GLACIAL GEOLOGY

CLAST ANALYSIS

Objective:

This laboratory exercise is an introduction to sampling and data analysis techniques. It will focus on the measurement and analysis of pebbles as a tool in determining the position and process of transportation of clasts within a glacier.

Resources:

Roundness and sphericity charts, Zingg diagrams, (attached), and an existing data set for the location (on-line). In addition, Section 8.5 in Menzies, 1995 or Drewry p.105, 120-122 will help the data interpreation.

Background:

We will collect clasts from two different units to compare the erosional imprint on each. Since field work often involves the collection of samples for further analysis, such samples should be representative of an entire population. It is important to choose the proper sampling method, to accomplish this especially if the sample size is small. Sampling techniques will be discussed and employed to collect samples of clasts from the two units.

The size and shape of the clasts reveal something about the glacial system through which they travelled. Well rounded clasts reflect more abrasion that angular clasts. We will collect a small sample set and measure properties such as their size, roundness and sphericity. To make the stastics valid we will add data from previous years to estimate transport history of the clast population. Thus

you will also be able to compare your data set to the larger existing set, which may indicate how successful you were in choosing a representative sample. The problems of drawing conclusions from a small data set should become apparent.

Methods:

Record the following information about each clast:

1. Lithology - What type of rock is it? The following are the commonly found rock types near Cincinnati:

LS- limestone DO- dolomite SH- shale

SS-sandstone CH- chert I - igneous

M- metamorphic O- other (everything else).

2.Axis Length

A - the longest axis of the rock

B - the intermediate axis (perpendicular to the A-axis)

C - the short axis (perpendicular to both the A and B axis)

3. Visual Sphericity - how close is the clast to a sphere? (1-.9)

4. Visual Roundness - how rounded are the edges? (1-.9)

5. Striated - does any face of the clast show striations? (Y or N)

Needed Definitions:

Pebble - a clast with a B-axis of length between 4 and 64 mm.

For the Sphericity and Roundness we will use a visual comparison chart om page 4. Note that other methods to calculate these parameters have been used.

Zingg Diagram - a plot to determine the shape of a clast using the ratios of the A,B, and C axes. For example a clast with nearly equal A, B, and C lengths has a spheroidal shape. Thus to find the shape of a clast calculate the ratio of B/A and C/B and plot on the Zingg diagram. The following chart gives the four shapes we will use.

Tasks:

1. For each unit make a brief description of its grain size, color and any strucutre. Decide how to collect a representive sample of the unit. Locate your sample locations on your sketch of the outcrop.

2. Collect 15 pebbles from each unit. Make a preliminary comment on differences between pebbles from each sample.

3. Record the following attributes (page 5): rock lithology, length of A, B, and C axis (mm), visual sphericity, visual roundness, and striations for each rock.