This might be a little crazy, but I actually took DEM data from the NOAA grid extract tool, converted it to a height image using MicroDEM, pasted the image in Photoshop and adjusted it to the proper size, then took it in G16 to convert it to a 16-bit format to work with Unreal’s landscapes, and imported an actual real-world model of the ocean floor.
There even exists a free online tool to download DEM data for any place in the United States that you want. If you literally want the Rocky Mountains in your game, select the area you want to use, convert it to a format Unreal can read, and you can literally have the heightfield of the Rocky Mountains in your project.