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I'm convinced there is a word for this. I was in the Hoodoos at Writing on Stone this weekend and kept noticing what looked like reddish quartzite boulders laying around in the sand, or sometimes sticking partially out of the hoodoos.

When a non-sedimentary rock gets washed out into silt which later lithifies, what's it called? It's kind of like a conglomerate, except there's only a couple of really big rocks, which eventually fall out out the rock because all the sandstone around them eroded away.

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    $\begingroup$ I love the idea of "non-sedentary rock". Is this one of Pratchett's trolls? (not editing in case it is actually a geological term and not a typo, but if a geologist comes along, check the title...) $\endgroup$
    – Semidiurnal Simon
    Commented Feb 28, 2016 at 6:06
  • $\begingroup$ @SimonW it actually fits :) $\endgroup$
    – Gimelist
    Commented Feb 28, 2016 at 7:53
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    $\begingroup$ @SimonW Auto correct is the bane of my existence... $\endgroup$
    – ShemSeger
    Commented Mar 2, 2016 at 4:25

4 Answers 4

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The technical term for a sedimentary rock that has a lithified fine-grained sediment with larger pieces of rocks suspended in it upon lithification is a conglomerate. The fine-grained interstitial part is called the matrix, and the large pieces suspended in it are called clasts. Clasts can range from gravel- to boulder-size. These are technical terms used by sedimentologists.

It is tempting to refer to these fragments as xenoliths but as that word has a very specific meaning in igneous petrology, it is best to avoid it to remove any confusion.

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  • $\begingroup$ Then what do you refer to the stones as after they've been freed from the sandstone? There are large stones out there just laying in the sand like eratics, but they are there because they eroded out of the hoodoo. $\endgroup$
    – ShemSeger
    Commented Feb 29, 2016 at 13:28
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    $\begingroup$ @ShemSeger I'm not a sure there is a term for "rocks that used to be lithoclasts but are now just boulders sitting there". If there is, it's mostly likely an obscure term that no one knows unless you're an extremely old professor that insists on using excess jargon. $\endgroup$
    – Gimelist
    Commented Feb 29, 2016 at 21:29
  • $\begingroup$ If they're loose stones that have been eroded out and mobilised from being in a conglomerate, then they are referred to as whatever the relevant grain size is, from here: en.wikipedia.org/wiki/Grain_size The ones that you're talking about would just be boulders. $\endgroup$
    – Ben MS
    Commented Feb 29, 2016 at 22:38
  • $\begingroup$ I agree, it's just a clast. But I see the point of naming it something else after it's been released from the matrix material. In this case, the original sandstone acts like a carrier which then releases the boulders after. It reminds me of a glacial boulder which is sometimes called a "glacial erratic". $\endgroup$
    – Antonio
    Commented Mar 1, 2016 at 9:16
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    $\begingroup$ @Antonio I'm very familiar with erratics. Actually, the Big Rock in Okotoks, Alberta pictured on that wikipedia article isn't far from here, only about a 2hrs drive. There's another one 40 minutes away, called the Glenwood erratic, both are popular for bouldering, I've climbed on both. I vote for calling the released clast an, "emancipated clast." $\endgroup$
    – ShemSeger
    Commented Mar 2, 2016 at 4:23
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Nodules, as in chert nodules are common in limestone, but not confined to any particular sedimentary rock.

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The word xenolith (from ancient Greek, meaning stranger stone or foreign rock) is used to describe stones entrapped with an igneous matrix. However, as the reference states,

a broad definition could include rock fragments which have become encased in sedimentary rock

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    $\begingroup$ As xenolith is a technical term in igneous petrology it's best to avoid its use in other areas to steer clear of confusion. I answered this as well, but wanted to comment on your answer. Technically correct is the best kind of correct! $\endgroup$
    – Ben MS
    Commented Feb 29, 2016 at 3:32
  • $\begingroup$ In this context I don't like the use of either 'xenolith' or 'erratic', both of which have highly specific connotations in igneous and glacial contexts, respectively. There isn't a specific term for the boulders you refer to, so resort to the time-honoured geologist's tradition of coining a new term. How about 'Terogoclast'? = large chunk of eroded / weathered rock. $\endgroup$
    – Gordon Stanger
    Commented Aug 10, 2016 at 1:11
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I believe the term you are looking for is "Intraclast". That is, an allochthonous clast (one not formed in situ) that is not a single grain or mineral, but a rock composed of multiple other grains or minerals. An example would be a granule of siltstone lithified into a coarse sandstone or a granule of granite similarly lithified in a sandstone. To borrow the conglomerate example from above, a pebble or cobble of quartzite, or sandstone, or any rock included in that conglomerate is an intraclast. The quartz grains or clay or mica in that same conglomerate are all just clasts. These are chunks of other rocks (not individual minerals) included in a new sedimentary deposit.

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