rock in la milagrosa canyon

what type of rock is it? sandstone?

Definitely not sandstone. My guess is granite or granite gneiss. Since it is so close to the Catalinas, maybe it is part of the same intruding pluton. I am hardly an expert on such things so hopefully someone that really knows about the geology responds.

i also thought gneiss at first, but it's a bit different from the rock on lemmon. i'm not an expert either, so maybe it's the same stuff, dunno.

It's gneiss. Gneiss is a metamorphic rock which means that it was something else,in the case of Mt. Lemmon it was granite, which got heated and/or pressurized enough to make the minerals re-crystallize. The banding which around here is sometimes close to horizontal, makes it look similar to sedimentary rocks (like sandstone) which are typically deposited in discrete beds. Cool stuff

huh. i'm not doubting you, but why is the gneiss in milagrosa so different from the gneiss on lemmon?

That's a good question Jon and I'm not up on the local geology enough to give you a reliable answer. But I'll tell you what It think anyway...(hehe) I know for certain that the rock in Milagrosa and Mt. Lemmon are both gneiss and that they were likely deformed at the same time. Both are a type of rock called mylonite gneiss that forms along faults that are deep enough in the crust that the rock deforms ductilely (@ least 10km down)as opposed to brittlely.

My guess is that the parent rock had some variability in its mineral composition and that the forces at work during the deformation event were not evenly distributed resulting in the variety of rock you see all through the Catalinas. You have probably noticed that the rock higher on the mountain does not have as pronounced banding as it does at Windy Point or lower. Probably the higher you go the farther you move from the fault zone where the deformation was most intense.

right on hampton. the rock on lemmon is 'peraluminous granite' which means it has a high aluminum content. the rock in sabino canyon and milagrosa has a lower aluminum content and was subject to greater metamorphic pressures than the rock higher up. there is a good section in the falcon guide for AZ about mt lemmon geology. there is a killer section about the geology of lemmon in the guide by john steiger. the geology section was written by george allen. maybe it's somewhere on the interwebs.

"Sabino Canyon is noted for its rock bands that alternate from fairly good to incredibly bad rock. The difference between fairly good and very bad is a result of the interfingering layers of the Oracle Granite with the Wilderness Granite outcrop. The fairly good climbing is on the Wilderness Granite and the incredibly bad climbing is on the Oracle Granite. Part of the reason the Wilderness Granite does not appear to be as sound here as higher may be because it is notably less peraluminous than at higher elevations.."

"The mylonitization gradually decreases until near the summit where there is no evidence for mylonitic deformation...understanding mylonitization is critical to explaining the systematic variation in climbing quality. At intermediate degrees of mylonitization, the Wilderness Granite changes climbing character from broad smooth faces with clean cracks, to more jagged cracks and steep gnarly faces with horizontal holds. The gneissic banding or foliation which the process of mylonitization imposes is responsible for the greater abundance of horizontal holds as seen at Windy Point in contrast to Rappel Rock. At Goosehead, just 2 miles up the highway from Windy Point, mylonitization has had little effect; most routes follow vertical cracks. At Chimney Rock, four miles down the highway from Windy Point, gneissic bands resulting from mylonitization are more pronounced and closer together. The resulting higher concentration of horizontal cleavages lends a distinct face-climbing character to much of the rock."

"Windy Point Overlook: As in Bear Canyon, some rock bears a hard dark-colored veneer, in other places the veneer is absent and the rock is incredibly poor..Beware of white rock under roofs; apparently the lack of runoff on some surfaces prevents case hardening"

Just a few random selections, there's pages and pages of this stuff in the Steiger guide.

i see. it's interesting that similar rock can have such different characteristics due to the forces that create it.