Scours in sedimentary rocks

Scours are sometimes visible in sedimentary rocks, and if you could identify one it allows you to conclude a few things from looking at an outcrop. The most important thing that you can figure out with a scour is which direction the water was flowing as it was created.

These marks in sedimentary rocks are created when the flow of water is suddenly increasing. Mudstone (or sandstone just relatively softer)  is deposited when there is little flow of water, and the the clay particles have time to settle to the bottom. As water speed picks up, the flow can knock pebbles onto this muddy surface.

Directly behind the pebbles that lie on the mud surface, a small suction is created where the water picks up higher speeds as it travels around the pebble. This faster moving water in turn begins to erode away the mud behind the pebble. More pebbles tend to fall in these eroding holes, speeding up the process and resulting in a common indicator of scours, the coarse infill. Knowing that the mudstone is below the coarse infill, you can determine stratigraphic up, or which way was vertical when the rocks were deposited and before they were uplifted.

Scours make a very typical shape that allows you to determine the flow direction of the water. On the upstream edge of the scour, the contact between mudstone and infill will be very steep, almost vertical. On the downstream edge, the slope is gentle into the trough. Using this information, you can determine which way the water was flowing, which can provide interesting insight about the history of the rock.

North Table Mesa Zeolite Crystals

Looking across Clear Creek Valley to South Table Mesa from one outcrop of amygdalar basalt.

Zeolites are microporous minerals formed from aluminum and silica. Microporous means that their mineral structure has large holes in it, and this feature gives way to the usefulness of zeolites.  They can be used to purify water by the means of ion exchange beds that consist of zeolite minerals. For the same reasons they are commonly used in laundry detergents.

Although the zeolites found on Table Mesa aren't useful for purifying water, they are quite spectacular in their occurrence. The delicate and intricate zeolites from this locality are prized rare specimens that rarely occur together in other parts of the world.

They are found in the amygdalar basalts in this locality. Amygdalar simply means that there were holes (or vesicules) in the rock that have been filled in by some material, which is usually calcite or zeolites. If there were no material in the holes, it would be simply a vesicular basalt. The zeolites are reported occur in seven different varieties, but the most common seem to be thompsonite, analcite, chabazite, okenite and mesolite.

A cavity of thomsonite (the tan balls) that has been opened and exposed to weathering.

The basalts of Table Mesa are very difficult to break up. Attempting to break these rocks with a 16 or 22 oz. rock hammer would be rather futile, and its doubtful that your hammer would weigh the same after so much steel would get taken off.  Even my chisels would show a new imprint of the sledge's face after each impact, and the top of the chisel would mushroom out a bit and curl back with each hit. Usually, after smashing at a face for a few minutes with the sledge and chisel, you will get a fracture in the rock.

After a fracture has been propagated, which usually tends to be along the long axis of the zeolite pockets, you have to go and break out the ends of the fractures so that the rocks can be removed. Otherwise, they all fit in just like puzzle pieces that despite being loose do not want to be removed. After demolishing the rock to removable pieces, you get the pure pleasure of removing each piece and inspecting it for crystal-filled pockets.

This amygdule contains analcite, chabazite and thomsonite.

Sometimes you get nothing, and sometimes there are some large cavities. One indicator of finding pockets seems to be oxidation of the rock immediately surrounding the pocket. These little trails of oxidation can lead your hammer from one pocket to the next.

I believe this to be an okenite crystal (~2.5 cm) among chabazite (square white and clear crystals), this specimen (in-situ) was destroyed in the attempt to retrieve.

The zeolites are somewhat zoned, and in some areas different types of zeolites will be found more frequently. For example one area was characterized by large cavities of purely thomsonite. Other areas had cavities filled with thomsonite and analcite, and some contained purely chabazite. The amygdules in the thompsonitic area were commonly 6" long and 2-3" in height and tending to be stretched in a NW/SE direction. The largest was over a foot long and about six inches tall, with two separate cavities attached. It had however been broken out a long time ago and was therefore quite weathered.

Chabazite crystal showing its pseudo-cubic structure.

Also of note was the immediate change of color when the minerals are brought out of pocket. Within five minutes of exposure to air, my translucent green thomsonites had dulled to a tan coating outside. The more weathered thomsonites are completely tan. Also the chabazites, which in their cubic form resemble NaCl or salt crystals, are clear and transparent when first broken out but quickly gain a slightly white coating on the crystal after exposure.

An analcite crystal with thomsonite on basalt matrix,  the analcime is quite large for this locality(~1.5 cm wide)

Although a lot of effort oft results in cracked and damaged crystals, there is a technique to collecting these crystals from the pockets. A hard day's work may result in only one or two nice specimens, and sometimes the best ones escape you. If you decide to head up to Table Mesa, be sure to check that collecting is allowed before you go. Bring along some soft packaging to transport your crystals back to the car, because the jolting of carrying them can also damage the crystals.

Bailey Creek

I just stumbled across this picture from summer '11 on Bailey Creek, southwest of Denver, CO.  Its a beautiful and continuous class V kayak run through a remote canyon. Towards the end of the run, the canyon flows through a granitic wonderland, with huge (200' wide) boulders scattered in the river channel to create small rapids. While the beginning of the run is definitely all Colorado boating, with tight, steep canyons and quick moves to be made, the granites at the end of the run inspire thoughts of Cali boating.

I'm in the white Jefe in the front center of the picture. My cousin, Ricky Hoberg is performing the move that I had just failed to do, and as a result, am facing upstream. The desired move is to slide off the dry side of the rock back into the current. Otherwise, you get pushed against the rock sticking out of the water just to the left of my bow, which can cause problems.

Just out of view in the bottom of the picture is a large hole, which by the time I had gotten my boat turned back around was right below my feet. The plunge was considerable, probably 6', and I had rolled into it without any forward speed. As my boat slipped over the edge and into the white foam below, I knew I was in trouble. I plunged vertically into the hole below, going so deep that almost my entire boat was submerged before returning to the surface upside-down. After waiting to be flushed from the hole, which didn't take long, I had to try and roll my boat back right-side up, which took about three or four exhausting and breathless attempts.

 On the edge of the river below, when you look upstream to what you have just run, that is the sweetest part. The steepness is apparent when looking back upstream. The adrenaline was flowing and the sun was shining. I don't think that I will ever forget that run.