Thursday, December 21, 2006

Life in the Macleay Valley - the glacial age

Update One (5 January 2007): I am adding references on the geology of the Valley to the end of this post.

A few years ago I read a very good book by John Mulvaney on the aborigines. This dealt, among other things, with the nature of geological change.

Unfortunately I did not take notes. I was reading just for pleasure, so I missed recording his key points. However, now that I am again writing with purpose, I need to look at New England's geological history.

As in my first post on the Macleay Valley, the material in this post is drawn especially from Marie H Neil's book, Valley of the Macleay. The History of Kempsey and the Macleay River District (Wentworth books, Sydney 1972). While my focus is on the Macleay, I am also using the post to set a broader context.

Over the period from 20,000 to 800,000 years ago there have been four ice ages in New England during which sea levels rose and fell.

Around 125,oo years ago the sea level was around 25 feet higher than it is now, so much of the lower Macleay Valley was under water.

In the fourth ice age which began about 100,000 years ago, the sea level began to fall. This moved the shore line out about six to ten miles, creating a large coastal plain that stretched along the current New England coastline.

Around 20,000 years ago, the sea level began to rise again, submerging the coastal plain. This rise continued until about 5,000 to 6,000 years ago, then slowed down. I do not have dates for the arrival of the first aborigines in the Macleay Valley, but it seems likely that they would have experienced this change.

During this period, great quantities of sand were deposited to the north of the emerging coastal islands, creating great spits. These eventually grew into large sand borders or dunes, linking the coastal islands (now headlands) along a line approximating the present coastline.

The current Macleay River deltaic plane came into existence as the River changed its course several times seeking new ways to the sea through the growing sand barriers. In so doing, great deposits of silt were carried down and left inside the sand barriers, creating the delta we can see today. Over large areas these alluvial deposits are more than 100 feet deep.

Additional References on the Macleay

Dr Tim Cohen, The geomorphology of the Macleay River Estuary, study prepared for the Kempsey Shire Council, September 2005
Damon Telfer, Macleay River Estuary Data Compilation Study, GECO Environmental, October 2005


Rod said...

Hi Jim,
I'm developing a blog that has a very strong geological theme for the northern rivers area. I dont know how interested you or your other readers are but please have a look and make any comments. If you think it worthy please add to your blog roll too!

Jim Belshaw said...

Rod, what a great blogging idea! I will run a story.