I am presently writting my Masters Thesis on the Assessment of A High School Geology Field Camp that I conduct at my High School in Indiana.  I struggled with finding resources on this subject as well, these are some of the better examples I have found so far.

Elkins J. & Elkins N. (2007) Teaching Geology in the Field. Journal of Geoscience Education, v.55, n.2 p. 126-132.

Huntoon J.E., Bluth, G.J.S., & Kennedy, W.A. (2001) Measuring the Effects of a Research-Based Field Experience. Journal of Geoscience Education, v. 49, n.3. p. 235-248.

Miller, George (1913) A High-School Course in Field Geography. The School Review, Vol. 21, No. 3 pp. 171-179

Mogk, Dave W., (1997) Field Notes, Monograph 3: Teaching Mineralogy, p. 47-51

Orion, Nir. (1993). A model for the development and implementation of field trips as an integral part of the science curriculum. School Science and Mathematics. v.93 p. 325-331.

Orion, Nir & Kali, Yael (2005) The Effect of an Earth-science Learning Program. Journal of Geoscience Education, 53(4) p. 377-393

Orion, Nir & Hofstein, Avi (1991) Factors Influencing Learning Ability during a
Scientific Field Trip in a Natural Environment. Presented at the Annual Meeting of the National Association for Research in Science Teaching. (Lake Geeneva, WI April 7-10, 1991)