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<br />;-\ <br /> <br />wi <br /> <br />(103331 <br /> <br />During the course of drilling the Dowds No.1 landslide was studied using <br />standard field methods, Prior to field study I had some preliminary ideas, <br />based on aerial photograph analysis that proved to be partially or entirely <br />incorrect, The most surprising discovery made is ,that approximately the upper <br />one half of the landslide area shows no field evidence whatsoever for modern <br />movement, Also notable is the fact, that in this upper area, where the <br />landslide is composed of very large blocky material consisting of Minturn <br />Formation arkosic sandstone with individual blocks ranging to 30 ft in <br />giameter, the snowmelt water was observed to percolate rapidly into the <br />~round, I attribute this to the high porosity and permeability of this blocky <br />material, The fact that the distribution of these blocks on the surface <br />appears to be chaotic combined with their modern static condition leads me to <br />believe that this upper part of this landslide was formed by a massive ancient <br />rockfall collapse. The condition of the lower one half of this landslide <br />below drill holes 11,12,13, appears to be entirely different however. In <br />this area there is definite field-observable evidence for continuing but <br />relatively slow modern movement. The morphology of the ground surface, <br />vegetation changes and types. drilling data, and the limited known history of <br />movement all confirm this interpretation, An apparent scarp within the <br />landslide mass coincides with vegetation change from coniferous to deciduous <br />forest. Water was issuing from the ground along this scarp in early May, The <br />composition of material on this part of the landslide is similar to that in <br />the upper part, except that the very large blocks are absent, This suggests <br />that either the mechanism that formed the upper part is completely different <br />from that which formed the lower part or, and perhaps a more likely reason, <br />modern movement of the lower part can be attributed to seasonal increase of <br />hydrostatic pressure in the lower part caused by percolation of water from the <br />~pper part and into it, The origin of material in the lower part then could <br />l~ considered to be the same as that in the upper part, with the largest <br />blocks being absent because of their disintegration by periodic movement. <br /> <br />With this interpretation of the, field data and the budgeted dri11ing- <br />program constraint of locating three,water-level-monitoring holes in the Dowds <br />No.1 landslide, I selected the drill hole locations in the highest <br />accessible places below the scarp. A permit had to be secured from the U.S, <br />Forest service to construct an access road. The amount of tree damage and <br />esthetic-degradation effects had to be considered also, Three holes were <br />drilled to about 100 ft each as the program called for. The basal landslide <br />shear zone was certainly not encountered in holes 11 and 12, In hole 13 we <br />en~9unt~reg~~~e~reg,~an~~t9ne ~oulders but the diagnostic test of going <br />through such a zone ana then bacl(Hirito unshearedDe'dr'ock wa'shot'ii'chi'eVed;No <br />free water or artesian flow occurred during or after dri 11 i ng" but si nce these <br />were the last holes to be drilled in the Dowds Junction investigation it was <br />well past the snowmelt season, The entire drilling program ended on July 19th, <br /> <br />- 9 - <br />