Icelandic Horse and the Ort

Photo by F. Howell, Cornell University Library

Taken around 1900, this print is labelled as "'Bloc,' Hvítárvatn. One of the 'orts' of Langjökull."  I had to do some research to figure out what this meant since "ort" means a bit of uneatten food remaining at the end of a meal.  Langjökull is the second largest ice cap in Iceland located in the western part of the Icelandic interior.  And Hvítárvatn is a lake near this glacier.  So my interpretation of the caption is that this Icelandic horse is taking a rest near one of the huge "bits" thrown out by an active volcano at one point in time.  How would you interpret the caption?