From Lajitas to the Cuevas Amarillas Locale, Part 4: My Favorite Hoodoo in Its Greater Context | Big Bend Ranch State Park, Texas, USA
Taken, like the previous one in this series, along the north side of FM 170, about 1 road mi / 1.6 km southeast of the Madera Creek bridge. Facing northeastward.
This is the companion and the context of the Part 3 photo. That shot focused squarely on the big hoodoo here shown in the left foreground. Now, however, we can better behold the bizarrely sculpted hillside on which that amazingly architectural landform stands.
As I reported in Part 3, my main source for interpreting what these fantastical shapes are made of is Christopher D. Henry's Geologic Map of Big Bend Ranch State Park, Texas (that state's Bureau of Economic Geology, undated). The only version I've found is the one printed on two sheets of paper. If it has been digitized or pdf-ized, I can't find it.
Be that as it may, this is one of my favorite geologic maps. I've spent hours poring over it. It's highly detailed, manifests an excellent sense of appropriate color use, and is packed full of information. It's true that the formation-name symbology and legend texts are in a font size so small that one practically needs a scanning electron microscope to read them, but that's the price to be paid for this level of precision.
The site featured here is located on the map's eastern half, but the western sheet is also required, because it's the one with the legend. And that legend is extremely helpful, in that it gives detailed descriptions of each rock unit. I've already quoted part of the one for the Eocene-to-Oligocene Chisos Formation exposed at this location, but here I go again:
tuffaceous sediment and conglomerate, massive to bedded, pumiceous sandstone, siltstone, and conglomerate.
The term tuffaceous does not refer to the quality of being able to stand up to whatever the forces of weathering or erosion throw at you. Rather, it signifies that you're a sedimentary rock type directly derived from the igneous extrusive rock known as tuff.
It's tough getting tuff straight, because it's often confused with tufa. The latter is a deposit of calcite laid down by calcium-rich water dripping or flowing over a surface. Its close relative, travertine, forms in the same way, but its source is usually warmer water issuing from geothermal springs.
In contrast, tuff is a blanket of volcanic ash that has settled on the ground and hardened to the consistency of soft stone. Geologists sometimes also use the term lithic tuff to identify, well, to identify tougher tuff.
When it's weathered or eroded, tuff becomes an unconsolidated sediment. If that material lithifies again after being transported and redeposited somewhere else, it becomes one kind of detrital sedimentary rock or another, depending on its particle size.
On this hoodooiferous hill it seems as though the tuffaceous rock is conglomeratic at least in part, with sandstone probably present, too.
To see the other photos and descriptions in this series, visit my From Lajitas to the Cuevas Amarillas Locale album.
From Lajitas to the Cuevas Amarillas Locale, Part 4: My Favorite Hoodoo in Its Greater Context | Big Bend Ranch State Park, Texas, USA
Taken, like the previous one in this series, along the north side of FM 170, about 1 road mi / 1.6 km southeast of the Madera Creek bridge. Facing northeastward.
This is the companion and the context of the Part 3 photo. That shot focused squarely on the big hoodoo here shown in the left foreground. Now, however, we can better behold the bizarrely sculpted hillside on which that amazingly architectural landform stands.
As I reported in Part 3, my main source for interpreting what these fantastical shapes are made of is Christopher D. Henry's Geologic Map of Big Bend Ranch State Park, Texas (that state's Bureau of Economic Geology, undated). The only version I've found is the one printed on two sheets of paper. If it has been digitized or pdf-ized, I can't find it.
Be that as it may, this is one of my favorite geologic maps. I've spent hours poring over it. It's highly detailed, manifests an excellent sense of appropriate color use, and is packed full of information. It's true that the formation-name symbology and legend texts are in a font size so small that one practically needs a scanning electron microscope to read them, but that's the price to be paid for this level of precision.
The site featured here is located on the map's eastern half, but the western sheet is also required, because it's the one with the legend. And that legend is extremely helpful, in that it gives detailed descriptions of each rock unit. I've already quoted part of the one for the Eocene-to-Oligocene Chisos Formation exposed at this location, but here I go again:
tuffaceous sediment and conglomerate, massive to bedded, pumiceous sandstone, siltstone, and conglomerate.
The term tuffaceous does not refer to the quality of being able to stand up to whatever the forces of weathering or erosion throw at you. Rather, it signifies that you're a sedimentary rock type directly derived from the igneous extrusive rock known as tuff.
It's tough getting tuff straight, because it's often confused with tufa. The latter is a deposit of calcite laid down by calcium-rich water dripping or flowing over a surface. Its close relative, travertine, forms in the same way, but its source is usually warmer water issuing from geothermal springs.
In contrast, tuff is a blanket of volcanic ash that has settled on the ground and hardened to the consistency of soft stone. Geologists sometimes also use the term lithic tuff to identify, well, to identify tougher tuff.
When it's weathered or eroded, tuff becomes an unconsolidated sediment. If that material lithifies again after being transported and redeposited somewhere else, it becomes one kind of detrital sedimentary rock or another, depending on its particle size.
On this hoodooiferous hill it seems as though the tuffaceous rock is conglomeratic at least in part, with sandstone probably present, too.
To see the other photos and descriptions in this series, visit my From Lajitas to the Cuevas Amarillas Locale album.