I have a blend of p. rye, ky blue and fine fescue. Back when my lawn was
seeded in the Spring a lot of run off created some large erosion gullies
and lots of smaller ones that are hidden by the grass now.

On side note I filled, seeded and burlapped the large gullies but
repeated deluges have removed about half the soil I laid, so I must wait
till the Spring to fix it. I think I will use sod instead. If anyone
feels the new soil under burlap should have withstood the heavy rains and
that I must have laid it wrong I welcome any tips. Most books say, lay
straw or burlap or some of that new green stuff.

My real question is that I notice when I mow the smaller gullies still
are free of grass even though they are not very deep (1/2 inch at most
and an 1-2 inches in width). The grass around them is thick and green.
Given the grass is growing now why do these areas not fill in?

Thanks for any advice.