If you do this type of degree could you proceed to do a post-grad of one of those subjects?
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If you were doing a JH in physics and maths, you would normally be well placed to go on to any physics PhD (and better placed for some topics). You would however be more limited on the Maths side to most likely only applied maths courses, unless you were able to develop the necessary pure maths background in that JH course structure (which is possibly unlikely).
Alternately if you were doing say, chemistry and spanish, you might find your choice of PhDs more limited. It's quite likely a major aspect of the course would have to be left to the wayside to allow you to take the necessary modules for the spanish half, and so research topics building from that would probably not be available. For example, if to fit in the Spanish stuff they dropped all the inorganic content, you probably would find most inorganic, industrial, environmental and some theoretical chemistry topics inaccessible.
There would be similar analogues for non-STEM subjects, but in general this is a bit less clear cut, as they're more likely to be able to get you to pick up the relevant topics through focused independent reading, whereas in STEM some degree of teaching is usually necessary.
Cheers for replying. I am considering doing Geology and Physics or Geology and Biology. So they are very STEM orientated. That's good to know because I really like Geology but I feel like I would be really missing out on either of those which I really enjoy especially nuclear physics and the lymphatic bits of biology.
However neither of the areas you've indicated are really relevant to Geology - the only aspect of nuclear/particle physics relevant for Geology is as far as radiocarbon dating, which doesn't require specialist nuclear/particle physics knowledge (or any of the quantum mechanics required to get to that point), and lymphatic physiology will normally be taught in terms of human biomedical science, which is also unrelated to Geology.
So, while there are ways the two are related, and complement each other, your interests seem fairly disparate. I mean actually a Biomedical Physics or similar course would probably be better preparation for those, and even then you would probably struggle pursuing high energy physics, as HEP is a very competitive area to research in. I'd also note that even just doing single honours mathematics is better preparation for a research career in HEP than a joint honours with geology (or biology, or anything), and if the single honours physics or related doesn't have some pretty hefty mathematical options you might find it very challenging at any rate.
You may want to think about why you're considering these areas, and make sure they're due to intrinsic interest and not some external consideration of supposed "prestige" that makes you interested in them.