The objective of this study is to examine the mineralogical transformations across the profile of incipient metamorphism in the Mancos Shale in order to develop a better understanding of the significance of clay mineral assemblages and thermally induced changes for the interpretation of the low temperature history of sedimentary rocks. Particular attention is given to the thermal dehydration of mixed-layer illite-montmorillonite and the relationship this has to similar transformations observed in deep bore-holes. The principal conclusions regarding the preintrusive conditions in the Mancos Shale, the effect of contact metamorphism on the Mancos Shale, and the thermal history of the Crested Butte area are: (1) A preintrusive temperature of about 15°C was attained in the Mancos Shale in the Crested Butte and Glenwood Springs areas as a result of depth of burial. (2) The low temperature contact metamorphic transformations observed in the Mancos Shale are compatible with the transformations observed in the Gulf Coast bore-hole studies when variations due to differing bulk compositions are taken into account. (3) The irreversible decrease in expandability of mixed-layer illitemontmorillonite to a completely dehydrated state occurs as a continuous step with increasing temperature. (4) The complete irreversible loss of expandability of the mixedlayer illite-montmorillonite is the first change in the Mancos Shale which signifies the onset of metamorphism. (5) The process of irreversible dehydration without structural reorganization of the mixed-layer illite-montmorillonite has predominated in the contact metamorphosed Mancos Shale in the Crested Butte area. (6) Excellent correlation is obtained between the temperature dependent changes which have been observed in the organic matter contained in the Mancos Shale and the irreversible dehydration of the mixedlayer illite-montmorillonite. (7) The structural reorganization of the mixed-layer illite-montmorillonite and/or its breakdown with the formation of new phases did not occur in the Mancos Shale until a temperature nearly high enough to form metamorphic biotite was attained. (8) Biotite is the first new product of recrystallization in the Mancos Shale. (9) Theoretical calculations of the thermal distribution around the laccolithic intrusions indicate that they could not have caused the contact metamorphic effects observed at large distances from the contacts. (1) It is apparent that there is an intrusive mass at a relatively shallow depth which is large enough to cause significant contact metamorphic effects over a very large area.