is this pack made of water shedding material?

It is somewhat resistant, yes. It is made from a variety of different cordura fabrics, most of which include a plastic-like coating on the inside and a repellent finish on the outside. After numerous uses, however,the repellent outside finish will wear down and require reapplication using a third-party treating solution. The inside coating can break down as well and become less effective.

Get a rain cover and keep it with your pack for those surprise storms!
In short, most good packs will repel a modest rain storm for hours, but will leak at the seams and zippers over time, or if exposed to extreme weather.

On Ospreys website, they suggest Nikwax so you can spray on your bag after heavy rainstorms or when you feel the need for more water resistance.

This product works great:

http://www.backcountry.com/store/NIK0041/Nikwax-Tent-and-Gear-Proof.html
View Details: Osprey Packs Argon 85 Backpack - 5100-5500cu in >