ACL Project List

The material investigations at the Holly Tower support rock identified inherent geological properties of the monolith, such as vertical bedding and surface case hardening, which have caused large-scale loss and potential destabilization. Detached from its original location on the ledge of a canyon rim, the support rock sits on its side and the vertical orientation of its bedding planes has allowed natural weathering processes to accelerate. The rock has begun to lose material in vertical sheets of laminae, exacerbated not only by the bedding orientation but by the high evaporative rates in the desert climate, which promotes mineralogic surface migration, otherwise known as case hardening. As this phenomenon works to create a dense crust on the surface of the sandstone, the inner core becomes depleted and ultimately this crust detaches from the main body of the rock in the form of slabs that are 3” to 4” thick. This leaves vulnerable, friable surfaces behind that weather at a much faster rate until the process of case hardening begins again.

An anti-swelling ethyl silicate consolidant was tested for application to the friable surfaces of the support rock to prevent further erosional loss and slab detachment. This product was evaluated for strength improvement, depth of penetration, water vapor permeability and water absorption, among other properties, and was found to be an appropriate stabilization treatment for use on the Dakota Sandstone. The next phase of research will evaluate mechanical solutions to large scale loss, such as grouting and pinning techniques.

  total station
Image of the weathering Dakota Sandstone boulder, the focus of the conservation efforts. ACL, 2012.
Hypothetical schematic showing case hardening and loss over time.
Hypothetical provenience of the Holly Tower support rock, with yellow surfaces denoting original top surface, green indicating original area of aattachment, and blue marking geological joints..

Complete condition glossary and drawing sets are also available:

conditions glossary (6.69 MB)
conditions survey (127.5 MB)