You have to start at the regional scale. Where was the photo taken (what Physiography)? Is there a state published geologic map? Most states have regional maps that will show you the regional geologic formation names, their ages and the types of rocks found within the formation (recognizing that some formations are massive and have one principal rock type and other formations are stratified and have similar, but varied rock types).
Clearly this is some lithic conglomeration. That said, the lithic pieces (large rocks floating in the matrix) may be sedimentary, metamorphic or igneous origin. Also, how hard is the matrix and what is its mineralogy?
How would a rock like this form? The answer to this may be found from the regional publications or the local setting (I mean is this rock type seen hear and there over a large area, or is the more typical rock type one thing, but here's this odd-ball item). If just an odd-ball rock mass, I mean it could be a cemented rock slide.
So, I'm not saying what it is - I mean why call it an agglomerate if there are no volcanos about?
If given the option you can always call it, "leaverite" - as it leave 'er right there - ha!
f-d
p.s., I'm also a registered geologist, became an engineer after doing geologic field work for 8 years.
¡papá gordo ain’t no madre flaca!