Very solid. They are man-made, composite stuff. Won't compress, even wet, like wood can. They are 9"+ and tapered very nicely. Plus, score them a couple times, and they readily snap off. Can't go wrong with 'em.
Wish I could link them, but even with the code from my receipt, they won't come up on the Home Depot site where I bought them.
It's these, but grey instead of brown
http://www.homedepot.com/buy/lumber-...it-152940.html