Papago Park

I visited another local park here in Phoenix today. Its called Papago (pap-a-go) Park. It is located not far from ASU, but has some very interesting geology.

This perticular butte is fascinating to many people for mostly non-geological reasons. It has a hole in it that when standing in offers spectacular vistas of downtown. Anyway, the history of the rock. Formed at the same time as Hayden Butte, Hole-in-the-rock (thats actually its proper name) is formed from totally different processes. 17 million years ago, somewhere within 2 km or so, there once stood a mountain fromed from granite and quartzite. One very violent day there was a massive landslide that came off the mountain and headed toward an ancient river. The landslice eventually became lithofied (turned into stone) and formed the butte. Interesting enough, the mountain that formed the landslide had eroded completely and cannot be found anymore. The butte is composed out of a composite sedimentary rock called breccia (bretch-a) which is an amalgamation of mud, sand , and broken up parts of granite and quatrzite (all of which came from the parent mountain) some of the chunks of granite are as large as a house.

The holes in the rock were formed when these boulders fell out of the breccia, and then were exposed to what is called cavernous erosion. This erosion over the course of millions of years formed the caves that so many people enjoy today. On an even more interesting note, this butte has the Great Unconformity running right through it! Remember from my trip up to payson, how where was the section of 1.2 billion years missing. Well that same gap exists at the base of the butte. The base of the butte is made up of granite that is 1.8 billion years old (as old as the oldest rocks found at the bottom of the Grand Canyon). however i found a xenolith of some kinda ash metamorphic that is even older. I had to take a sample of that back for my collection (as oldest rock)