I had found mention of this a number of years ago and was interested enough to pursue a search in an attempt to identify which bear image from southern Colorado this might be. During a subsequent conversation with Larry Loendorf we agreed that it might be the large Picketwire bear. This figure was prominent, had been discovered and publicized early on - its photograph had been printed in newspapers. Loendorf also pointed out that it was originally known as the “cinnamon bear” because rain runoff from the canyon rim had dyed it red with the red dust of the soil. This seems to match the description of it being “apparently ‘painted’ with red iron on the face of a soft rock”.
On May 13, 2009, I received from the Darwin Correspondence Project a transcription of the letter, which described the picture and its location. “The image is painted – as it were – on a perpendicular face of a very soft grey sandstone rock, about 40 feet from its base & 38 feet from its top, but may be easily reached – to the level of the bottom of the picture – by climbing over the dèbris at the foot of the bluff. . . . The coloring matter appears to be iron (probably Fe3O4) and penetrates the rock to a depth of more than ½ inch. . . . The image is in length, from nose to tail, about 8½ feet”. (This preliminary transcription has yet to be published in the Correspondence of Charles Darwin.)
(I wish to extend an extra thank you to Rosemary Clarkson of the Darwin Correspondence Project for her generous assistance with my inquiry.)"