Logging tasks
1-Observe the road cut on the North side, are there many dikes? are they uniform in size?
2-Pull a "pebble" from a dike, is it rounded or rough? What might this say about how far it rose?
3-Other than the pebbels, is the other material dike material soft or hard. Does the dike appear to be harder or softer than the rock around it?
Dikes are normally formed by a magma pushing through cracks in the rock that is above it. Sometimes the fault may already be there, and other times the pressure from the magma may actually force the the material apart.
Whatever the case this eventually forms a wall or barrier of a different kind of material that is usually a younger volcanic rock in the middle of the older rocks.
However in this instance we are looking at rock that is formed in a far different way. This area was volcanicly active at one time. A number of calderas, and the faults made by those collapsing volcanoes exist in the area. The mountains here are made up of many different ages of rock, scattered in a haphazard pattern. A younger stone may be right next to something far older geologically as the area collapsed.
As a giant pluton (a mass of lava below the surface) cooled water seeped in and the pressure built. Eventually the pressure built and it needed to escape. Rather than a controlled and even eruption (like a geyser). The giant explosion took place blowing up faults and seams that existed or were created in the stones above.
This pressure explosion blew large amounts of material from up those seams. Some of these appear to be 6000 feet or more in length, though most in this area are near 2,000 feet.
These explosions blew stones off the quartzite walls. They bounced and wore as they traveled upward. Wearing them into pebbles. The deeper down the pebble dike the rougher the and sharper edged are the pebbles.
There are many mines in the area. They regularly cut into these pebble dikes. If the pebbles have a high concentration of gold or silver they would dig deep shafts at that location looking for those gold/silver veins.
Because the stones are far older that are blown upward, the pebbles in the dikes tend to be far older than the stone above.
The pebbles themselves are usually quarzite, though they can be made of any of the stones that are below the level of the dikes. Sometimes some limestone is thrown up if the dike travels through it. Because of the sudden heating and cooling the quarzite can take on an "onion skin" properties. Outer shells breaking off like an onion, from the inner stone.
Information from General geology and mines of the East Tintic mining district, Utah and Juab By Hal Tryon Morris, Thomas Seward Lovering, 1979