January 06, 2007
A Holocene conglomerate from the Mohawk Valley of New York is shown above at three different scales. The left image shows large-scale cross stratification (or deltaic foresets) in the outcrop of this kame conglomerate. On the center image two gneiss cobbles (erratics) encased in calcite cement are displayed. The uniform thickness of the cement around the clasts indicates that the cement precipitated in the zone of saturation, below the water table. Because the conglomerate is well above the present water table, it's likely that these cements precipitated while Lake Albany still filled the Mohawk Valley. The image on the right shows a detail of the center image in which the “dog tooth” cement crystals are nucleated on a sandstone pebble. Cement crystals coarsen into the pore space, but do not fill it completely. A meter stick provides the scale in the first two images. The top of a pen is the scale in the right hand image. This conglomerate has never been buried more deeply than it is at present. Therefore, compaction did not play a significant role in the lithification of this essentially modern rock.