Matt... I have seen this many times in benches... the gravels are cemented together and if your finding them in a gold bearing area they probably have some pretty gold in them because that is virgin my friend...Streams move around over time and sometimes will cut through old stream beds or benches and can be found in chunks in the stream, while in the bench itself is almost all cemented... I found a bench once that was so cemented together with black sands and iron oxides it was almost impossible to work, with what I had to work with, but was well worth the effort... I also know of some small streams near me that at the head is caliche but it is usually white here in Ga, decomposing quartz  with a lot of calcite and sometimes  clay holding it together that is very hard to work but sometimes worth it...