user81101. Welcome to Worldbuilding!
From a certain point of view, this question can be answered "anything that works." For example, you could use the traditional triangle (upside-down "V") and fill it with polka-dots. It would work as well as any other symbol.
There are cartographic icons that identify different types of mountains. Here's a sample:
I found that on a Pinterest page and don't know its provenance. You'll note that it doesn't identify the specific type of mountains you're dealing with. This is probably because mountains like that are found in some Asian countries, for example, but are not found in the U.S. or western Europe, where that table was probably created.
Part of the problem is that you don't identify the context for your map. Do you need it to reflect actual catrographic icons? For quite a while now, geologic maps simply show regions of color and texture. Check out the map of China below (courtesy, and much bigger at, ikkrest.org).
See the colors and textures? Even at smaller levels, this is becoming popular. There are no more icons, just colors, textures, and involved keys. That would be the most efficient way to express your map, it's just not the most intuitive because what you're looking at doesn't look like a mountain.