Pete;
Perhaps you could use some of the Woodland Scenics small rock molds and Hydrocal plaster pre-mixed with a reddish clay colored powder paint. Once the Hydrocal sets, you could soften some of the harder angles with a file. Kind of leave some of the dust from filing. Use an “El-cheapo” hair spray to hold it in place. Should look pretty much like dollops of clay from a steam shovel scoop.
Just a thought, David Meashey