Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of construction materials maker Vulcan Materials (NYSE: VMC) erupted today, jumping as much as 34.1% amid crushingly heavy trading.

So what: Rival Martin Marietta Materials (NYSE: MLM) just posted a $4.7 billion hostile takeover offer for Vulcan. The companies discussed a merger last year, but nothing came of it, so now we have the smaller company running the show with a stock-swap bid for the larger one.

Now what: Martin Marietta's shares also gained nearly 5% today as investors saw value in the proposed deal. Since it's a stock-for-stock transaction, Vulcan's market cap is now larger than the bid would indicate. Combining these all-American materials specialists would unlock economies of scale. Vulcan investors who were disappointed by the recent dividend cut should welcome this offer with open arms.

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