Second sidebar border top on media 640px and 960px

When, there is no First sidebar, Second sidebar border top is above to Content.
I had to modify css :

@media screen and (max-width: 960px) {
  #sidebar-second {
    clear:both;
  }
}
@media screen and (max-width: 640px) {
  #sidebar-second {
    clear:both;
  }
}