Sasquatch and other local websleuths can speak to the history of Roslyn Lake better than I. The boulders are all similarly sized and lined up in a row very deliberately, running east to west on the north side of the turnout. My assumption has been that they were placed there when the lake was drained, to keep people from trying to drive out on the lakebed.
They're fairly hidden by the grass at the moment but I'll try to snap some pics when I go out to water tomorrow (unless Sasquatch beats me to it

)
After looking at the Wiki article about the history of Roslyn Lake, I noticed that PGE might not own the property in that turnout anymore. The article states:
"Powerhouse ReGen completed the acquisition in 2011, the property by then consisting of the powerhouse site, the historic former Bull Run elementary school, and portions of the former Roslyn Lake Recreational Area. Since the transference of ownership, restoration has commenced on various elements of the sites, but no specific repurposing has been determined."
Bull Run Hydroelectric Project - Wikipedia, the free encyclopedia
The Bull Run Elementary School, is just a stones throw East of the turnout. Metro bought the other portion of Roslyn Lake to use as a Remote Elephant Center. I have not seen a released map, showing how the property is divided.
http://portlandtribune.com/pt/9-news/125183-elephants-pack-trunks-for-roslyn-lake
I am not sure when the boulders were placed in the turnout. I do know that they bulldozed some dirt around after the lake was drained in 2008. La Louve is right, that the boulders were placed, so that people would not drive into the lake bed. There are a lot of recreational 4wheeling people in the area. I do know that there were similar boulders on the NorthEast side before 2008, where I used to fish. I just didn't fish the South side enough, to remember if the boulders were already placed there.
That would look pretty neat having a bench made from one of the boulders. I wonder if I can get one of the local stores to donate a cement bench? That is a great idea to have one placed there. It would be nice to clear a spot and place a bench, so that people could enjoy the garden while seated!
Roslyn Lake Powerhouse Blog
http://bullrunhydro.blogspot.com/
Bull Run Hydroelectric Time Line/Including Roslyn Lake
1906 Planning begins on BRHEP
1908 Construction on LSRD and flume begin
1912 Little Sandy River Dam completed
1912 Bull Run begins generating power
1912 MHR & PC merges with PRL & PC (PGE)
1913 Marmot Dam completed
1925 Bull Run Reservoir renamed Roslyn Lake
1989 Marmot wooden crib dam replaced with new concrete dam
1999 PGE announces it will remove Marmot dam
2004 Marmot Dam operating license expires
2007 Marmot Dam demolished
2008 Little Sandy River Dam demolished
2008 Roslyn Lake ceases to exist