Bridge And Tunnel Turtle

local Length = ...
if Length ~= nil then
    Length = tonumber(Length)
else
    print("enter Dig Length")
    Length = tonumber(io.read())
end
if Length == nil or Length < 0 then
    print("INVALID NUMBER")
    Length = false
end

local bannedBlocks = "yes"

function findBlock()
    for i=1,16,1 do
        turtle.select(i)
        if turtle.getItemCount() > 0 and turtle.getItemDetail(i).name ~= "minecraft:gravel" then
            return i
        end
    end
    return false

end

if Length ~= false then
    while Length > 0 do
        if Length % 15 == 0 then
            print(Length," left to Bridge")
        end
        if turtle.detectDown() then
            if turtle.detectUp() then turtle.digUp() end
            if turtle.detect() then turtle.dig() end
            turtle.forward()
            Length = Length - 1
        else
            if turtle.getItemCount() > 0 then
                if not turtle.placeDown() then
                    turtle.dropUp()
                end
            else
                local Slot = findBlock()
                if Slot ~= false then
                    turtle.select(Slot)
                else
                    break
                end
            end
        end
    end
end

if findBlock() then
    print("done")
else
    print("out of blocks")
end
turtle.select(1)