# What's the youngest sedimentary rock? **URL:** \ **Category:** Factual Questions\ **Created:** [April 28, 2009, 7:53pm UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660 "2009-04-28T19:53:03Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [April 28, 2009, 7:53pm UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660/1 "2009-04-28T19:53:03Z")
Now, igneous rock can obviously be from the Holocene. But what is the youngest age of a large-scale collection of sedimentary rock? In other words, how long does sand or organic material take before it can be turned into sandstone or limestone? I assume that there are some evaporites or conglomerates forming as we speak that could count as “large-scale”, but I’m more talking about sandstone and limestone here. But while we’re at it how long does it take for the fastest conglomerate to be naturally cemented into what we’d call a conglomerate form of rock? (I assume orders of magnitude less than for limestone and sandstone, but can it take on the order of years rather than centuries or millennia?) ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [April 29, 2009, 2:51am UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660/2 "2009-04-29T02:51:10Z")
It’s my understanding that sedimentary rock is continuously being created as sediments flow out to sea and settle on the continental shelf (as well as the oceanic basins). In other words, it’s somewhat of a continuum. If you were to dig into sedimentary deposits, you would find unconsolidated deposits on the topmost layers, which would be more and more cemented together into rock the deeper you went. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [April 29, 2009, 9:07am UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660/3 "2009-04-29T09:07:40Z")
It depends. Much sedimentary rock is itself derived from eroded rock, which is cemented together by clays or other chemically distinct soils that can harden in mere years. For things like limestone and sandstone, you have to decide how hard and dry it has to get before it’s called rock. ---
**Author:** ![Yossarian](https://avatars.discourse-cdn.com/v4/letter/y/9de0a6/32.png) [@Yossarian](https://boards.straightdope.com/u/Yossarian)\ **Post date:** [April 29, 2009, 12:44pm UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660/4 "2009-04-29T12:44:27Z")
Limestone? Easy: every active reef out there. The Bahamas, for instance: [http://strata.geol.sc.edu/Bahamas/BahamasGalleryIndex.html](http://strata.geol.sc.edu/Bahamas/BahamasGalleryIndex.html) (You might want to page **lieu**. He likes his rocks soft.) ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [April 29, 2009, 2:48pm UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660/5 "2009-04-29T14:48:52Z")
> [@Ludovic](#): > > Now, igneous rock can obviously be from the Holocene. Are you forgetting active volcanology? New igneous rock is being created every second in Hawaii, among other places. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [April 29, 2009, 3:32pm UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660/6 "2009-04-29T15:32:23Z")
> [@Duckster](#): > > Are you forgetting active volcanology? You wouldn’t consider us to be in the holocene right now? ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [April 29, 2009, 4:10pm UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660/7 "2009-04-29T16:10:50Z")
I understand that there’s an enormous amount of aggregate being formed in a matter of days throughout urban areas on a regular basis. 🙂 Ignoring concrete, I agree with the others that it’s going to depend on your definition of when something becomes a rock. It’s a continuous process going on in numerous places throughout the world. ---
**Author:** ![Stana\_Claus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stana_claus/32/13874_2.png) [@Stana\_Claus](https://boards.straightdope.com/u/Stana_Claus)\ **Post date:** [April 29, 2009, 4:19pm UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660/8 "2009-04-29T16:19:55Z")
What about stalactites, stalagmites, and other cave formations? Those things are continuous works in progress in any active cave system. ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [April 29, 2009, 4:57pm UTC](https://boards.straightdope.com/t/whats-the-youngest-sedimentary-rock/494660/9 "2009-04-29T16:57:00Z")
> [@Ludovic](#): > > You wouldn’t consider us to be in the holocene right now? I’m not into the nightclubbing. :smack::smack: