Making maps

Thoroughly settled into the chilly winter term, I am starting to digitize some of the excavation maps we drew in the field. Of course, we took hundreds and hundreds of photos to document what we were digging, but as we begin interpreting some of these deposits, our schematic maps are proving very helpful in clarifying patterns that are difficult to make out in photos.

A months-old shot of Sophia and Jess (my crack team of mappers) creating a plan or "bird's eye" view map of the postholes that appeared during our excavations around the earthwork ditch.

Case in point: in the south wall of Unit 8 (the 5×3 m block we opened over the earthwork), we exposed a only a small portion of the ditch in cross section. Given the orientation of the entire feature — essentially coming out of the southwest corner of the unit — this clipped, outer edge was tricky to photograph. In person, you could clearly see distinct strata of ditch fill, but the lack of light in the corner of the unit made it hard to capture on camera.

The problematic south wall is at the top left. In the west profile (right) you can still see some of the intact fill layers.

So, field crew member Alicia Michalski and I made a schematic profile map, documenting the boundaries between major fill zones and other stratigraphic information we observed. To do this, we set up a level line above the edge of the wall, from which we measured the depth of various points along these boundary lines. Back in the lab, I scanned our original pencil-on-graph-paper map, traced the image digitally, and produced this map:

Although this map is not quite finished (among other things, I’ll eventually include more thorough descriptions of the zones of ditch fill), it still conveys some things that were tricky to see, both during excavations and in the photograph. For one thing, the east edge of the ditch (the boundary between Zone C-2 and sterile subsoil) is much steeper in reality that was suggested by our final excavation cuts. The eastern edge was a major challenge to isolate as we were digging for two main reasons. First, it was throughly obscured by earth worm burrows and the worms themselves. Second, the subsoil was very similar in color to Zone C-2 — a point which merits further consideration.

As you can see in the digitized map, the bottom-most layer of ditch fill is divided into Zone C-1 and Zone C-2. During horizontal excavation, it appeared as mottled sediment — dark, organic loam in some areas, and stickier, yellow clay in others. As I mentioned above, the clay was nearly the same color as the subsoil, but it was very different in texture. Instead of the extremely dense, sterile clay, which troweled away in ribbons, Zone C-2 was quite soft and cake-y. The “sterile-ish-ness” of Zone C-2 caused me and Jess no end of confusion during our horizontal excavations, but luckily, the vertical profile view offers some clarification.

The top of Zone C, with the bases of some of the rock filled postholes still in place. Notice the mottling of the yellow and dark brown sediments in the ditch, but please don't pay much attention to the patches of light soil just outside the ditch. It was late afternoon, the dirt had baked out and lightened in the sun, and our water sprayer was empty. Mea culpa.

The relatively steep angles of the Zone C deposits suggest that this sediment slumped into the ditch, probably as a result of erosion. Zone C-2 is likely run-off from the steep walls of the original ditch walls, while Zone C-2 may be top soil that washed into the ditch. The alternating layers of Zone C-1 and Zone C-2 suggest that after the ditch was completed, it was left open long enough for several erosional episodes to occur. If the stability of the walls of our excavation units are any indication, it would take a some pretty serious rainstorms or quite a long time for that much erosion to happen; Garden Creek’s clayey soils are tough! In contrast, Zones A and B, both of which had higher artifact densities that Zone C, were very homogenous, and may have been deposited very quickly. It will be interesting to see if future micromorphological analyses support or refute these hypotheses. Of course, what this all means with regard to on-site activities remains to be seen… but that’s for a later (and hopefully more concise!) blog post.

Advertisements

About Alice Wright

Alice is an Assistant Professor in the Department of Anthropology at Appalachian State University. She tweets about archaeology, Appalachia, and cats @alicepwright.
This entry was posted in Excavation, Mapping, Maps and tagged , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s