This is not called the South Glacier. 

It is called the S Glacier because it looks like an S when viewed from across the valley. 

Here are a few sources: 

https://en.wikipedia.org/wiki/S_Glacier

https://en.wikipedia.org/wiki/Hurry-up_Peak

Also see Fred Beckey's Cascade Alpine Guide Volumne 2: Stevens Pass to Rainy Pass. Pages 267-268

Also reference USGS and USFS maps.

Thank you.