Hummocky Cross Stratification (HCS)

Hummocky cross stratification (HCS) forms from the passage of aggrading symmetric ripples.  HCS is formed by the movement of 3-dimensional symmetric ripples.  Because the ripples are symmetric and the flows are simply moving back and forth, rather than downstream, the stratification is characterized by laminae that both fill in swales and that arch upward into hummocks. The stratification, therefore, contains laminae that are oriented both convex- and concave-upward.  When HCS beds become amalgamated, this stratification can be confused with TCS because the convex-upward laminae are often erosionally truncated.

	Here's an example of a very subtle hummocky surface.  The higher topographic features are called hummocks, while the topographic depressions are called swales.
	This hummocky surface is somewhat better developed than in the above photo.  Note that the spacing of hummocks is such that a new hummock is present approximately every 120 degrees, and that the depression between three hummocks defines the swale.
	Here's a somewhat unique feature, wherein a smaller hummock is preserved within the swale formed by three larger hummocks.  Such a bed geometry will result in more complex hummocky cross stratification.
	HCS in the Ferron Sandstone, Gentile Wash, Utah.  This is a classic example of HCS.  In the lower part of the bed (beneath the pencil), you can see planar laminae, indicating high speed flows, probably in an oscillatory environment.   These laminae must have been deposited during the main phase of the event, and then as the flow waned, and flow speeds decreased, HCS began to form.  You can see the gradual transition to the gently arching laminae that define the hummocky stratification.   Look closely at these laminae and notice that the laminae fan and become more widely spaced as you move toward the swale, and pinch to become more narrowly spaced as you approach the hummock.  As you progress upward through the bed (particularly noticable on the right side of the photo, about 3/4 of the way from the base of the bed), you'll notice some irregular spacing between laminae.  What's happening here is that the laminae are becoming amalgamated, perhaps because of discrete shifts of the bedform over the sediment surface during the depositional event.  Near the top of this bed, particularly on the left, you'll see that the laminae are disrupted by burrowing organisms.