Near an old fire-trail I found a single fist-size igneous rock that I identified (as an amateur geologist) as belonging to a formation that is not known of in the area but is known to occur in the neighbouring district, although not common there. Naturally I was interested if there was an outcrop and looked for more float samples, but twenty minutes of searching yielded no more samples of the kind. My question is, how reliable are float samples? Is there probably an outcrop? Or assuming my ID was correct, could I just have been that the bulldozer that cut the road all those years ago came directly from the small part of the neighbouring district where these rocks are found. Or did the bulldozer move it from elsewhere on the hill, but there still probably is an outcrop in the immediate vicinity.

Apologies for the rather specific (and long-winded) question, I understand that this Stack Exchange is looking for more general questions, but I would just like to add if it was possible for an intrusive igneous rock such as syenite to be shed from quartz conglomerate? It seems unlikely to me, but that is the main formation above the spot where it was found and could explain its occurrence. (Should I have created a seperate question for this?)


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    $\begingroup$ Suggest you take the last part of the question and make it a separate question indeed, as it seems separate in general $\endgroup$ Jun 14 at 10:39

1 Answer 1


Syenite is a granite like igneous rock that is deficient in quartz. If it contains quartz the amount of quartz is less than 5 percent.

Conglomerate is sedimentary rock that contains clasts (rock fragments - the sedimentary component) that have were once washed down a river and are held together by quartz or calcite. The rock fragments (clasts) with the matrix of the conglomerate are rounded, which indicates they tumbled down a river.

The clasts within the conglomerate can be from a variety of different rock types, including: quartz or feldspar, quartzite, sandstone, limestone, granite, basalt, and gneiss. It is possible that syenite can be a component of a conglomerate, it depends on the local geology.

Regarding the floater, with only one sample for an area it is difficult to tell, but is seems unlikely to indicate a local outcrop. As you indicate it may have been deposited there during road construction works.


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