Scanned Topo Maps (Digital Raster Graphics)

In the viewer's left-side table of contents, select the Overlays tab, then the Other Featured Data panel, then click the Scanned Topo Maps from USA Topo layer.
Information on the historical topographic map collection is available at The National Map: Historical Topographic Map Collection site.
The phrase digital raster graphic, or DRG, can mean several things.  It is a generic term used to mean almost any scanned image of a paper map.