I found this composition while searching for what Bleecker St. looked like in the 1970s. I clicked on a photo of Morton Subotnick, who had a studio on Bleecker back then, and found a site, Critical Terrain. This site also had a blurb on Terry Riley's In C. The constant motion and layers made me feel a simplistic happiness.
Here's what Terry's website says about the composition:
"IN C 1964 - for any instruments - (usual length 45” to 1’45”)score which is the part for all players...The composition that launched the Minimalist movement "In C", with performing directions, in Terry's own manuscript and autographed."
The composition also has its very own Wikipedia entry. Check it for a more in depth look at In C.