Here is the full list of all 30 Valley achievements. It takes around 8-10 hours to unlock all of the achievements on Windows.
Kill 50 trees
Drop down the pit located in Proving Grounds
Gain entry to the Sussurian Pyramid
Spend 20 minutes of accumulated time in mid-air
Collect 100 acorns
Complete the Astra Facility
Complete game without the valley dying
Stop the Astra Facility from killing the valley
Defeat the Alpha
Complete Smoke Lake
Complete the Proving Grounds
Complete Camp Henday
Defeat 25 dark creatures
Take 10,000 steps
Find Virginia's music box
Take life from 20 animals
Revive 100 trees
Complete the Soma Reactor
Complete the Soma Facility
Complete Titan Rock
Complete Susurrus Valley
Pacify 10 amrita swarms
Complete Wendigo Lake
Collect enough medallions to access all pyramid chambers
Acquire all upgrades for the L.E.A.F. suit
Gain entry through all the acorn doors
Acquire the Lifeseed
Collect all energy upgrades
Discover the secret Landshark encampment
Find all 8 pages