Sculpting a pool in Landscape

Hi,

Am new to UE & trying to sculpt a landscape for a pool which i modeled in 3DS Max. I needed a landscape since its a exterior scene & i am planning to paint foliage on it.

The landscape tends to mould with my sculpt. I tried Clay brush & ZERO Falloff. Am i missing something obvious? or is there a better way to do it?

http://postimg.org/image/40piducd1/

Thanks in advance,