I second that. Rock on top of the ramp is a bad news for off road cars. Not too much a problem for nitro car metal chassis but a real killer to electric cars.
During hot sunshine day, you can splash water on the ramp to soften the earth. Afterward, you can remove the rocks easily.
I am waiting for the day HPC's off road track to be smooth enough that electric buggy with low ground clearance can drift nicely on it. Perhaps, IFMAR race in the future