I don't have a Clue

[TinaMarieEast]

I have found a few examples like this. Mostly in road cuts located near the military range. Different kinds of minerals have formed the same designs. This cobble is the largest. I am relatively sure that it came to Yuma from elsewhere in the ancient Colorado Rivers wanderings. Ideas??

[DPS Ammonite]

It also might be bryozoan. If the rock is chert it might be from the Kaibab Limestone that was eroded to help form the Grand Canyon.

[ynot]

It is a stream tumbled cobble, this makes it hard to say where it came from or exactly what it is. A scale would help narrow down the possibilities.

The rock has the texture of a sandstone or siltstone.

Doubt it is a chert

[Ludwigia]

I tend to think that those are little corals in there.

[TinaMarieEast]

Here are a couple of other pics with the rock and, aand as requested, a size comparison.

[abyssunder]

I would agree with corals (maybe colonial rugose corals) if the geological time and the dimensions might be proper.

[DPS Ammonite]

Let’s figure out what the rock is. Will a steel knife scratch it? I bet so. 

 

What I see is a piece of well rounded chert with Paleozoic fossils. Note the rounded fractures in the center of the photo that are indicative of chert and not a silt or sandstone. Since there are not very many Paleozoic fossil bearing cherts that crop out near Yuma this rock probably came from far away. Ordinary siltstones, sandstones and limestones from the Paleozoic outcrops in central and northern Arizona are not durable enough to survive a long trip in a river. A USGS paper suggests that Kaibab chert nodules occur in the Colorado River gravel in Yuma County. My best guess is that it is Kaibab since the cherts are usually light colored.

 

What is this fossil?

These are molds in chert. Note the impressions where the calcareous creature once was. The semi parallel columns of chert that are normal to the surface of the fossils are the infillings of the individual bryozoan zooecium. The oval cross sections of the fossils indicate that this probably was a branching bryozoan species and not a horn coral. I doubt that it is coral because I have never seen any Paleozoic branching colonial corals in Arizona with such small calices. Branching bryozoans with small zooecium are common in Arizona Paleozoic rocks including the Kaibab Limestone. 

[Rockwood]

9 hours ago, DPS Ammonite said:

The oval cross sections of the fossils indicate that this probably was a branching bryozoan species and not a horn coral.

Nail on the head. The second photo down just can't be made to fit the rugose symmetry.

[TqB]

I agree with bryozoan, the rods being zooecial tube internal moulds normal to all outer surfaces.