The Three Sisters are made of Arkose, a type of sandstone that contains a significant amount of feldspar.

Here's why:

* Sandstone: The most obvious component, the rock is formed from cemented sand grains, which makes it a sandstone.

* Feldspar: The presence of feldspar gives it a pinkish or reddish hue. This is why the Three Sisters are known for their distinctive red color.

* Arkose: The high feldspar content puts it into the specific category of arkose.

So, the Three Sisters are actually huge arkose formations, formed by erosion and weathering over millions of years.