Speckled Rocks

[Mans]

Hi everyone 

What are these dark spots (light brown)? 

[GotAU?]

Although at first  I though it could be fossiliferous limestone with shells mixed in,  looking closer at the mix of other types of rock stuck in it (like that black basalt looking one and others) I now think it’s breccia with some large fragments of darker colored sandstone mixed in. 

[jasong]

I'm going with GotAU's first guess (I think the black is some kinda lichen?) - some kind of calcareous rock like limestone or even something like massive caliche. The darker spots would probably be little areas of higher iron concentration oxidizing more than the surrounding rock. Another option is desert varnish, but the crumbly, unstable nature of these rocks would seem to preclude that. 

One example in the desert SW that comes to mind is the Hualapai Limestone, which also looks like caliche (it often has caliche sitting on top of it), and has a lot of spots of darker oxidation in it in places. It can be a semi congolmerate in some places, as I believe it was the remants of an extinct lake or sea. 

A photo of it by Lake Mead, swiped from Google images. You can see the darker coloration on the exposed cliff face. If it showed the rock debris closer, you'd see the same thing speckling the rocks too on smaller scale. 

[Geologyhound]

Is the rock calcareous?  If you put dilute hydrochloric acid (0.1 molar) on it would it fizz?  If so it would be a limestone with shells or at least shell replacements (shells dissolved away and the void space was then filled later).  If it is not calcareous, then it is more likely a volcanic tuff or possibly a debris flow deposit.  It looks like there are a lot of vesicles (air pockets in a volcanic rock), so I am leaning towards a tuff.  In this case the lighter colored material would be whatever surrounded the volcanic vent and got ripped up in fragments during an eruption.  The heat and force of the eruption could deform the included clasts.  If the rock is from a debris flow, then the lighter colored inclusions could literally be chunks of mud that were eroded during the flow.