Small Basic - More "natural" terrain generation for simple Minecraft clone I've found online

This is the main focus of the question:

For BlockX = 0 To 900 Step 30 'Try to carve out Hills
 GraphicsWindow.BrushColor = "SkyBlue"
 depth = Math.GetRandomNumber(5)
 GraphicsWindow.FillRectangle(BlockX,BlockY,30,depth*30)
EndFor

And the whole program:

GraphicsWindow.Clear()
GraphicsWindow.CanResize = 0
GraphicsWindow.BackgroundColor = "SkyBlue"


gw = GraphicsWindow.Width
gh = GraphicsWindow.Height
gw = 900
gh = 600

For BlockX = 0 To 900 Step 30
For BlockY = 420 to 270 Step -30
    b = Math.GetRandomNumber(2)    
    If b = 1 Then 'Stone
      GraphicsWindow.DrawImage("C:\AllVersions\Versions\v0.4\SBCraft\assets\rock.png",BlockX,BlockY) 
    EndIf
If b = 2 Then 'Dirt
      GraphicsWindow.DrawImage("C:\AllVersions\Versions\v0.4\SBCraft\assets\dirt.png",BlockX,BlockY)
    EndIf
  EndFor 
EndFor

For BlockX = 0 To 900 Step 30 'Grass Layer
  BlockY = 240
  GraphicsWindow.DrawImage("C:\AllVersions\Versions\v0.4\SBCraft\assets\grass.png",BlockX,BlockY)
EndFor

For BlockX = 0 To 900 Step 30 'Try to carve out Hills
  GraphicsWindow.BrushColor = "SkyBlue"
  depth = Math.GetRandomNumber(5)
  GraphicsWindow.FillRectangle(BlockX,BlockY,30,depth*30)
EndFor   

Explanation: I found a very simple Minecraft clone online, and I'm trying to expand to it by adding a terrain generator. The terrain generator supports 3 blocks dirt, grass, and stone. For the bottom 6 layers, dirt and stone randomly generate, and a layer of grass on top. The main focus of this question is the last For statement. For each column, a random number is selected. The random number selected is how many blocks are deleted in each column. But this makes very jagged and unnatural terrain. Is there some weird trigonometric thing I can do to make hills / natural terrain, or another way?