To make the tapered parts you could use black squares to cut off a parts of the shape. To keep it symmetrical you only have to do one side, and then use a mirror node.
You could also try the svg node and just draw out one side and use the mirror again to make it symmetrical.
As for the tessellation: very fine details, like cracks between stones, won't show up unless you have a high enough polycount. Even if you subdivide with a factor of 64 this can be to little sometimes. The high-res plane has a very high polycount to start with (higher then a regular plane with a factor of 64), so that plus an increased tessellation factor makes for an extremely high polycount. You can get a better image of it by turning on the wireframe (Display>wireframe. Do mind: this can cause some slowdowns/ possibly crashes at very high polycounts)
What happens if you use something less detailed, like a perlin noise, for a heightmap? If still nothing happens there's something else wrong.