Found this and it sounds like other things that I've heard in the past.
Rock debri from jyarb #11210 1/4/2009 9:44:04 AM
From Michigan Sportsmen:

When shipping channels were blasted into Lake Erie's lakebed, the broken rock was loaded aboard large barges and towed away. Much of this material was used to develop breakwalls and harbors along the Detroit River, but a portion of the debris was simply dumped some distance away from the blasting sites. This created a type of "smallmouth heaven." Smallies congregate and hold in large numbers among these dumping grounds, most of which are located one-half mile east of the Banana Dike off Gibraltar. Bass aren't the only species to take advantage of this hotspot. Walleyes, pike and perch all call this area home and can be regularly caught here throughout the summer months. Also contributing to this location's good fit into the smallmouths' needs are the schools of baitfish that are drawn to these rocks. In all, the rock-infested dumping grounds may just be one of the top locations for smallmouths in our state and certainly in Michigan's portion of Lake Erie.