A Trench
A ditch with deep sides is commonly referred to as a "gorge" or "ravine." These terms describe narrow, steep-sided valleys that are typically formed by the erosion of rock and soil by a river or other water flow. In some contexts, a "trench" may also be used to describe a similarly deep and narrow excavation.
Platue
name of a polygon with 12 sides = dodecagon
A shape with three or more sides is a polygon. Not sure there is a special name for 4 or more sides.
Try Fjord.
Mesa means a hill that has a flat top and steep sides and that is found in the southwestern U.S. Its antonym could be:canyon,ditch, orgulley
A ditch with deep sides is commonly referred to as a "gorge" or "ravine." These terms describe narrow, steep-sided valleys that are typically formed by the erosion of rock and soil by a river or other water flow. In some contexts, a "trench" may also be used to describe a similarly deep and narrow excavation.
A Canyon
steep sides
"Gully" refers to a narrow valley with steep sides, often formed by running water erosion. It can also refer to a water-worn ravine or a drainage ditch along the side of a road.
the type of landform is a plateau.
A cliff
A cliff
On sidehill cuts or steep grades, ditch relief culverts should be installed at an angle of 60º to the centerline for more direct entrance of water into the culvert.
A hill has steep sides because of how the hill formed it formed with steep sides. I don't agree. Hills have steep sides because of the Earth's plates. When the plates push against each other, they sometimes push up, creating a hill or a mountain. Whether the hill has steep sides or not depends on the factors such as weathering and erosion, and on quite simply how it forms.
It is called a plateau
You might be thinking of a plateau