Ads scrolling over navigation bar

I’m using WordPress 4.3.1 and my website is https://www.gnarledcarrots.com .

I just got ads with AdSense and when you scroll through my page, the advertisements scroll over my navigation bar.

I tried changing the z-index, but that didn’t do anything.

My navigation coding is:

.menu-toggle {
        display: none;
    }
.main-navigation {
	margin-top: 0px;
	text-align: center;
    	width: 100%;
	position: fixed;
	top: 0;
	left: 0;
	background-color: #292929;
	padding: 0;
}

.main-navigation a {
	font-size: 14px;
	font-weight: 100;
	text-transform: uppercase;
	text-decoration: none;
	color: #fff;
	font-family: 'Lucida Grande', 'Lucida Sans Unicode', 'Lucida Sans', Geneva, Verdana, sans-serif;
	letter-spacing: 1.2px;
	word-spacing: 0.8px;
	line-height: 25px;
	padding-top: 10px;
	padding-bottom: 6px;
	padding-left: 25px;
	padding-right: 25px;
}