Two rows in a DIV, one with fixed height and another with fluid height

Two rows in a DIV, one with fixed height and another with fluid height

I have a DIV like below
<div class="parent">
<div class="fixedHt"></div>
<div class="fluidHt"></div>
</div>
I have written CSS like below
.fixedHt{
height:30px;
}
.fluidHt{
margin-top:30px;
}
I want to achieve the same with columns I can achieve with floats, how can
I achieve this in rows?