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On the field, I find it very difficult to differentiate the two types of rocks just by looking at them.I know that you should do some analysis of the sample before any conclusions. But, for example in structural geology, I have to draw a geologic cross section and differentiate sedimentary layers.

By looking at pictures I find it very hard to tell whether the rock is limestone or sandstone.

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    $\begingroup$ Have you actually handled, seen & felt examples of sandstone & limestone? Because if you have the differences are obvious. I get the impression you haven't & that you are trying to determine the difference between the two from looking at pictures A from reading. $\endgroup$ – Fred Apr 22 '17 at 3:56
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    $\begingroup$ I don't really agree that it's always so obvious (eg. Dunham mudstone vs siliciclastic claystone can be rather tricky sometimes), but an acid test would never fail. It's hard to put words on how limestone feels compared to siliciclastic rocks. It usually weathers differently. Remember, "Carbonates are born not made" and try to imagine how they are formed, that would be my best advice. $\endgroup$ – Tactopoda Apr 22 '17 at 13:01
  • $\begingroup$ I've seen acid tests fail in the British Lower Palaezoic of NW England. Basically the groundwater was at some point very carbonate rich (probably from Carboniferous Limestones that have since eroded) and these impregnated all the mudstones/slates/sandstones with carbonates. $\endgroup$ – winwaed Apr 25 '17 at 18:02
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The main difference is micro-texture. You cannot differentiate between both from a picture or a view, you need to go to the field and:

  1. Break the rock with your hammer to observe the rock in fresh-cut.
  2. Add a bit of water to remove dust and correctly observe the colour and texture of the rock.
  3. Finally use a magnifying glass to determine what the rock is made of.

You should differentiate them as sandstones are always composed of grains. The texture is always grain-supported.

Limestones are composed of calcite. The texture use to be different, but it migth have also carbonated grains.

Dunham classification of carbonates Rebooted Dunham classification of carbonates. Image from: commons.wikimedia.org

You can differentiate them in all cases unless you have a grainstone limestone with sand-grade grains (63μm-2mm). Then you need to identify the carbonated grains that can be fossils or oolites.

Oolites Oolites. Image from: commons.wikimedia.org

Fossils are clearly differenciated from sandstone grains with a magnifying glass, and oolites have a concentrical structure as shown in the picture.

Alternatively you can use dissolved hydrochloric acid or vinegar. As limestones are composed by calcite you will see a reaction, but you could also see the reaction on a sandstone if it has carbonated components.

Once you have determined the composition of a rock bed you can do the cartography at the lab with a stereoscope and aerial pictures or do it at the field again painting the limits on your map.

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