问一逻辑题的语法结构

Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska, is known to be 11,200 years old.  Researchers reasoned that, since glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago, humans must have come to the Americas more than 18,000 years ago.

gwd的一道cr，我不确定阴影部分的结构，推测意识是“阻止人类从Alaska-Siberia”大陆桥向南迁移，migration和south和from是什么修饰关系？这是什么语法点呢？