Rio Grande Gravels

[HankRocks]

I did poke around Amistad a couple of times, found no real material around the lake as everything around the lake shore is Limestone. Did not explore the River below the dam that might hold some interesting gravel beds.

If I had to speculate, the gravel agate around Falcon is very old. It's probably older than some of the current agate bearing formations in the Big Bend area. If we wait another 100-200 million years( I think I will have a few beers while I wait) the agate formations in Big Bend will be converted to gravel beds somewhere, if the geologic conditions are right. The only other question is the Pet Wood ad Palm. Possibly it was laid down on top of the agate bearing gravels and then weathered out in some later period.

Anxious for the Texas Geology book to get here and start reading and learning.

Henry

[jamesp]

I believe Matt Dillon has a great handle on the origination of Falcon gravels.
But he also collects gravels along other sections of the Rio well upstream.

Gravel deposits way down in the low land of S. Texas could have no telling how many layers.
And no telling how many times it has been rearranged.

Mel mentioned the upper layer of the caliche. Venture to say the upper layer around the mid point of L Falcon is most productive.

A farmer had built a ramp up to his gate. About 200 feet long and about 15 feet tall.
I believe he scraped the large cobbles off the top of the adjacent hill.
I must have found a quarter of my agate and wood on that one small built up road bed.
The problem in that area is that a lot of the gravel deposits on the surface have been removed. Easy to remove

[jamesp]

This is about 25 acres of el primo L Falcon ranch land that I know intimately.
The elevation at the lighter spots reaches the magic 370+ number. Note the baldy spots running SW to NE about center.
The rest of the field drops to 340.
To the west say 300 yards is another hill approaching 390. It too is capped with yet bigger cobbles.
Any spot on this 200 acre ranch above 370 was thick in cobbles and the vegetation suffered for it.
This ranch had been deep disced and the vegetation told the story by looking at google earth.

Just a micro-environment situation for this area.



And here is the ground view looking south from top of above photograph.
You can see the gentle hill/ridge where the bald spots are, about 330 elev in the draw and back up to 370 on the ridge/horizon.
The whole ridge was gravel rich. Mind you the car is sitting on agate too. Just smaller cobbles than over on the ridge.
All those rocks you see sitting on the ground are agate and wood rich.



Same ranch. This is about 5 acres. Highest knob for 2 miles in all directions. Elev 388. Packed with ave. 6-8 inch cobbles, the biggest and thickest accumulation.
It just so happened I had permission to collect this high spot on this small ranch.

Note vegetation missing, directly related to the elevation. Deep cobble layer capped knob. Fill yo bucket.



There was 5 or 6 high spots on this 200 acres and every high spot was capped with cobbles.
So you simply walked to the high spots and collected, very basic.
Dumb easterner figured it out.(Real dumb, my first western agate collecting trip)
This is basically flat land. So it made sense.

Point bar deposits:



Photo of point bar deposit



Theory says the knobs in this area are simply point bar deposits from the ancient river.
Knobs that defied modern erosion because they are capped with deposits of cobbles.
Never tried mapping them, guessing you would end up finding an ox-bow river.

Henry says it's an old deposit. Have to agree. Point bar deposits sitting on the highest knobs ? Yes, that would be an old deposit for sure.
If they are point bar deposits sitting on high, then this area has seen extreme erosion by serious forces.
Note that the hill top deposits are NOT encased in caliche. Just cobbles laying loose.

Be aware this ranch is on a 'high' peninsula pointing out in the lake.
All the land around this ranch is lower, and the lake pool is at ~306 elev.
So if the old river was at 390 elev., where did all the surrounding ground go ?
Probably washed into the fertile farm valley below Lake Falcon and perhaps into the Gulf of Mexico.

[jamesp]

Rio Grande Gravels

The gravel beds along the Rio Grande have particularly diverse materials, because this river has its headwaters far to the northwest in the western Rocky Mountains. Most of the gravels found along the river in south Texas probably don't come from quite that far away. A more likely source area for some of it is the Big Bend area west of the Pecos. And other materials were picked up downstream, including reworked Uvalde gravels, and well as sandstone and other rocks found in bedrock outcrops breached by the Rio Grande. Among the rock types found in the South Texas Plains stretch of the Rio Grande are chert, chalcedony, sandstone, limestone, as well as various metamorphic rocks, such as quartzite, and volcanic rocks, such as rhyolite.

While the materials making up Rio Grande gravels may be particularly diverse, most of the cobbles are relatively small and made out of tough materials that survived long journeys. Large pieces of fine-grained material are rare. It is probably no coincidence that the predominance of the Unstemmed Point Tradition of the South Texas Plains is more or less centered on the Rio Grande, where rocks suitable for making large stemmed dart points are hard to come by.

Rio Grande gravels, some ancient and some more recent, can also be found in the terraces above the river and on the uplands miles away. The upland gravel deposits in the Rio Grande valley are sometimes called "Uvalde gravels" and there is probably little meaningful distinction.

[jamesp]

You can get quite a headache studying sediment deposition lol.

Over the years, a great amount of thought and a large number of papers have dealt with finding stratigraphic traps. Various schemes and systems have been devised. The broad purpose of this paper is to present a unified picture of one of these methods—the environmental method.
The approach of the method is to study recent beach arid bar deposits along the Gulf Coast from the Rio Grande River to Mobile Bay. These deposits present an interesting regional depositional history, when considering the basis for the method—the Doctrine of Uniformitarianism. This doc- trine simply says that we can go out today, see sediments being deposited, and know that sedimentary rocks have always formed in the same ways.
Generally, there is no hesitation in applying the doctrine. However, direct comparisons of the recent to the past can be dangerous if the conditions at the present are not well understood. The physical and chemical processes responsible for sedimentation change very little, but unlike the processes themselves, the conditions prevailing upon the earth's surface are variable. An excellent example is the Rio Grande River which no longer is building its delta seaward simply because very little sediment is being supplied to the basin of deposition. On the other hand, the opposite is true in the Mississippi River Delta area. The Mississippi River is supplying an abundance of sediment to the basin of deposition.
Balanced against the sediment supply that rivers bring to a basin of deposition is the amount of energy (waves, longshore currents, and tidal currents) available to do work in sorting and trans- porting it. Furthermore, the balance between supply of sediment and available energy in certain environments can easily be upset by relatively minor" changes in one or the other. Changes in balance result in situations which are of sedimentational and stratigraphic interest to us in terms of trans- gression and regression.
Utilizing the Gulf Coast as a model, it can be noted that it is possible for transgression and regression to occur simultaneously in nearby areas. Since the two do occur simultaneously, it is clear that transgression cannot be related simply to rise in sea level, nor regression to fall.
Considering these important environmental factors observed in the recent, a comparison, though not direct, can be made to regressive and transgressive beach and bar deposits in the ancient sequence of rocks in central Utah and southwestern Colorado.

[Sabre52]

Super interesting bunch of posts James. Fun to speculate about Texas geology in that part of the world. Can really give you a headache trying to make rhyme or reason out of all that stuff all right. Sure wish we had those gravel beds here locally. Get real tired of all the limestone and chert. Find a tiny bit of pet wood once and awhile and that's it round here.

Cliff: The Devil's Claw is about four feet wide and growing like wildfire but the Sphinx moths have found it now and their caterpillars are eating away at it. Have a ton of seed pods though so I will get plenty of good seed for next year.....Mel

[jamesp]

Jul 13, 2017 6:25:46 GMT -5 Sabre52 said:

Super interesting bunch of posts James. Fun to speculate about Texas geology in that part of the world. Can really give you a headache trying to make rhyme or reason out of all that stuff all right. Sure wish we had those gravel beds here locally. Get real tired of all the limestone and chert. Find a tiny bit of pet wood once and awhile and that's it round here.

Cliff: The Devil's Claw is about four feet wide and growing like wildfire but the Sphinx moths have found it now and their caterpillars are eating away at it. Have a ton of seed pods though so I will get plenty of good seed for next year.....Mel

There could be many reasons for having a cobble layer at a given elevation. I was just listening to the point bar theory.
It seems like there could have been a continuos layer of cobbles throughout at a given elevation and the rest of the soil washed away around them.
When ever there are round cobbles(deposited) from the highest to the lowest elevation with multiple drainages feeding a given territory things get complicated.
Those hard stones did some serious travel.
The resulting wide variety makes the Rio very special collecting spot.
And delivered closer to us agate desperate easterners.

I still have to ask if flower garden agate is a one source agate from the Woodward Ranch. It is so unique and I have never heard of anyone finding it on any other location.
But there it is peppered all over Lake Falcon territory. Wish I were a fly on the wall when all this material was being displaced.
Shame there is no such thing as rock DNA.

Funny, I approach a coral shoal in a Florida river. The front of the shoal is fresh coral imbedded in clay and sealed from the chemicals and salts from the water.
Walk across it on tailing side and dig the older deposits that have washed over the shoal and has deposited in the black muck behind the shoal for thousands of years and find all kinds of colors.
Pluck coral from the high flow in the shoal that is partially exposed to the river water and the top half has been deeply stained by the water and the bottom half is still protected by the clay.
Move toward the centerline of the river and find the oldest coral that has been eroded out of the clay for the longest time and has the deepest stains.
Move to the banks and again find coral still encased in the protective clay. Check the varying clay colors and find varying coral colors.
Point being, no great forces seemed to ever have moved the coral very far from where it was silicified.
And it apparently has never seen freezes in most cases.
No freeze damage and no impact damage from high speed erosion.

[Sabre52]

I'm guessing that the flower garden is mainly a Woodward area thing as Clayton does bring in some definite Woodward type plume too. However, there is also a good chance that flower garden in general is just one of the many varieties of moss/plume formation and occurs a bit wherever you find moss and plume because, in the Rio gravels, it does seem to me you find a lot of intermediate forms. I seem to remember Matt Dillon lists flower garden types from other locations too and I've seen similar types from the Cadys and Lavic too....Mel

[coloradocliff]

Jul 13, 2017 6:25:46 GMT -5 Sabre52 said:

Super interesting bunch of posts James. Fun to speculate about Texas geology in that part of the world. Can really give you a headache trying to make rhyme or reason out of all that stuff all right. Sure wish we had those gravel beds here locally. Get real tired of all the limestone and chert. Find a tiny bit of pet wood once and awhile and that's it round here.

Cliff: The Devil's Claw is about four feet wide and growing like wildfire but the Sphinx moths have found it now and their caterpillars are eating away at it. Have a ton of seed pods though so I will get plenty of good seed for next year.....Mel

Sabre52 Hit them caterpillers with some 7 dust. Bet you got some. Problem is you reduce your moth population. Have great pictures of humming bird moths     White lines. Have grape eater moths chewing on a woodbine that I use for cuttings so will hit them. They prefer it over my grape vines. good !!

My white lined Sphinx's love the 4 O'clocks.

jamesp What a real labor of love you put up Jim. Mike and I are eating this stuff. Always look and dig for anything. Ancient river flows are really what has my deepest interests. Need to get all wrapped up and get yor help trained for selling the Fire Pits so you can disappear for a few weeks .

[coloradocliff]

Jul 13, 2017 4:05:56 GMT -5 jamesp said:

This is about 25 acres of el primo L Falcon ranch land that I know intimately.
The elevation at the lighter spots reaches the magic 370+ number. Note the baldy spots running SW to NE about center.
The rest of the field drops to 340.
To the west say 300 yards is another hill approaching 390. It too is capped with yet bigger cobbles.
Any spot on this 200 acre ranch above 370 was thick in cobbles and the vegetation suffered for it.
This ranch had been deep disced and the vegetation told the story by looking at google earth.

Just a micro-environment situation for this area.



And here is the ground view looking south from top of above photograph.
You can see the gentle hill/ridge where the bald spots are, about 330 elev in the draw and back up to 370 on the ridge/horizon.
The whole ridge was gravel rich. Mind you the car is sitting on agate too. Just smaller cobbles than over on the ridge.
All those rocks you see sitting on the ground are agate and wood rich.



Same ranch. This is about 5 acres. Highest knob for 2 miles in all directions. Elev 388. Packed with ave. 6-8 inch cobbles, the biggest and thickest accumulation.
It just so happened I had permission to collect this high spot on this small ranch.

Note vegetation missing, directly related to the elevation. Deep cobble layer capped knob. Fill yo bucket.



There was 5 or 6 high spots on this 200 acres and every high spot was capped with cobbles.
So you simply walked to the high spots and collected, very basic.
Dumb easterner figured it out.(Real dumb, my first western agate collecting trip)
This is basically flat land. So it made sense.

Point bar deposits:



Photo of point bar deposit



Theory says the knobs in this area are simply point bar deposits from the ancient river.
Knobs that defied modern erosion because they are capped with deposits of cobbles.
Never tried mapping them, guessing you would end up finding an ox-bow river.

Henry says it's an old deposit. Have to agree. Point bar deposits sitting on the highest knobs ? Yes, that would be an old deposit for sure.
If they are point bar deposits sitting on high, then this area has seen extreme erosion by serious forces.
Note that the hill top deposits are NOT encased in caliche. Just cobbles laying loose.

Be aware this ranch is on a 'high' peninsula pointing out in the lake.
All the land around this ranch is lower, and the lake pool is at ~306 elev.
So if the old river was at 390 elev., where did all the surrounding ground go ?
Probably washed into the fertile farm valley below Lake Falcon and perhaps into the Gulf of Mexico.

Interesting  hypothesis. Really trying to get up to speed. Mike and Henry are also strongly interested.

    Love the Texas Pinstripes you applied to the Honda's quarter panel. What you call the design? Desert Mesquite??

[coloradocliff]

Jul 13, 2017 4:05:56 GMT -5 jamesp said:

This is about 25 acres of el primo L Falcon ranch land that I know intimately.
The elevation at the lighter spots reaches the magic 370+ number. Note the baldy spots running SW to NE about center.
The rest of the field drops to 340.
To the west say 300 yards is another hill approaching 390. It too is capped with yet bigger cobbles.
Any spot on this 200 acre ranch above 370 was thick in cobbles and the vegetation suffered for it.
This ranch had been deep disced and the vegetation told the story by looking at google earth.

Just a micro-environment situation for this area.



And here is the ground view looking south from top of above photograph.
You can see the gentle hill/ridge where the bald spots are, about 330 elev in the draw and back up to 370 on the ridge/horizon.
The whole ridge was gravel rich. Mind you the car is sitting on agate too. Just smaller cobbles than over on the ridge.
All those rocks you see sitting on the ground are agate and wood rich.



Same ranch. This is about 5 acres. Highest knob for 2 miles in all directions. Elev 388. Packed with ave. 6-8 inch cobbles, the biggest and thickest accumulation.
It just so happened I had permission to collect this high spot on this small ranch.

Note vegetation missing, directly related to the elevation. Deep cobble layer capped knob. Fill yo bucket.



There was 5 or 6 high spots on this 200 acres and every high spot was capped with cobbles.
So you simply walked to the high spots and collected, very basic.
Dumb easterner figured it out.(Real dumb, my first western agate collecting trip)
This is basically flat land. So it made sense.

Point bar deposits:



Photo of point bar deposit



Theory says the knobs in this area are simply point bar deposits from the ancient river.
Knobs that defied modern erosion because they are capped with deposits of cobbles.
Never tried mapping them, guessing you would end up finding an ox-bow river.

Henry says it's an old deposit. Have to agree. Point bar deposits sitting on the highest knobs ? Yes, that would be an old deposit for sure.
If they are point bar deposits sitting on high, then this area has seen extreme erosion by serious forces.
Note that the hill top deposits are NOT encased in caliche. Just cobbles laying loose.

Be aware this ranch is on a 'high' peninsula pointing out in the lake.
All the land around this ranch is lower, and the lake pool is at ~306 elev.
So if the old river was at 390 elev., where did all the surrounding ground go ?
Probably washed into the fertile farm valley below Lake Falcon and perhaps into the Gulf of Mexico.

 The plain that was being eroded by the slow moving, oxbow river slowly had the Uvalde gravels that were present, eroded and dropped further downstream while the point depositions remained. As the slow moving river slowly meandered over the plain eroding it for millions of years. Then the only original, agate-cobble rich gravels were left at much the same elevation, less stream gradient fall, were from the original depostition onto and to form the plain or possibly and ancient delta of sorts.  So then your magic 370+ number, is the original elevation during the heavy deposition of the agates before time and erosion dropped many off the 370 bench and randomly scattered them for the umpteenth time. The un-eroded 370 foot  level is the Goldilocks zone of higher concentration. Know that you are speaking mainly of gravels but the other lap  type minerals ie, Jaspers and wood have a bit different specific gravity so then their dispersal would be close to, but some what different than the agate type rock.

     Jasper cobbles are rounder. What can we infer from that   as applicable to rock movement and relocation? Longer distances traveled or more a factor of the weight and hardness differences.  The cherts must have flown down the river in geologic time scale.

Rock Dna. Any type of chromatography or such measuring devices to record chemical compostions from different sources upstream? Might make it possible to create a map with mucho man hours, and college grant monies. Might help unravel some of the mysteries. Seems if the chemical composition of the rock in the magic, uneroded after original deposition, in the 370 goldilocks zone then we would have a better idea of ancient water and river movement and quantities of water moving and from what direction or directions.

Thanks @hankrocks for taking Amistad out of the present puzzle. Still need to get info on whats in the gravel bars below the dam. Any good guesses Hank?

[jamesp]

"Jasper cobbles are rounder. What can we infer from that as applicable to rock movement and relocation? Longer distances traveled or more a factor of the weight and hardness differences."

There is a very simple answer for this one.
Moss agates and other AGATES formed in lava bubbles are often biscuit shaped.
Look for hockey puck/biscuit shapes and you will find a high percent agate.

Keep in mind a point bar deposit has a third dimension of depth, it is not a 2 dimensional deposit.
Most of those knobs had a slope on one side much more thick in loose cobbles. Makes the point bar deposit theory much more plausible.
I really tried to pay attention to the point bar theory. Especially on Fernando's ranch since he had deep disced it and removed the foliage.



So you could probably connect the dots if you mapped the knobs and the side of the slope with the dense cobble slopes.
Dense cobble slopes would be the inside of the turn of the old river.

However, as erosion occurred, you may have point bar deposits at many elevations as erosion continued.
Because they quarried pockets of gravel from 380 down to 310. So it may get more complicated, point bars at many elevations. Erosion complicated.
I would like to cruise that lake in a boat when it is at lowest level. Then you could see untouched gravel deposits without the foliage to obscure.
I know super agate man Matt Dillon collects in the lake pool at low levels. That may be the absolute densest population of imported western agate.
The only problem with the agate that stays underwater is the alkaline coating that obscures ID of cobbles. Portable pressure washer ?

[jamesp]

The bass fisherman work the gravel shores. They are the ones knowledgeable of the cobble shore lines.
Great wisdom from the fisherman.

The lake is a big bass making machine.

Shoulda hired one of the local guides. "Hey, take me fishing and drop me off where the gravel shores are, take a client too and y'all fish while I rock hunt. Then pick me and my rocks up at end of day"
The guide could have made two paychecks; me the easy paycheck.

You need a nice fast boat to cover that big lake.

Lake Falcon Bass tournaments are serious:

[jamesp]

Being a dumbass from Georgia I drove the little Honda car all over the desert floor off roading.
Noticed tires started leaking air slowly.
Took car to tire service shop and the fellow removed one of my tires and drug his rag up the inside of the tire.
Rag grabbed. Inside of tire looked like the inside of a pin cushion.
Hundreds of mesquite thorns run clean thru the Michelins. He suggested I finish my rock trip, keep adding air as needed till I get back home.
And replace all 4 tires. 4 fine tires went to the dump.

Next trip I stayed off the desert floor and kept it on the gravel roads.

[Sabre52]

Boy they got pounded by rain down there on the Rio last night. Flash flooding all over. Too bad we ain't down there rock collecting. I imagine lots of new stuff exposed.

Yep, those mesquite thorns are nasty. I've even seen tires popped down on the Mojave when you go cross country. Not as much bad stuff there but some will poke a tire....Mel

[jamesp]

Jul 14, 2017 6:37:13 GMT -5 Sabre52 said:

Boy they got pounded by rain down there on the Rio last night. Flash flooding all over. Too bad we ain't down there rock collecting. I imagine lots of new stuff exposed.

Yep, those mesquite thorns are nasty. I've even seen tires popped down on the Mojave when you go cross country. Not as much bad stuff there but some will poke a tire....Mel

This spot had a lot of pig trails that my car would navigate.
I used satellite on cell phone to see the baldish areas. The thin lines of mesquite along the road hid the open patches with exposed ground. Cell phone images solve that.
Driving down the pig trails, well, I carefully dodged the sticks and young mesquites this last trip and had no punctured tires.

The red line is full pool. The land to the left/west is about useless for collecting except about and up to 200 feet to the left of the line due to wave action at high water exposing cobbles.
The upland to the right of the red line has had quarry activity of various ages, age a function of the age/size of the foliage.
Walking between the denser population of mesquite is a skin remover, lol, damn the Texas thorns of razor blade sharpness with poison injections.

The upper knob in the photo, where heavy wave action removed the foliage to left of line was dense with wood. Why there I have no idea.



Looking back at upper knob in photo where wood was thick



Select wood and agate within 10 foot radius. Sitting on bone hard caliche.
Wood in middle row, bleached biscuits(moss) and jasper nuggets. Wood easy to separate, more blockish plus markings
About 60% of it was reject.



The biggest wood, 3rd from left, viewed 3 directions