present, but there wasn't time for the mineral bonds to form, resulting in the formation of volcanic glass.


Molten magmas, below the surface solidify more slowly, resulting in larger crystals, especially when more water is present. The crystals of the Elberton Granite, northeast of Athens, GA generally measure 1 - 2 mm and the estimated cooling time for the granite to solidify was 1 million years (laboratory experiments with high-pressure furnaces provide some of this information). In a molten magma, there is a defined progression in which minerals crystallize, as the magma cools, different minerals solidify. The Bowen Reaction Series shows the temperature range in which certain major silicate minerals solidify. Quartz is the last major mineral to solidify and the first to melt. In the above sample, the order is biotite, potassium feldspar, muscovite, and quartz.

Generally, minerals that form as well-defined crystals do so because they have "room to grow", i.e., they are the earlier minerals to solidify in a magma (or lava) so they can grow within the remaining molten material or they have a cavity (a fracture, a gas-bubble, or other void) into which to grow. The later minerals solidify, the less room there is to allow crystal growth.

More info will be added to this post or to a following post.

Thursday, March 5, 2015

A Few of My Favorite (Geology) Things - Part 1 (Originally published 7/12/07)


My academic work and employment have covered a number of different branches of Geology.  This particular vignette falls within the science of Paleontology - the study of fossils. Studying fossils such as the Class Echinoidea (at left) can also extend into the study of layered rocks and the study of ancient ecosystems.

Echinoids compose a particular class within the Phylum Echinodermata.  Echinoderms, in broad sense include Asteroids (starfish), Crinoids (sea lillies, sea feathers), Echinoids (sand dollars, sea urchins), etc., and some extinct, weird critters such as Blastoids and Cystoids.

They are characterized by a pentameral symmetry, shown in the five "petals" of the fossil sand dollar at left. The echinoids are divided into Regular Echinoids (sea urchins, pencil urchins) and Irregular Echinoids (sand dollars, sea biscuits, sea cookies, and heart urchins). And they are one of my geo-hobbies".  I have been lucky enough to have had a couple of good collecting sites in my field mapping areas in SW Georgia.

The Regular Echinoids appeared in the fossil record during the Ordovician Period.  Generally the Paleozoic urchins are discovered as crushed specimens with some of their spines and plates present. Carefully cleaned and restored examples appear on eBay from time to time. Many of these types come from the Pennsylvanian shales in the Brownwood, TX area.  The Irregular Echinoids (heart urchins) began appearing in the fossil record during the Mesozoic Era, in the Jurassic Period.  The sand dollars began appearing in the fossil record during the latest part of the Cretaceous Period or early Tertiary Period.

The specimen above Periarchus pileussinensis is one that I collected from the Late Eocene Tivola  Limestone (approx. 40 million years old) in the old Medusa Cement Company quarry near Clinchfield, Houston County, Georgia, on the inner Coastal Plain. It was during one of my undergraduate field trips, probably in 1973 or 1974.  The Eocene Epoch (56 to 34 million years ago) of the Tertiary Period was a time of global warmth (palm trees in Alaska, crocodiles in the Dakotas), high sea levels, and great echinoid biodiversity.

Late Eocene sedimentary rock units on the Georgia Coastal Plain include the Clinchfield Sand, the Tivola Limestone, the Twiggs Clay, the Ocmulgee Formation, the Sandersville Limestone, and others, extending from south of Augusta southwestward to the Bainbridge, GA area. Much of the Florida Peninsula is underlain by Eocene limestones, such as the Ocala Limestone and in many surface exposures of the Ocala Limestone, echinoid collectors find their idea of heaven.

The Periarchus genus is found in North Carolina, Georgia, Alabama, Florida, and Mississippi and was present from the earliest Late Eocene until the end of the Eocene, when the genus, along with others, became extinct, perhaps due to the impact of the "Chesapeake Bolide".

In Georgia, the preceding species to P. pileussinensis was the Periarchus lyelli in the Clinchfield Sand (and elsewhere).  Also in Georgia, the next succeeding species in the lineage was Periarchus quinquefarius in the Sandersville Limestone.  Most of the Periarchus specimens found in Georgia measure about 3.5 to 4 inches in diameter, the size of the modern day Atlantic Coast sand dollars.   The smaller Protoscutella in the older Middle Eocene of the Carolinas, Alabama, and Mississippi may represent an ancestor to Periarchus, though most Protoscutella specimens are generally 1/2 to 1/3 the diameter of Periarchus.  To my knowledge, Protoscutella has not yet been found in Georgia (though I have tried).  And to my knowledge Protoscutella and Periarchus have not been found in the same layers.  When there are multiple fossil-bearing layers present in an area, Periarchus is always found in units younger than Protoscutella.

[Another time, I will post more photos of some of my Favorite Things.]

What a Geologist Sees - Part 1 (Originally published 7/3/07)

No, I don't need glasses... nor a therapist, just 'cause I see stuff like this when I look at Grand Canyon photographs.  We  geologists are afflicted in this manner.  So just politely say "that's nice" to humor us.  The photo is from the south rim of the Grand Canyon (maybe from Hopi Point or somewhere near there).

Normal people are not expected to remember all of these terms, so I will explain a few of them so next time you visit the Grand Canyon (or the first time), even if you can't remember the specifics, you can revel in what geologists call "deep time". [Seriously, if you spend a little time reading articles on the internet before visiting these National Parks, you will enjoy them more.] 

Above the Inner Gorge, virtually all of the rocks are sedimentary rocks, the earliest of which were deposited on a continental shelf and when the bulk of the continent was flooded by a shallow inland (epeiric) sea, the rest of them were deposited, though there is a 100 million-year-long gap in the layers (more on that later).

In the geologic science of Stratigraphy (the study of layered rocks), when there is an interruption of the geologic record, represented by an erosion surface, we call these "Unconformities, of which there are four types; 1) Nonconformities; 2) Angular Unconformities; 3) Disconformities; and 4) Paraconformities. Types 1, 2, & 3 are visible in this Grand Canyon photo.  On the left side of the photo, an arrow points to a "Nonconformity", where the Cambrian-age Tapeats Sandstone overlies the Proterozoic Brahma Schist (metamorphic rocks). To the right of the "Normal fault", the Brahma Schist is overlain by the Proterozoic Bass Limestone (also a <strong>Nonconformity</strong> here), the reddish-colored Hakatai Shale, and the cliff-forming Shinumo Sandstone, all of which are very old sedimentary rocks.  The Brahma Schist, the Vishnu Schist and the Zoroaster Granite represent the first of two mountain ranges that were formed, then eroded away before the Tapeats Sandstone was deposited approximately 540 million years ago.

After these mountains were weathered and eroded, the rising seas covered the area and deposited a thick sequence of sedimentary and volcanic rocks, called the "Grand Canyon Supergroup".  These rock layers were then "block faulted" (tilted, but not folded), yielding the second mountain range, which was then eroded prior to the Cambrian Period sea-level rise. When horizontal sedimentary rocks overlie older, tilted sedimentary rocks, the erosion surface between them is called an "Angular Unconformity". Where that particular arrow is pointed, the Hakatai Shale (and the cliffs of the Shinumo Sandstone) represented an island surrounded by the Tapeats Sandstone. When sea level continued to rise and the shoreline moved toward the present-day southeast, the Bright Angel Shale was deposited over the exposed Shinumo Sandstone. If you enlarge the image, you may be able to see the layers of the Hakatai Shale tilting to the right (in relation to the horizontal Bright Angel Shale above). 

As sea level continued to rise (for millions of years) eventually flooding most of the continent, the Muav Limestone was deposited on top of the Bright Angel Shale. These three oldest sedimentary rocks units compose the "Tonto Group" and represent a continual rise of sea level that can be traced to 100 miles SE of El Paso, TX.  After the Muav Limestone was deposited, there was an interruption in the sequence, sea level dropped (and/or the land rose). The next time the sea covered the area and left layers behind was 100 million years later, with the Redwall Limestone, deposited during the Mississippian Period.  The eroded surface between the Muav Limestone and the Redwall Limestone is a "Disconformity" and rocks representing the entire Ordovician, Silurian, and Devonian Periods are missing. [There are some Devonian rocks elsewhere in the Grand Canyon.]

The coverage of the area by the sea that deposited the Redwall Limestone happened very rapidly. Limestones are generally deposited in shallow, tropical waters at a distance from any large landmasses. Normally, when sea level rises gradually, the sequence is sandstone overlain by shale overlain by limestone - telling you of the gradually deepening waters.

I hope this little geology lesson hasn't put you to sleep. The Grand Canyon is a fascinating place as it is unusual to see so much geologic time represented in such a small area. The oldest metamorphic rocks in the Inner Gorge are about 1.7 billion years old. The oldest horizontal layer is about 540 million years old and the youngest horizontal layer (in this photo) is the Permian Kaibab Limestone, deposited about 240 million years ago, so the horizontal layers in the Grand Canyon represent about 300 million years (estimated) of geologic time, with 100 million years missing (remember the Disconformity).  The canyon itself is thought to be only about 5 to 10 million years old. Why the entire Colorado Plateau underwent this rapid uplift, but remained essentially horizontal and largely un-deformed and largely un-faulted is something of a mystery to geologists and the source of many lively discussions. [Yeah, I left out some details, otherwise this post would be ten paragraphs longer.]</p>

What a Geolgist Sees - Part 2 (Originally published 7/24/07)




This scanned-slide was taken during my UTEP Geology Summer Field Camp field trip 30 years ago. I don't remember exactly where it was, but it is probably in the vicinity of Canyonlands National Park, near Moab, UT.

This is to further illustrate the definition of an "Angular Unconformity" and while we are "at it", the concept of  "Superposition", and "Slope-forming" vs. "Cliff-forming" sedimentary rock units.

To address "Superposition" first, that is the concept that when observing a sequence of layered rocks (usually sedimentary), if the sequence has not be severly deformed, then the oldest layers are at the bottom and the youngest are at the top. Perfectly logical when you think about it, but it was a big deal when conceived by Nicolas Steno in the 1600s.

Another of Steno's principles was "Original Horizontality", i.e., when originally deposited virtually all sedimentary rocks are horizontal.  So when we look at the above photo, we can see that the reddish-colored layers are slightly inclined to the right and are overlain by the horizontal grayish-colored layers. [It may be a little more complex than this, but as I don't remember the exact location, I will keep it simple.] 

In reconstructing past events, the reddish-colored shales (which form slopes in dry climates) were deposited, then the area underwent some minor geologic deformation, which resulted in the gently tilting shown here. Then after some weathering and erosion of the reddish layers, the gray layers were deposited above the eroded surface (the Angular Unconformity).

Above the gray shales are the cliff-forming layers, probably siltstones and sandstones in this case. In dry climates, limestones can form cliffs, too, but these are likely siltstones and sandstones. The differences in colors relate to the amounts of iron in the sediments and the oxygen conditions in the environment at the time of deposition, as related to atmospheric exposure in a river delta or floodplain setting. Generally, the more oxygen there is, the redder the sediments are. [Again, I am leaving out some details for the sake of brevity.]  So using these concepts, without knowing exactly which rock units (and ages) are present here, at least we can define some of the events that led to the particular landform that we see in this photo.

My Geologist "Wish List" (Perhaps a "Geo-Bucket List") (Originally published 7/24/07)

or at least part of it. [Yeah, this kind of stuff is only interesting to fellow geologists. I am trolling for more geology readers.] Maybe someday I can mark off at least a few of these things.

I may have alluded elsewhere to a few of thing things I still would like to see or do, relating to Geology (and other sciences).

As for collecting things, I would like the opportunity at some point to collect:

A decent zircon crystal, a decent topaz crystal, a complete trilobite, and a complete ammonite. A crinoid calyx with at least a portion of the column would be nice, too.

As for the minerals, I seek not gem-quality, but rather specimen quality, to be able to show kids what a topaz crystal (or at least a piece of topaz looks like, likewise with the zircon. I have collected a diamond in Arkansas; aquamarine in New Hampshire; green and pink tourmaline in Maine; golden beryl in Georgia; rhodonite, rhodochrosite, and huebnerite in Colorado; rutile and platinum in Georgia, yada, yada.

As for things I would like to see (and photograph) (taking my family, if they want into some of the back-country areas, in a 4 X 4):

Monument Valley, AZ/UT.  My parents went through there in the summer of 1980 and my Dad passed away in November of 1980.  I scanned some of his slides last year and for some of the Monument Valley photos, as I don't know the exact location and orientation of the photograph view, I do not know which mesas and buttes I am looking at.  Some of them I have been able to ID from the internet, but some are given different names by different photographers. I would like to be able to identify them myself. I would also like to take in the magnificent scenery seen in so many John Ford movies.

Grants, New Mexico basalt flows.  As you drive on I-40 between Albuquerque and the Arizona line, in the Grants, NM area, there are basalt flows that seem to have "rolled up" to the edge of the freeway, when in fact, the flows were already there. As these flows are geologically young, in an arid climate, their features are well-preserved. I would just like to stop and get some close-up photos of these flows.

The Jemez Mountains, NM.  In the Los Alamos, New Mexico area, there is a young, caldera-type volcano, which I visited on a field trip in 1985, but I misplaced my field notebook, so a few of my slides have left me wondering "what is that?". I would like to collect from some of the volcanic ash deposits along the roadway and get some more examples of pumice.

The Davis Mountains, Texas.  I visited the Davis Mountains several times as a UTEP grad student, but again, some of my slides are not labeled and I cannot identify the particular volcanic unit names nor the exact locations. You have to learn that your memory will not last forever. To a geology student, a neat photograph only has so much value if the subject is not well-defined.

Vinton Canyon, Franklin Mts., Texas.  I visited this locality several times as a grad student and it is a good locality for collecting Pennsylvanian-aged fossils and on the gradual slopes further to the west of the canyon's mouth, there are places to collect Permian-aged fossils. I would like to photograph the localities and re-identify which limestone and shale formations they are (this stuff contributes to the scientific value of fossils). And maybe collect a few more fossils.

The Eagle Mountains, Texas. I spent 10 weeks in the Eagle Mountains (a large caldera-type volcano) during the summer of 1978 and revisited the area in 1979 or 1980 for a weekend. I shot hundreds of good slides while there, but for a few of them, I can't remember exactly where in the mountains I was (of course I want to be there with topo maps. My field area was in a specific part of the mountains, which I remember well, it was when I ventured into other areas that my memory fails me. I would also like a few more close-up shots of some of the volcanic textures. There is also an area where volcanic ash and other eruptive debris was washed into a small lake and solidified as siltstone layers.  I would like to go back and walk the margins of those deposits and collect some more samples.  It is not often that you see sedimentary rocks deposited inside of a volcano.   Ideally, I would like to write a short article about this unusual occurrence.

Yellowstone National Park.  When I was there in 1974, I had an Instamatic camera and an undergrad's understanding of what I was seeing (which was very little).

The Snake River Plateau, Idaho.  Same 1974 trip, same deal.

Yosemite National Park.  Same 1974 trip, same deal.

Central Wisconsin. While traveling the area in 1982, I forgot to reel in my completed roll of 35 mm film back before opening the back of the camera (Do'h!), whereby I lost all of my slides of glacial features.  My brewery slides were on other rolls, but I lost all of the geology stuff.

Aden/Afton Basalts, southern New Mexico.  I did my Master's Thesis in these young volcanics and I became sick of the "sameness" of the flows, while I was mapping some unusual craters.  I shot hundreds of slides and dozens of print photos. There are some other features that I saw, but did not photograph in other parts of the area, i.e., other volcanic features that one might have to otherwise see in Hawaii.  As they were not directly related to my field work, I said I would "get them later".

I guess the best thing I can do is pass along to current geology students (and others interested in nature photography/study) is - make good notes and label your photos/specimens. Despite your passion, you won't remember all of these details 10 or 20 years from now.

What a Geologist Sees - Part 3 (Originally published 8/12/07)

When molten rock is below the Earth's surface, we call it "magma". When it reaches the surface, depending on its mode of eruption, we call it "lava" or "pyroclastics".

Below the Earth's surface, magma bodies rise through the crust due to their bouyancy. In the lower part of the crust, because the crust is hot and "plastic" (pliable), magma bodies push aside and melt their way upwards.

When the magma bodies reach the shallower sub-surface, the existing rock is cooler and more brittle. This is when the process of "stoping" (rhymes with "roping") takes place. The rising magma body slowly fractures and forces aside the host rock(s). Often during this process, fragments of the host rock will break loose and become incorporated into the magma. If the magma is not hot enough to completely melt the incorporated fragment, it is preserved as a "Xenolith".

In the case of this xenolith, it is fairly small, with a maximum dimension of about 5 inches. The host rock is the Elberton Granite, from Elbert County, GA and the xenolith is a biotite gneiss.

The presence of the xenolith is referred to as an "inclusion" and using the Concept of Inclusions as articulated by James Hutton and Charles Lyell, without knowing the radiometric ages of either rock, we know that the xenolith is older than the host rock, as the xenolith was solid when it "fell into" the molten granite. [This is a brief glimpse into the concept of "Relative Age Dating", wherein we seek to establish a sequence of events without knowing radiometric ages.]

[As a further aside, at any give time, most magmas are probably not 100% molten, but rather a mixture of solidified minerals and still-molten minerals. The darker-colored minerals generally crystalize first, as the magma gradually cools. This progression is presented in the Bowen Reaction Series.]

When visible due to difference in color, sometimes individual crystals "xenocrysts" can be preserved in lava flows and solidified magmas ("plutons" or intrusions), having been "plucked" from older rocks as the lava/magma moved through.

[If nothing else, I have given you several new words to use in playing "Scrabble". "Xenolith" or "Xenocryst" would be worth a lot of points.]

Granites, being intrusive igneous rocks, only become "available" for study at the surface by way of faulting and/or erosion, as they form thousands of feet below the surface. The Stone Mountain Granite, of similar age to the Elberton Granite, is estimated to have been 10,000 feet below the surface when it solidified 300 to 325 million years ago. BTW, there are xenoliths exposed, in various places, on the weathered surface of Stone Mountain.

So, when you walk past a stone building facade, if you see a distinctly different mass of rock surrounded by "matrix", this may well be a xenolith.

What a Geologist Sees - Part 4 (Originally published 8/19/07)



Have you ever noticed rounded river pebbles, on a hill-top or on some sort of plateau? Not just a few scattered pebbles, related to landscaping, but widespread occurrences and hillside exposures of seemingly intermixed soil and pebbles? On top of a hill?

How far did your sense of curiosity take you? Have you considered that "rivers move" over time? I am sure that many non-scientists have learned a little about how rivers meander and migrate, vis-à-vis the Mississippi and other rivers with broad valleys. But how often do we stop and think about the hilly terrain around us and how it changes over time?

This particular Topozone map shows the area in which both of these exposures were found. Notice the present location of the Chattahoochee River, approximately 1/2 mile north and west of this portion of Peachtree Industrial Boulevard in NW Gwinnett County, GA.

The upper photo area (Site 1) is now covered by a housing development. The site was approximately 200 yards due North of the intersection of Peachtree Industrial Blvd. and Abbotts Bridge Road, which crosses the Chattachoochee River. The present day Chattahoochee River channel is approximately 40 - 50 feet below the elevation shown at the highway intersection. From the lower left quadrant of the map, where Peachtree Industrial Blvd. enters the map, I have traced gravels for about 2/3 of its extent on this map, on both sides of the highway, to about the midway point between "Industrial" and "Blvd" notations on the map.

Exposures can be on eroded hillsides, in roadside ditches, creek valleys, and in construction sites. Because of present-day development of the area, it is difficult to trace the gravels for a greater distance to the southwest along P'tree Industrial Blvd, off of this particular linked map.

The lower photo area (Site 2) can be found in a small creek valley adjacent to Peachtree Industrial Blvd, probably 1/4 mile from Site 1. If you look at the linked map (if it works), in the lower left quadrant of the map, you will notice a series of long, parallel buildings (a storage area). Site 2 is across the small creek and up a side-valley. There are other exposures of river gravels overlying saprolite along this creek, directly across from the storage area site.

[While on the subject, saprolite can be described as "rotten rock", i.e., rock that has been so totally chemically-weathered, it has lost all of its structural integrity and can be crumbled by hand, though you can still see structures and textures in the outcrop.]

In both cases, the upper surface of the saprolite was once the eroded bottom of the river, which was then covered by the river sands and gravels. The contact is easily seen in the Site 1 photo, while at Site 2, the contact is shown by the dashed line. At one point, this essentially was "the lowest point in the valley". The present-day course of the river was, at that time, upland areas that had not yet been eroded by the lateral migrations of the river, as it also cut into the Piedmont soils and saprolite.

[In the Site 1 photo - "Poorly-sorted" refers to the wide variety of grain sizes, ranging from clay size to coarse-gravel sized particles. This is normal for mature rivers in this particular setting. The "Wentworth Scale" is one way of classifying grain sizes. If you have been to the ocean (or have seen sand dunes), where the sand particles are essentially all the same size, that is classified as "well-sorted".]

Now if I try to explain this to local residents, i.e., if I point out to someone that river gravels underlie this local area and all that is within sight, they might find that interesting, for the moment, but they will never see the fascination with trying to understand the nuances of "where the river used to be" versus where it is now. And other than the momentary "Wow, I didn't know that.", more thought will not be given to the subject.

That is why geologists are notorious for "talking shop" when we get together at parties and such. Cause almost no one else finds this stuff fascinating. Maybe that is why we become more eccentric as we get older (or maybe as our brains petrify). Maybe this is why we talk to ourselves (aside from teachers "practicing" their lectures).

There can be instances where tracing old river channels (and related sediments and sedimentary rocks) - in the subsurface - can be useful. Sometimes there can be mineralized zones associated with old river channels (which once-covered) served as conduits for the mineralized fluids. Sometimes the porosity and permeability of the sediments/sedimentary rocks may make them suitable for aquifers or oil reservoirs. If such river sediments lay beneath a proposed landfill, the sediments might serve as a conduit for leachate (landfill leakage) to reach an aquifer, thus rendering the site unsuitable for landfill use.

[Oops, I did it again. Yammered on for too long.]