For example, traveling west from Nogales, Arizona, Baboquivari Peak captured my attention. The most prominent geological feature visible, it stands significantly higher than the surrounding mountain ranges (which includes Kitt's Peak, with its observatory at the top). Baboquivari Peak's sides rise at a steep angle; on the north side, there is a notch that seems to defy gravity, because the vertical side leans over the horizontal. At the point where the angle of the sides increases dramatically, there is a tilted stripe that reminds me of Columbia River basalt flows; it is different in texture from the peak above, and the mountain range below.
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My questions are: why are the sides so steep? Why is it tilted? How did the notch on the north side form? How old is it?
Wikipedia's article is not very helpful in answering my questions.
A BLM document says(page 16):
The Baboquivari Mountains were uplifted in the mid-Tertiary during formation of the Basin and Range Physiographic Province. The wilderness is underlain by early Jurassic sedimentary and volcanic formations consisting of conglomerate, sandstone, siltstone, volcanic flows, and flow breccias. These formations are intruded by late Jurassic granite which forms Baboquivari Peak. The above formations are cut by numerous igneous dikes, which crisscross the entire wilderness. The Coyote Mountains Wilderness is underlain by granitic plutons of various ages ranging from Jurassic to early Tertiary. The granitic rocks are cut by numerous pegmatite dikes. Metamorphic rocks form roof pendants in the granite in the north-central part of the wilderness. These rocks were originally Paleozoic carbonate and sandstone rocks that were recrystallized from the heat of the granite intrusion and metamorphosed into schist and quartzite. Copper and minor amounts of gold and silver were mined from the Bonanza Mine (Cavillo Camp) located in this area.This passage describes the features I have questions about, and names them ("pegmatite dikes", "roof pendants"), but doesn't really explain how they were formed, or why.
This page on Roof Pendants provides a nice graphic, but I'm not sure they're talking about the type of notches I have questions about. (I think they're talking about layers of metamorphic rock overlying older rock, not the type of overhang I'm curious about.)
From Facts about Baboquivari Peak:
Baboquivari is the most sacred place and mountain to the Tohono O'odham people. The tall rock mountain is the center of Tohono O'odham cosmology and the home of I'itoli, their Creator and Elder Brother. The Tohono O'odham tribe, formerly called the Pagago [sic] or "Bean Eaters," still occupy their ancestral homeland in southern Arizona. Their religious traditions are based on this stark desert landscape, which is dominated by monolithic Baboquivari. I'itoli or Elder Brother Lives Inside Baboquivari The rock god I'itoli, also spelled I'itoi, lives in a cave on the northwest side of the mountain that he enters by a maze of passages. Legend says he came into this world from a world on the other side, leading his people, whom he had turned into ants, through an ant hole. He then changed them back into the Tohono O'odham people. The Tohono O'odham still regularly make pilgrimages to the cave, leaving offerings and prayers for I'itoli. I'itoli often appears in basketry as a male figure above a maze (Man in the Maze symbol) teaching the people that life is a maze of obstacles that must be overcome along life's path or himdag.Sometimes, the geological explanations full of jargon, speaking of magma uplifts (why does magma uplift the land horizontally in some places and at an angle in other places, often closely adjacent?), minerals, metamorphism, trying to describe dikes and sills that defy logic (because they occur so close to each other, and the conventional explanations don't say why the intrusions don't split rocks...), seem just as far-fetched as the ancient Indian legends. A thousand years from now, will today's geology survive, or will they look at it as we look at epicycles?
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The stratified ones are of the Atascosa mountains. This layering reminded me of the Grand Canyon; I guessed they were sedimentary, laid down at the bottoms and shores of oceans. However the linked paper says they are volcanic. Which leads to my questions: where did the volcanic layers come from? Why are they so level? What eroded them? Why did the erosion leave the current peaks?
In nearby Sycamore Canyon, why are the linear features on the side of the canyon vertical in some places, and immediately adjacent, slanted? Why is there a cave outlined by very black rock, as if buckets of black paint had been thrown over the beige-colored rock?
These are questions that I don't find answered explicitly in the geologic tracts I've come across, which seem to hide behind jargon and an academic know-it-all attitude that belittles a layman's curiosity.
The Geology and Natural History sections of a USDA page may answer, in very general terms, some of the questions: there was water in the Atascosa region, probably quite a lot of it, for example.
