Glacial landforms often occur in suites. This makes formerly glaciated landscapes predictable - somewhat predictable. Landforms have distinctive appearances. Each form links back to a dominant geomorphic process (flowing water, collapsing slopes, melt-out of stagnant ice). Geomorphic processes, landform units, landscape position, and the nature of the sedimentary deposit often correlate. If there the landscape is ordered, then mapmaking is possible. And landscapes are always ordered.
Geologists see the order in landscapes.
Some geologists are skilled enough to delineate the units. The really bright ones make maps that are so good that they remain unchanged for generations. Once you tune into the local pattern of adjacent landforms - how a terrace escarpment rises to its correlative flat, which in turn ascends to an alluvial fan, a bedrock slope, or a moraine - you can begin to put pencil to paper with some confidence. Its amazing how quickly your brain starts to recognize patterns and predict what's around the next corner.
This map was a student assignment. I handed out 11" x 17" photocopies of a portion of this USGS topo map (blank topo maps) to my Remote Sensing students at Boise State. "Map the landforms you see - active floodplains, terraces, landslides, closed depressions, bedrock-controlled hillslopes, etc." I gave them a list of about 10 units to work with. They looked at the map, then back at me, and seethed silently with rage. "You want to start with the terrace surfaces and work your way up in elevation. Look for continuity in the flow of the contours. Pay attention to the places where the continuity is interrupted. Any questions?"