We were puzzled about the Supergroup so we bought a pdf map from the Geological Society of America that covered the eastern part. We found that a forest of inconsequential faults, strikes and dips, and other notations was distracting us from the trees. We converted it to Autocad with the notion of moving these nuisances to a different layer. This tediously done, we still did not like the hatches so we set about creating our own with better contrast.
This proved to be astonishingly difficult as the unit boundaries proved to be multiple crudely overlapping splines and polylines that confounded our new hatches. We wound up erasing duplicates and retracing the boundaries. Lots of Supergroup exposures lie west of our GSA map, so we used a similar approach with scaled screen captures from the USGS “Mapview”. These we positioned in
Arcmap to create the previously posted image below.
Arcmap allows crude two point geolocation of Autocad linework that is good enough for views from the stratosphere as above, but when we dug into the schists we really wanted more. We had been unable to accurately position the GSA map in Autocad. Autocad warps the underlying Bing image to match the linework from a single geolocation point, and none of their projections matched.
During the schist work we realized that the Autocad imagery was good enough that we could SEE the layers and began drawing directly on the imagery.
Thus the Unified Map of the Grand Canyon was born.
It was a lot of work. It is a completely different kind of geological map based on the river runner’s encounter with the TOP of each layer. Top means when you can’t see it any more because it is buried by the layer above, not when there is air above it. If there is air above it, you are somewhere below the top and above the bottom; which here is the top of the layer below. Using only the top forces constant mindfulness of the stratigraphy.
We use no regions or hatches. The imagery itself becomes the hatch, true to the nature of the layer. We ignore land slides and faults not critical for understanding. Recent lava is a distraction. It is plainly visible. We die into it when it is thick, but often find a line that telegraphs through.
The map is unified in that the USGS Mapview uses many different maps compiled over half a century with different groupings and different names. Our interpretation groups Esplanade with Supai, Surprise Canyon with Redwall, and Temple Butte with Muav. The latter two are discontinuous, and well, we didn’t need any more lines.
We will get further into the features of this map in the next post.