The Nacimiento uplift to the west and the volcanic San Juan Uplift to the east formed the Chama Platform.
Initial glaciation appears to have covered a broader area, perhaps even over the Continental Divide to the west.
As the Watersheds' erosion focused glaciation eastwardly most of the oil and gas-bearing strata found in the San Juan “Basin” were stripped away, and the remaining Mancos shale and sandstone were crushed and flushed clean, by repeated glaciation and melt runoff, creating the Watersheds' glacial “flume” to the south and east.
In contrast, the San Juan Basin drains northwestly.