Identifying Bottom Composition on Fishfinder

[fishinggeek]

Anyone have any tips on how to identify what kind of bottom is under the boat, such as mud, gravel or rock? I can clearly identify boulders, weeds and submerged trees, but not sure about the rest.

[bow slayer]

thin line on bottom of finder indicates hard bottom ,thicker line as bottom =mud.....at least on older lcd finders like I have