Is conglomerate sandstone, like the flatirons, ok to climb after rain?

Always wondered. It doesn't seem to absorb water like other types of sandstone.

Dries way faster - much more compact. The purple parts are particularly slippery- dry or wet! 

As a general rule, the sandstone east of the continental divide is fine after rain and basically all west of it is not 

For clarification purposes (and not to deride the OP):  There is no such thing as "conglomerate sandstone".  Conglomerate and sandstone are two different types of rock, classified by the size of the predominant grains.  You might have alternating layers of conglomerate and sandstone, but they cannot, by definition, be within the same layer or unit.  You can have a conglomerate with some sandstone-sized grains (or vice versa) but the rock unit would still be classified by the size of the predominant grain as either as conglomerate or a sandstone.

More specifically, the Flatirons are comprised of the Fountain Formation, an arkosic sandstone (or arkose) based on the predominant grain size (< 2 mm) and the feldspar content.

^^^touche'^^^  Conglomerate and Sandstone are two distinct rock types.

tl;dr - sorry, there isn't really a broad rule of thumb here. Geology is fascinating and complex. Also, it's probably absorbing a lot more rain than you think it is.

The local geological history of an area matters just as much as the global history that created it. You want to know about how the rock got there (depositional environment, overburden causing compaction, mineralogy of the source providing the sand grains), plates smashing together to uplift the Rockies, and history (did something cement the rock in one place but not another, did any metamorphism happen before the Rockies were created).

The sandstones of the desert Southwest, such as the Creek, are typically Aeolian (deposited by wind, like the Sahara). Bullet-hard stuff like Horseshoe Canyon ranch often comes from marine deposition (like the silt being laid down in the Gulf by the Mississippi) and then cemented more strongly by fluid flows, compaction, and/or some  metamorphic experiences.

The Garden of the Gods and the Flatirons are both the Fountain Formation which was alluvial deposition (laid down by rivers and deltas). Alluvial deposition means rock properties can vary quickly over relatively short distances because rivers are a significant change compared to the land around them. On geological timescales, they move extremely rapidly. They wear down and shape the bits of eroded rock as they transport them. Fluid flows can flush rock with dissolved minerals, cementing the grains more strongly, or they can dissolve cement, leaching it out of surrounding rock. All of these processes affect the mechanical properties you are wondering about.

All that technical information is really interesting. A few years ago someone did a great write up on how water affects all rock. Basically he said all rock gets weaker after rain but to different degrees.

What’s most important today in the age of Internet shaming is knowing which sandstone you can climb on after rain without getting a nasty gram on your windshield or a long thread shaming you. 
Here is a list to get you started. Feel free to add on:

Shame on you:
Red Rock
Indian Creek
Moab
Garden of the Gods
Zion

No Shaming:
Eldo
Flat Irons
New River Gorge

Tim Schafstall wrote: For clarification purposes (and not to deride the OP):  There is no such thing as "conglomerate sandstone".  Conglomerate and sandstone are two different types of rock, classified by the size of the predominant grains.  You might have alternating layers of conglomerate and sandstone, but they cannot, by definition, be within the same layer or unit.  You can have a conglomerate with some sandstone-sized grains (or vice versa) but the rock unit would still be classified by the size of the predominant grain as either as conglomerate or a sandstone.

More specifically, the Flatirons are comprised of the Fountain Formation, an arkosic sandstone (or arkose) based on the predominant grain size (< 2 mm) and the feldspar content.

Honest question because geology is rad. How is the rock classified in cobble places like Maple Canyon and Echo Canyon in Utah where it is hard metamorphic like river stone cobbles cemented in sandstone? I've always thought those cliffs as conglomerate since there sees to be two very obvious types of rocks cemented into one layer of stone.

Jordan Wilson wrote:

Honest question because geology is rad. How is the rock classified in cobble places like Maple Canyon and Echo Canyon in Utah where it is hard metamorphic like river stone cobbles cemented in sandstone? I've always thought those cliffs as conglomerate since there sees to be two very obvious types of rocks cemented into one layer of stone.

Maple Canyon is conglomerate.  I am not familiar with Echo Canyon.  Do not think of conglomerate as "two very obvious types of rock".  Conglomerate will almost always have different sizes of rock.  But to be a conglomerate, a rock must be comprised mostly of rounded grains > 2 mm in size (angular grains > 2 mm is breccia).  The matrix in which the larger sizes are imbedded is often of smaller grain size.

And conglomerate is a sedimentary, not metamorphic rock.  To be metamorphic, the original rock unit must have been altered by pressure and or temperature.  For example, Seneca Rocks in WV is a quartzite - sandstone that has been subjected to pressure and or temp such that the original grains and depositional layers have been altered.  Sedimentary rock is subjected to pressure and temp too (that is why, for example, the wind-blown sandstone of Red Rock, NV is consolidated), but not so great that the original grain and layering structure are gone.

Alot of it comes down to how the sandstone is cemented, the fountain formation around boulder is cemented with silica (and slightly metamorphosed in places?),  where around garden of the gods it's cemented with calcite

Ross Hokett wrote: Alot of it comes down to how the sandstone is cemented, the fountain formation around boulder is cemented with silica (and slightly metamorphosed in places?),  where around garden of the gods it's cemented with calcite

...and thus it matters if the cementing agent is water soluble or not.  If the cementing agent is water soluble, the rock weakens when wet.

dnoB ekiM wrote:

...and thus it matters if the cementing agent is water soluble or not.  If the cementing agent is water soluble, the rock weakens when wet.

Water isn't a cementing agent, lime or silica is. But point taken. Water is a catalyst. 

Tim Schafstall wrote:

Maple Canyon is conglomerate.  I am not familiar with Echo Canyon.  Do not think of conglomerate as "two very obvious types of rock".  Conglomerate will almost always have different sizes of rock.  But to be a conglomerate, a rock must be comprised mostly of rounded grains > 2 mm in size (angular grains > 2 mm is breccia).  The matrix in which the larger sizes are imbedded is often of smaller grain size.

And conglomerate is a sedimentary, not metamorphic rock.  To be metamorphic, the original rock unit must have been altered by pressure and or temperature.  For example, Seneca Rocks in WV is a quartzite - sandstone that has been subjected to pressure and or temp such that the original grains and depositional layers have been altered.  Sedimentary rock is subjected to pressure and temp too (that is why, for example, the wind-blown sandstone of Red Rock, NV is consolidated), but not so great that the original grain and layering structure are gone.

Maybe I need to start PMing you but this is very fascinating, so this is what Echo looks like https://www.mountainproject.com/photo/114813350 its a bunch of hard stones that look like polished quartzite set in to bands of soft sand stone similar to San Rafel Swell. One day while climbing there the Men in Black of geology came rolling up in these black suvs and came filing out and were telling us all about how the cliffs were formed and that they're one of a kind. Ever since then I've been trying to figure out more about the rock. 

So as for me who is eager to learn, I see the round cobbles as separate from the sand stone but they are to be viewed as one type of rock?

G K Chesterton wrote:

Water isn't a cementing agent, lime or silica is. But point taken

Nor did my statement say water was a cementing agent.  It stated that the differentiating factor is if the cementing agent (be that silica, calcite, etc)is  water soluble or not.  

dnoB ekiM wrote:
...and thus it matters if the cementing agent is water soluble or not.  If the cementing agent is water soluble, the rock weakens when wet.

Castle Rock Conglomerate, such as at Castlewood,  is a fascinating type of conglomerate.
It loses no tensile strength when wet. None. Those sharp crystals don't even rub off with your feet with standing water.
The reason why is that it is literally naturally occurring concrete. Concrete is made by cement, aggregate and water. What makes cement bond it so strong is because of the origins of it being limestone that has been subjected to high heat from a kiln and thus converted to lime.
The matrix of CRC is volcanic ash and ejecta deposited from the area of Mt. Princeton northeast to the southern part of what is now the Denver metro area. For a 100 million years there was a volcanic field centered around the San Juans, producing the earth's largest known volcanic explosions. We climb at Penetente on that history. The primordial Rockies had layers of limestone, some of this was vaporized as ejecta and coated the ancient front range, which rose up from an inland sea. Subsequent flood events then added water to this mix of lime, sand and river rocks which flooded a wave of concrete down the slopes into the inland sea between Douglas and El Paso counties. It settled in and quickly solidified, preserving the air pockets from the violence of the flood. 

Greg D wrote: All that technical information is really interesting. A few years ago someone did a great write up on how water affects all rock. Basically he said all rock gets weaker after rain but to different degrees.

What’s most important today in the age of Internet shaming is knowing which sandstone you can climb on after rain without getting a nasty gram on your windshield or a long thread shaming you. 
Here is a list to get you started. Feel free to add on:

Shame on you:
Red Rock
Indian Creek
Moab
Garden of the Gods
Zion

No Shaming:
Eldo
Flat Irons
New River Gorge

 No shaming:

Red River Gorge (Climb while wet/downpour if you want to)
Chattanooga (T-Wall, Foster’s, etc)

Shaming even though it’s unwarranted because Midwesterners are stupid: Midwestern Sandstone (Devil’s Lake Old Sandstone, Necedah)

To me, the rule of thumb is: “does this place normally get tons of rain?”  If so, it’s probably fine; if not, it’s probably not.

Jordan Wilson wrote:

Maybe I need to start PMing you but this is very fascinating, so this is what Echo looks like https://www.mountainproject.com/photo/114813350 its a bunch of hard stones that look like polished quartzite set in to bands of soft sand stone similar to San Rafel Swell. One day while climbing there the Men in Black of geology came rolling up in these black suvs and came filing out and were telling us all about how the cliffs were formed and that they're one of a kind. Ever since then I've been trying to figure out more about the rock. 

So as for me who is eager to learn, I see the round cobbles as separate from the sand stone but they are to be viewed as one type of rock?

So, after a little research, I found this:

The Echo Canyon is dominantly reddish brown and forms bold cliffs. In exposures near the mouth of Echo Canyon, the formation is mainly conglomerate but includes sandstone and mudstone beds. Fossils, including Inoceramus and brackish-water mollusks collected from fine-grained beds near the mouth of Echo Canyon, indicate that this part of the formation was deposited in or near a sea (Madsen, 1959). Clasts in the conglomerate range in size from pebbles to small boulders, and more than 60 percent by volume are subangular to rounded fragments of carbonate rock, sandstone, and siltstone. Most of the remaining clasts are rounded cobbles and small boulders of tan and pink quartzite that resembles the quartzite in the Cambrian Tintic and Pennsylvanian Weber Quartzites of the autochthon of the Willard fault. 

Mullens, T. E., 1971.  Reconnaissance Study of the Wasatch, Evanston, and Echo Canyon Formations in Part of Northern Utah. Unites States Geological Survey Bulletin 1311-D. 

In summary, it is a conglomerate with some discontinuous sandstone and mudstone beds (the layers of smaller grained material in the photo).

Tim Schafstall wrote:

So, after a little research, I found this:

The Echo Canyon is dominantly reddish brown and forms bold cliffs. In exposures near the mouth of Echo Canyon, the formation is mainly conglomerate but includes sandstone and mudstone beds. Fossils, including Inoceramus and brackish-water mollusks collected from fine-grained beds near the mouth of Echo Canyon, indicate that this part of the formation was deposited in or near a sea (Madsen, 1959). Clasts in the conglomerate range in size from pebbles to small boulders, and more than 60 percent by volume are subangular to rounded fragments of carbonate rock, sandstone, and siltstone. Most of the remaining clasts are rounded cobbles and small boulders of tan and pink quartzite that resembles the quartzite in the Cambrian Tintic and Pennsylvanian Weber Quartzites of the autochthon of the Willard fault. 

Mullens, T. E., 1971.  Reconnaissance Study of the Wasatch, Evanston, and Echo Canyon Formations in Part of Northern Utah. Unites States Geological Survey Bulletin 1311-D. 

In summary, it is a conglomerate with some discontinuous sandstone and mudstone beds (the layers of smaller grained material in the photo).

Thanks! You're number one! Got me a rabbit hole to chase now.