Poster presented at the American Geophysical Union, 2006, San Francisco.
Dramatic changes in land use were associated with the rise of agriculture in the mid Holocene in the Mediterranean region. Both surface properties and drainage networks were changed along with direct modifications to surface properties (vegetation removal and change, sediment liberation and compaction); consequent drainage alteration (terracing, canals) and up and downstream responses in the watersheds communicated these changes throughout the landscape.
The magnitude, rate, and feedbacks with the growing human populations are critical questions in our effort to assess human-landscape interactions. To investigate these relationships, recent field work in the Penaguila Valley of southeast Spain included landform mapping, alluvial deposit description, and sample collection emphasizing areas of active erosion, remnant land surfaces and their relation to archaeological sites.
We have updated our geomorphic maps by refining the delineation of alluvial terraces, steep-walled (40m deep) drainages ("barrancos"), and hollows ("barrancos de fondo plano"). Hollows are curved, elongate, flat-bottomed gullies with steep walls (2-30m tall) and extend headward from the main barrancos. This work enables more accurate terrace correlations necessary for both landscape evolution modeling and interpretation of the development history of the basin.
Alluvial terraces are crucial to this research because they record periods of past stable topography. In the Penaguila, sites dating back to late Mesolithic and early Neolithic (around 6600 BP) and subsequent periods (Chalcolithic and Bronze Age) are exposed on a prominent terrace surface mapped as Terrace A. This broad low relief surface is scarred by deep barrancos and hollow formation that expose bedrock marls and overlying alluvial deposits. Stratigraphic profiles and texture analyses of Terrace A deposits reveal overland flow facies and channel networks in reworked and CaCO3-encrusted marls, and several organic-rich paleosols. Small remnant surfaces mapped as Terrace Z (below Terrace A) were observed within the main barrancos and indicate a later, brief accumulation period with subsequent incision to the modern channel.
Holocene landscape development in the Penaguila appears to have progressed from a period of stability to slope denudation with aggradation (stream infilling) followed by rapid incision which initiated sometime near the time of occupation. This change from a low relief alluvial surface to one cut by narrow channels may have been an important shift for local populations. Their response to that environmental modification may be associated with the horticulturalist to agricultural intensification noted in the archaeological record. Tighter chronology and better understanding of the driving processes for barranco incision and hollow formation will improve our ability to correlate the changing landscape with land use practices. Such an improved correlation leads to better understanding of human-landscape interactions.