ID sand on FF

How do I know if I am over sand?
I imagine I am over sand when the terrain is fairly flat, but how do I know or tell apart sand from grass from flat rock?

Is there a color to look for?
Humminbird 688 color dual beam

Please help, It will help my Hali hunting..... I think

I'm not an expert, but Bill is! 

I believe that its the difference in the width of what it shows as the bottom.  Say a thick 1" line would be sand, where rock would be like .25"?  Basically trying to show the density of the signal as it bounces back.

I'm not an expert, but Bill is! 

I believe that its the difference in the width of what it shows as the bottom.  Say a thick 1" line would be sand, where rock would be like .25"?  Basically trying to show the density of the signal as it bounces back.

Al nailed it pretty well.  The color difference too.  A grassy bottom will show "thicker" because of the "foggy" signal the grass sends back as opposed to rock.

When I first started fishing with a FF, I went over known areas, sand, grassy, structures, etc. learned what it looks like on my FF, that's the best way you can tell. I also, had my fishing buddy show me what a bait ball looks like on the FF.
Takes a lot of patience to really get to know FFs.
Hope that helps.

So the denser the material the thinner the line on ff because the sonar doesn't penetrate? So I am looking for thicker flat lines not cloudy. I guess I am gonna have to get it over sand I can see. So I know what it looks like at depth.

So the denser the material the thinner the line on ff because the sonar doesn't penetrate? So I am looking for thicker flat lines not cloudy. I guess I am gonna have to get it over sand I can see. So I know what it looks like at depth.

Thinner because the signal reflects ... or penetrate.

^ apparently has a problem with penetration^

  

Thanks to all who contributed.